^ 


9P 


AN  AMERICAN  TEXT-BOOK 


OF 


PHYSIOLOGY 


HENRY  P.  BOWDITCH,  M.  D. 
JOHN  G.  CURTIS,  M.D. 
HENRY  H.  DONALDSON,  PH.  D. 
W.  H.  HOWELL,  PH.D.,  M.D. 
FREDERIC  S,  LEE,  PH.D. 


BY 

WARREN  P.  LOMBARD,  M.D. 
GRAHAM  LUSK,  PH.D.,F.R.S.  (EDIN. 
W.  T.  PORTER,  M.D. 
EDWARD  T.  REICHERT,  M.D. 
HENRY  SEWALL,  PH.D.,M.D. 


EDITED   BY 


WILLIAM  H.  HOWELL,  PH.D.,  M.D. 

Professor  of  Physiology  in  the  Johns  Hopkins  University,  Baltimore,  Md. 


SECOND  EDITION,  REVISED 


VOL.  II. 

MUSCLE  AND   NERVE;   CENTRAL   NERVOUS  SYSTEM; 

THE  SPECIAL  SENSES;  SPECIAL   MUSCULAR 

MECHANISMS;    REPRODUCTION 


PHILADELPHIA  AND  LONDON 
W.  B.  SAUNDERS  &  COMPANY 

J90J 


BIOLOGY 
LIBRARY 


GENERAL 


COPYRIGHT,  1900, 
BY  W.  B.  SAUNDERS  &  COMPANY 


PRESS  OF 
W.    8.    SAUNDERS   A    COMPANY 


CONTRIBUTORS  TO  VOLUME  IL 


HENRY  P.  BOWDITCH,  M.D., 

Professor  of  Physiology  in  the  Harvard  Medical  School. 

HENRY  H.  DONALDSON,  PH.D., 

Professor  of  Neurology  in  the  University  of  Chicago. 

FREDERIC  S.  LEE,  PH.D., 

Adjunct  Professor  of  Physiology  in  Columbia  University  (College  of  Physicians  and 
Surgeons). 

WARREN  P.  LOMBARD,  M.D., 

Professor  of  Physiology  in  the  University  of  Michigan. 

HENRY  SEWALL,  PH.D.,  M.D., 

Professor  of  Physiology  in  the  Denver  College  of  Medicine,  Medical  Department  of  the 
University  of  Denver. 


1 1 7283 


PREFACE  TO  THE  SECOND  EDITION. 


ADVANTAGE  has  been  taken  of  the  necessity  of  issuing  a  second  edition 
of  the  American  Text-Book  of  Physiology  to  alter  somewhat  its  general 
arrangement.  The  book  has  proved  to  be  successful,  and  for  the  most  part 
has  met  only  with  kindly  and  encouraging  criticisms  from  those  who  have 
made  use  of  it.  Many  teachers,  however,  have  suggested  that  the  size  of 
the  book,  when  issued  in  a  single  volume,  has  constituted  to  some  extent  an 
inconvenience  when  regarded  from  the  standpoint  of  a  student's  text- book 
that  may  be  needecj  daily  for  consultation  in  the  lecture-room  or  the  labora- 
tory. It  has  been  thought  best,  therefore,  to  issue  the  present  edition  in  two 
volumes,  with  the  hope  that  the  book  may  thereby  be  made  more  serviceable 
to  those  for  whose  aid  it  was  especially  written. 

This  change  in  the  appearance  of  the  book  has  necessitated  also  some 
alteration  in  the  arrangement  of  the  sections,  the  part  upon  the  Physiology 
of  Nerve  and  Muscle  being  transferred  to  the  second  volume,  so  as  to  bring  it 
into  its  natural  relations  with  the  Physiology  of  the  Central  Nervous  System. 

The  actual  amount  of  material  in  the  book  remains  substantially  the  same 
as  in  the  first  edition,  although,  naturally,  very  many  changes  have  been 
made.  Even  in  the  short  time  that  has  elapsed  since  the  appearance  of  the 
first  edition  there  has  been  much  progress  in  physiology,  as  the  result  of  the 
constant  activity  of  experimenters  in  this  and  the  related  sciences  in  all  parts 
of  the  world,  and  an  effort  has  been  made  by  the  various  contributors  to  keep 
pace  with  this  progress.  Statements  and  theories  that  have  been  shown  to 
be  wrong  or  improbable  have  been  eliminated,  and  the  new  facts  discovered 
and  the  newer  points  of  view  have  been  incorporated  so  far  as  possible.  Such 
changes  are  found  scattered  throughout  the  book. 

The  only  distinctly  new  matter  that  can  be  referred  to  specifically  is  found 
in  the  section  upon  the  Central  Nervous  System,  and  in  a  short  section  upon  the 
modern  ideas  and  nomenclature  of  physical  chemistry,  with  reference  especially 
to  the  processes  of  osmosis  and  diffusion.  The  section  dealing  with  the  Central 
Nervous  System  has  been  recast  in  large  part,  with  the  intention  of  making 
it  more  suitable  to  the  actual  needs  of  medical  students  ;  while  a  brief  presen- 
tation of  some  of  the  elementary  conceptions  of  physical  chemistry  seems  to 
be  necessary  at  the  present  time,  owing  to  the  large  part  that  these  views  are 
taking  in  current  discussions  in  physiological  and  medical  literature. 

The  index  has  been  revised  thoroughly  and  considerably  amplified,  a  table 
of  contents  has  been  added  to  each  volume,  and  numerous  new  figures  have 
been  introduced. 

AUGUST,  1900. 


PREFACE. 


THE  collaboration  of  several  teachers  in  the  preparation  of  an  elementary 
text-book  of  physiology  is  unusual,  the  almost  invariable  rule  heretofore 
having  been  for  a  single  author  to  write  the  entire  book.  It  does  not  seem 
desirable  to  attempt  a  discussion  of  the  relative  merits  and  demerits  of  the  two 
plans,  since  the  method  of  collaboration  is  untried  in  the'  teaching  of  physi- 
ology, and  there  is  therefore  no  basis  for  a  satisfactory  comparison.  It  is  a  fact, 
however,  that  many  teachers  of  physiology  in  this  country  have  not  been 
altogether  satisfied  with  the  text-books  at  their  disposal.  Some  of  the  more 
successful  older  books  have  not  kept  pace  with  the  rapid  changes  in  modern 
physiology,  while  few,  if  any,  of  the  newer  books  have  been  uniformly  satis- 
factory in  their  treatment  of  all  parts  of  this  many-sided  science.  Indeed,  the 
literature  of  experimental  physiology  is  so  great  that  it  would  seem  to  be 
almost  impossible  for  any  one  teacher  to  keep  thoroughly  informed  on  all 
topics.  This  fact  undoubtedly  accounts  for  some  of  the  defects  of  our  present 
text-books,  and  it  is  hoped  that  one  of  the  advantages  derived  from  the  col- 
laboration method  is  that,  owing  to  the  less  voluminous  literature  to  be 
consulted,  each  author  has  been  enabled  to  base  his  elementary  account  upon 
a  comprehensive  knowledge  of  the  part  of  the  subject  assigned  to  him.  Those 
who  are  acquainted  with  the  difficulty  of  making  a  satisfactory  elementary 
presentation  of  the  complex  and  oftentimes  unsettled  questions  of  physiology 
must  agree  that  authoritative  statements,  and  generalizations,  such  as  are  fre- 
quently necessary  in  text-books  if  they  are  to  leave  any  impression  at  all  upon 
the  student,  are  usually  trustworthy  in  proportion  to  the  fulness  of  informa- 
tion possessed  by  the  writer. 

Perhaps  the  most  important  advantage  which  may  be  expected  to  follow 
the  use  of  the  collaboration  method  is  that  the  student  gains  thereby  the  point 
of  view  of  a  number  of  teachers.  In  a  measure  he  reaps  the  same  benefit  as 
would  be  obtained  by  following  courses  of  instruction  under  different  teachers. 
The  different  standpoints  assumed,  and  the  differences  in  emphasis  laid  upon 
the  various  lines  of  procedure,  chemical,  physical,  and  anatomical,  should 
give  the  student  a  better  insight  into  the  methods  of  the  science  as  it  exists 


PREFACE. 

to-day.  A  similar  advantage  may  be  expected  to  follow  the  inevitable  over- 
lapping of  the  topics  assigned  to  the  various  contributors,  since  this  has  led 
in  many  cases  to  a  treatment  of  the  same  subject  by  several  writers,  who  have 
approached  the  matter  under  discussion  from  slightly  varying  standpoints,  and 
in  a  few  instances  have  arrived  at  slightly  different  conclusions.  In  this 
last  respect  the  book  reflects  more  faithfully  perhaps  than  if  written  by  a 
single  author  the  legitimate  differences  of  opinion  which  are  held  by  physi- 
ologists at  present  with  regard  to  certain  questions,  and  in  so  far  it  fulfils 
more  perfectly  its  object  of  presenting  in  an  unprejudiced  way  the  existing 
state  of  our  knowledge.  It  is  hoped,  therefore,  that  the  diversity  in  method 
of  treatment,  which  at  first  sight  might  seem  to  be  disadvantageous,  will  prove 
to  be  the  most  attractive  feature  of  the  book. 

In  the  preparation  of  the  book  it  has  been  assumed  that  the  student  has 
previously  obtained  some  knowledge  of  gross  and  microscopic  anatomy,  or  is 
taking  courses  in  these  subjects  concurrently  with  his  physiology.  For  this 
reason  no  systematic  attempt  has.  been  made  to  present  details  of  histology  or 
anatomy,  but  each  author  has  been  left  free  to  avail  himself  of  material  of 
this  kind  according  as  he  felt  the  necessity  for  it  in  developing  the  physiolog- 
ical side. 

In  response  to  a  general  desire  on  the  part  of  the  contributors,  references 
to  literature  have  been  given  in  the  book.  Some  of  the  authors  have  used 
these  freely,  even  to  the  point  of  giving  a  fairly  complete  bibliography  of  the 
subject,  while  others  have  preferred  to  employ  them  only  occasionally,  where 
the  facts  cited  are  recent  or  are  noteworthy  because  of  their  importance  or 
historical  interest.  References  of  this  character  are  not  usually  found  in  ele- 
mentary text-books,  so  that  a  brief  word  of  explanation  seems  desirable.  It 
has  not  been  supposed  that  the  student  will  necessarily  look  up  the  references 
or  commit  to  memory  the  names  of  the  authorities  quoted,  although  it  is  pos- 
sible, of  course,  that  individual  students  may  be  led  to  refer  occasionally  to 
original  sources,  and  thereby  acquire  a  truer  knowledge  of  the  subject.  The 
main  result  hoped  for,  however,  is  a  healthful  pedagogical  influence.  It  is  too 
often  the  case  that  the  student  of  medicine,  or  indeed  the  graduate  in  medicine, 
regards  his  text-book  as  a  final  authority,  losing  sight  of  the  fact  that  such 
books  are  mainly  compilations  from  the  works  of  various  investigators,  and 
that  in  all  matters  in  dispute  in  physiology  the  final  decision  must  be  made,  so 
far  as  possible,  upon  the  evidence  furnished  by  experimental  work.  To  enforce 
this  latter  idea  and  to  indicate  the  character  and  source  of  the  great  literature 
from  which  the  material  of  the  text-book  is  obtained  have  been  the  main 
reasons  for  the  adoption  of  the  reference  system.  It  is  hoped  also  that  the 


PREFACE. 

book  will  be  found  useful  to  many  practitioners  of  medicine  who  may  wish  to 
keep  themselves  in  touch  with  the  development  of  modern  physiology.  For  this 
class  of  readers  references  to  literature  are  not  only  valuable,  but  frequently 
essential,  since  the  limits  of  a  text-book  forbid  an  exhaustive  discussion  of 
many  points  of  interest  concerning  which  fuller  information  may  be  desired. 

The  numerous  additions  which  are  constantly  being  made  to  the  literature 
of  physiology  and  the  closely  related  sciences  make  it  a  matter  of  difficulty  to 
escape  errors  of  statement  in  any  elementary  treatment  of  the  subject.  It  can- 
not be  hoped  that  this  book  will  be  found  entirely  free  from  defects  of  this 
character,  but  an  earnest  effort  has  been  made  to  render  it  a  reliable  repository 
of  the  important  facts  and  principles  of  physiology,  and,  moreover,  to  embody 
in  it,  so  far  as  possible,  the  recent  discoveries  and  tendencies  which  have  so 
characterized  the  history  of  this  science  within  the  last  few  years. 


CONTENTS  OF  VOLUME  I. 


INTRODUCTION  (By  W.  H.  HOWELL). 

BLOOD  (By  W.  H.  HOWELL). 

LYMPH  (By  W.  H.  HOWELL). 

CIRCULATION  (By  JOHN  G.  CURTIS  and  W.  T.  PORTER). 

SECRETION  (By  W.  H.  HOWELL). 

CHEMISTRY  OF  DIGESTION  AND  NUTRITION  (By  W.  H.  HOWELL). 

MOVEMENTS    OF    THE    ALIMENTARY    CANAL,    BLADDER,    AND 
URETER  (By  W.  H.  HOWELL). 

RESPIRATION  (By  EDWARD  T.  REICHERT). 

ANIMAL  HEAT  (By  EDWARD  T.  REICHERT). 

THE  CHEMISTRY  OF  THE  ANIMAL  BODY  (By  GRAHAM  LUSK). 


CONTENTS  OF  VOLUME  II. 


PAGE 

GENERAL  PHYSIOLOGY  OF  MUSCLE  AND  NERVE  (By  WARREN 

P.  LOMBARD) 17 

A.  INTRODUCTION 17 

The  general  property  of  contractility,  17 — The  movements  of  amosbee,  leucocytes, 

vorticella,  etc.,  19— The  general  property  of  irritability,  20— The  general  property  of 
conductivity,  20 — The  general  distribution  of  the  properties  of  conductivity  and  irri- 
tability, 21. 

B.  IRRITABILITY  OF  MUSCLE  AND  NERVE 23 

Definition  of  various  irritants,  23 — The  persistence  of  irritability  in  excised  organs, 
24 — Irritability  of  nerves,  24 — Demonstration  of  irritability  by  various  forms  of 
stimuli,  25 — The  independent  irritability  of  muscle,  25 — The  curare  experiment  to 
prove  independent  irritability,  26 — Other  proofs  of  direct  irritability  of  muscle,  27 
— Conditions  that  determine  the  efficiency  of  irritants,  28 — Irritating  effect  of  the 
electrical  current,  28 — Description  of  the  apparatus  used  in  electrical  stimulation,  29 
— Effect  of  the  rate  of  stimulation,  31 — Du  Bois-Reymond's  law,  32 — Irritating  effect 
of  induced  electric  currents,  33 — The  myogram,  34 — The  make  and  break  shocks  of 
the  induction  current,  35 — kathodal  and  anodal  contractions,  35 — Description  of  the 
commutator,  36 — The  closing  contractions  stronger  than  the  opening  contractions, 
38 — The  effect  of  variations  in  the  strength  of  stimuli,  39 — The  effect  of  density  of 
the  electrical  current,  41 — The  spread  of  the  electric  current  in  moist  conductors, 
41 — The  spread  of  electrostatic  charges,  42 — Means  of  preventing  the  spread  of  cur- 
rent, 44 — The  unipolar  method  of  excitation,  45 — The  effect  of  duration  of  current  on 
its  stimulating  action,  46— The  effect  of  the  angle  at  which  the  current  enters,  48 — 
The  effect  of  the  direction  of  the  current  (Pfliiger's  law),  49 — The  effect  of  battery 
currents  on  normal  human  nerves,  51 — The  conditions  that  determine  the  irritability 
of  nerves  and  muscles,  55 — The  effect  of  mechanical  agencies,  55 — The  effect  of  tem- 
perature, 56— The  effect  of  chemicals  and  drugs,  58— The  effect  of  the  electrical  cur- 
rent on  muscle,  61 — The  effect  of  the  electrical  current  on  nerve,  62 — Electrotonus, 
63 — Effect  of  rapidity  of  stimulation,  65 — The  effect  of  varying  the  normal  blood- 
supply,  66 — The  effect  of  separation  from  the  central  nervous  system,  69 — The  effect 
of  the  fatigue  of  muscles,  70 — The  fatigue  of  nerves,  75 — The  effect  of  use  and  disuse, 
76— The  effect  of  enforced  rest,  77. 

C.  CONDUCTIVITY 77 

The  necessity  of  protoplasmic  continuity  for  conduction,  77 — Isolated  conduction 

in  nerve-trunks,  79— The  distribution  of  the  excitation  by  the  branches  of  a  nerve, 
80 — conduction  in  muscles,  80— The  transmission  of  the  excitation  by  means  of  end- 
organs,  82 — Conduction  in  both  directions  in  muscles,  84 — In  nerves,  85 — The  rate 
of  conduction,  87— Transmission  of  the  wave  of  contraction,  87— The  length  of  the 
contraction  wave,  88 — The  rate  of  conduction  in  different  kinds  of  muscles,  89 — Eate 
of  conduction  in  motor  nerves,  89 — Rate  of  conduction  in  sensory  nerves,  91 — The 
effect  of  death  processes  on  the  conduction,  91— The  effect  of  mechanical  conditions 
on  conduction,  92— The  effect  of  temperature  on  conduction,  92— The  effect  of 
chemicals  and  drugs  on  conduction,  93 — The  effect  of  a  constant  battery  current 
on  conduction,  94— Practical  application  of  the  foregoing  effect,  95— The  relation 
of  conduction  to  the  fatigue  of  nerves,  95— Nature  of  the  conduction  process  (nerve- 
impulse),  97. 

D.  CONTRACTILITY 99 

Graphic  records  of    simple    muscle    contractions,   99— The    myograph,  100— The 

chronograph,  100— The  latent  period,  102— Optical  properties  of  muscle  during  rest 
and  contraction,  103 — The  elasticity  of  muscle,  105 — The  muscle  contraction  in  dif- 
ferent muscles,  108— The  effect  of  tension  on  the  curve  of  contraction,  109— The 
effect  of  rate  of  excitation  on  the  curve  of  contraction,  111 — Introductory  and  stair- 
case contractions,  112— The  effect  of  fatigue  from  repeated  stimulation,  113— The 
effect  cf  repeated  stimulations  on  the  form  of  separate  contractions,  115 — The  pro- 
duction of  tetanus  by  repeated  stimulations,  117 — The  cause  of  summation  in  tetanic 
contractions,  120— The  effect  of  two  excitations  following  rapidly,  121— The  effect  of 
support  on  the  height  of  contractions,  122 — The  effect  of  gradually  increasing  the 
rate  of  excitation,  123 — Summary  of  the  factors  producing  tetanus,  124 — The  number 
of  stimulations  necessary  to  produce  tetanus,  125— The  effect  of  very  rapid  rates  of 
excitation,  126 — Relative  intensity  of  tetanus  and  single  contractions,  126— Con- 

13 


12  CONTENTS. 

PAGE 

tractures  and  continuous  contractions,  127 — Contracture  following  frequent  excita- 
tions, 128 — Contracture  following  single  excitations,  129 — The  effect  of  fatigue  on 
contracture,  130 — Effect  of  the  constant  current  on  the  form  of  contraction,  131 
— The  effect  of  death  processes  on  the  form  of  contraction,  132 — Normal  physiological 
contractions,  132 — Muscle  sounds,  tremors,  etc.,  132 — Comparison  of  effects  of  normal 
and  artificial  stimulation,  134 — Fatigue  of  voluntary  muscle  contractions,  134 — 
Effect  of  temperature  on  muscular  contractions,  136 — Effect  of  drugs  and  chemicals 
upon  muscular  contractions,  137 — Liberation  of  energy  by  the  contracting  muscle, 
138 — Conditions  controlling  the  amount  of  work  done  by  a  muscle,  139 — The  thermal 
energy  given  off  by  a  contracting  muscle,  141 — Muscle-tonus  and  chemical  tonus,  143. 

E.  ELECTRICAL  PHENOMENA  IN  MUSCLE  AND  NERVE 144 

Liberation  of  electrical  energy  during  functional  activity,  144 — Description  of  the 

galvanometer  and  capillary  electrometer,  145 — The  current  of  rest,  147 — Theories  as 
to  the  cause  of  the  current  of  rest,  148 — The  current  of  rest  in  nerves,  149 — Currents 
of  action  in  muscle,  150 — Secondary  tetanus,  150 — The  diphasic  action  currents,  152 — 
Kelation  of  the  action  current  to  the  muscle  contraction,  153 — Currents  of  action  in 
nerves,  153 — Relation  of  the  electrical  phenomena  of  nerves  to  the  physiological  pro- 
cesses, 157. 

F.  CHEMISTRY  OF  MUSCLE  AND  NERVE 159 

The  condition   of  rigor  mortis,    159— Conditions  influencing  the  development  of 

rigor  mortis,  160 — The  cause  and  nature  of  the  contraction  of  rigor  mortis,  161 — The 
chemical  changes  occurring  in  rigor  mortis,  162 — Eigor  caloris,  164 — The  constituents 
of  muscle  serum,  muscle  proteids,  166 — Nitrogenous  extraction  of  muscle,  166 — The 
non-nitrogenous  constituents  of  muscle,  167 — The  gases  of  muscle,  168 — The  chem- 
istry of  nerves,  169. 

* 

CENTRAL  NERVOUS  SYSTEM  (By  HENRY  H.  DONALDSON) 171 

INTRODUCTION 171 

The  unity  of  the  central  nervous  system,  171 — Phenomena  involving  consciousness, 
172 — Growth  and  organization,  172 — Plan  of  presentation  of  the  subject,  172. 

PART  I. — PHYSIOLOGY  OF  THE  NERVE-CELL • 173 

A.  ANATOMICAL  CHARACTERISTICS  OF  THE  NERVE-CELL 173 

Form  of  nerve-cells,  173 — Peculiarities  in  structure  of  nerve-cells,  174 — The  volume 

relations  of  nerve-cells,  175 — Size  of  nerve-cells  in  different  animals,  175 — Relation 
between  size  and  function,  175 — The  growth  of  nerve-cells,  176— Maturing  of  nerve- 
cells,  177 — Classification  of  cells  by  means  of  the  form  of  the  axone,  177 — Growth  of 
the  branches  of  the  nerve-cell,  179 — Internal  structure  of  the  neurones,  179 — Medulla- 
tion  of  nerve-fibres,  179 — Growth  of  the  medullary  sheath  in  peripheral  nerves,  180 — 
Medullation  in  the  central  system,  181 — Changes  in  the  cytoplasm,  182 — Old  age  of 
the  nerve-cells,  182. 

B.  THE  NERVE  IMPULSE  WITHIN  A  SINGLE  NEURONE 183 

The  nerve-impulse,  183 — Direction  of  the  nerve-impulse,  184 — Double  pathways  for 

the  nerve-impulse,  185 — Significance  of  cell  branches,  186 — The  generation  of  nerve- 
impulses,  187 — The  rate  of  discharge  of  nerve-impulses,  189 — Points  at  which  the 
nerve-impulse  can  be  aroused,  189 — Irritability  and  conductivity,  189 — Summation  of 
stimuli  in  nerve-cells,  190. 

C.  THE  NUTRITION  OF  THE  NERVE-CELL  .......' 191 

C.hemical  changes  in  the  nerve-cell,  191— Fatigue  of  the  nerve-cell,  191— Atrophic 

influences  affecting  the  nerve-cell,  195 — Effects  of  amputations  in  man  on  the  nerve- 
cells,  196— Degeneration  of  nerve-elements,  197— The  nutritive  control  of  the  neurone, 
•     198 — Degeneration  of  the  cell-body,  199 — Regeneration  of  the  axone,  199. 

PART  II.— THE  PHYSIOLOGY  OF  GROUPS  OF  NERVE-CELLS 202 

A.  ARCHITECTURE  AND  ORGANIZATION  OF  THE  CENTRAL  NERVOUS  SYSTEM  ...       202 
General  arrangement  of  the  central  nervous  system,  202 — Arrangement  of  the  cells 

forming  the  several  groups,  205 — Segmentation  of  the  central   nervous  system,  205 — 
(  Relative  development  of  different  parts,  206— The  connections  between  cells,  206— 
Theories  of  the  passage  of  the  nerve-impulses,  207. 

B.  REFLEX  ACTIONS 207 

The  conditions  of  stimulation  controlling  reflex  actions,  207 — The  diffusion  of  cen- 
tral impulses,  208— Simple  reflex  actions,  208 — Influence  of  location  of  stimulus  on 
reflexes,  209— Segmental  reflex  actions,  210 — The  influence  of  the  strength  of  stimulus 

on  reflex  actions,  210 — Continuance  of  the  reflex  response,  211 — The  latent  period  of 
reflexes,  211  —The  summation  of  stimuli  in  reflexes,  211 — Reflex  reactions  from  frac- 
tions of  the  cord,  212 — Reflex  reactions  in  other  vertebrates,  212— Co-ordination  of  the 
efferent  impulses  in  reflex  actions,  214 — Purposeful  character  of  reflex  responses,  215 
—  Reflexes  in  man,  216 — Periodic  reflexes,  216 — Variations  in  diffusibility  in  reflexes, 
217 — Influence  of  strychnine  on  reflexes,  217 — Peripheral  diffusion  of  reflex  impulses, 
218 — Reflexes  in  the  sympathetic  system,  218 — Manner  of  diffusion  of  impulses  in 


CO-V'/'AYV'/X  13 

PAGE 

the  sympathetic  system,  -jilt  -Evidence  for  continuous  outgoing  impulses  from  the 
central   nervous  system.  :_"_'<>— -Kigor  mortis  as  affected    by  t  lie  nervous  system, 
Modification  of  rellexes  by  simultaneous  and  successive  afferent  impulses,  221 — Effects 
of  afferent  impulses  on  reflexes,  223— Inhibit  ion  of  reflexes,  223. 

C.  REACTIONS  INVOLVING:  TIIK  KN»  KPHALON 226 

The  path  of  afferent  impulses  in  the  central  nervous  system,  226 — Degenerations  in 

the  spinal  cord  after  hemisection,  228 — Physiological  observations  on  afferent  pathways 
in  the  central  nervous  system,  22!) — The  nerves  of  common  sensation.  -_':;o  The  nerves 
of  pain  and  their  pathway  in  the  cord,  231 — The  pathways  of  impulses  in  the  cord, 
233 — The  nuclei  and  courses  of  the  cranial  nerves,  236 — The  pathway  of  the  fibres  of 
the  optic  nerve,  238 — The  pathway  of  the  fibres  of  the  olfactory  nerve,  240. 

D.  LOCALIZATION  OF  CELL-GROUPS  IN  THE  CEREBRAL  CORTEX 241 

The  discovery  of  localization  of  function  in  the  cortex,  241 — Effects  of  stimulation 

of  the  cortex,  241 — Course  of  the  descending  impulses,  244 — Mapping  of  the  cortex, 
247 — The  size  of  the  cortical  areas,  247 — Subdivision  of  the  cortical  areas,  247 — Sepa- 
rateness  of  centres  and  areas,  248 — Multiple  control  of  muscles  from  the  cortex,  250 — 
Cortical  control  is  crossed,  251 — Course  of  impulses  leaving  the  cortex,  251 — Size  of 
the  pyramidal  tracts  in  different  mammals,  252. 

E.  LOCALIZATION  IN  THE  CEREBRAL  CORTEX  OF  THE  CELL-GROUPS  RECEIVING  THE 

AFFERENT  IMPULSES 252 

Sensory  regions  of  the  cortex,  252 — Delimitation  of  the  sensory  areas,  253 — Hemi- 
anopsia.  255 — Association-fibres  arid  association-centres  (Flechsig),  256 — Aphasia,  257 
— Relative  importance  of  the  two  hemispheres,  258 — Composite  character  of  incoming 
impulses,  260 — Variations  in  association,  260 — Latent  areas,  261. 

F.  COMPARATIVE  PHYSIOLOGY  OF  THE  DIVISIONS  OF  THE  ENCEPHALON .^262 

Methods  of  determining,  262 — Removal  of  central  hemispheres,  263 — Functions  of 

the  corpus  callosum,  270 — Functions  of  the  corpora  striata,  271 — Functions  of  the 
thalamus,  271 — Functions  of  the  cerebellum,  272. 

PART  III.— PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM  TAKEN  AS  A  WHOLE 274 

A.  WEIGHT  OF  THE  BRAIN  AND  SPINAL  CORD 274 

Weight  of  the  encephalon  and  spinal  cord,  274 — Weight  of  the  encephalon,  275 — In- 
terpretations of  weight,  277 — Weights  of  different  portions  of  the  encephalon,  277 — 
Effect  of  social  environment,  277 — Brain-weight  of  criminals,  277 — Brain-weights  of 
different  races,  278— Weight  of  the  spinal  cord,  278. 

B.  GROWTH  CHANGES 278 

Growth  of  the  brain,  278 — Relation  between  growth  of  body  and  that  of  encephalon, 

280 — Increase  in  the  number  of  functional  nerve-elements,  280 — Increase  in  the  fibres 
of  the  cortex,  282 — Significance  of  medullation,  283 — Increase  in  the  mass  of  the  neu- 
rones, 283 — Number  of  cells,  283 — Change  in  specific  gravity  with  age,  284. 

C.  ORGANIZATION  AND  NUTRITION  OF  THE  CENTRAL  NERVOUS  SYSTEM 285 

Organization  in  the  central  system,  285 — Defective  development  of  the  central  system, 

285 — The  central  nervous  system  of  laboratory  animals,  286 — The  blood-supply  of  the 
central  system,  286 — The  influence  of  glands  on  the  nervous  system,  289 — The  influ- 
ence of  starvation  on  the  nervous  system,  289 — Fatigue  of  the  central  nervous  system, 
289— Daily  rhythms  in  the  activity  of  the  central  nervous  system,  289— The  time 
taken  in  central  processes,  291. 

D.  SLEEP 291 

Conditions   favoring   sleep,   291 — Causes   of  sleep,   292 — Condition   of  the  central 

nervous  system  in  sleep,  293 — Effect  of  loss  of  sleep,  295. 

E.  OLD  AGE  OF  THE  CENTRAL  SYSTEM .    .    295 

Metabolism  in  the  nerve-cells  in  old  age,  295 — Decrease  in  weight  of  the  brain  in  old 

age,  296— Changes  in  the  encephalon  in  old  age,  296— Changes  in  the  cerebellum  in 
old  age,  296. 

THE  SPECIAL  SENSES 298 

A.    VISION  (By  HENRY  P.  BOWDITCH) 298 

The  general  physiology  of  vision,  298— The  mechanical  movements  of  the  eyeballs 
around  various  axes,  298— The  muscles  of  the  eye,  299— The  dioptric  apparatus  of  the 
eye,  300 — The  refracting  media  of  the  eye,  302 — The  optical  constants^of  the  eye.  301 
— The  mechanism  of  accommodation,  306 — The  range  of  accommodation.  312 — Myopia 
and  hypermetropia,  313 — Presbyopia,  314 — Spherical  aberration.  315  —  Chromatic 
aberration,  316 — Astigmatism,  317 — Intraocular  images.  320 — Musc;e  volitantes,  .')•-<> 
— The  retinal  vessels,  321 — Circulation  of  blood  in  the  retina,  322 — The  innervation 
and  movements  of  the  iris,  322— The  principle  of  the  ophthalmoscope,  326— The 
structure  of  the  retina,  327 — The  blind-spot  of  the  retina.  32S— ( 'hantres  produced  in 
the  retina  by  light,  330— The  production  of  the  sensation  of  light.  331— The  qualita- 
tive modifications  of  light,  332— Color-sensations,  333— Means  of  producing  color- 


14  CONTENTS. 


mixtures,  333  —  Color-theories,  335 —  Color-blindness,  338— The  intensity  of  light 
sensations,  339 — The  luminosity  of  different  colors,  340 — The  function  of  rods  and 
cones,  341 — The  saturation  of  color-sensations,  342 — Retinal  stimulation,  343 — The 
latent  period  of  light-sensations,  343 — The  rise  to  maximum  of  light-sensations,  343 
—The  fatigue  of  the  retina,  344— The  after-effect  of  stimulation,  345— After-images, 
346— Color-contrasts,  346— The  perception  of  space,  347— Irradiation,  349— The  false 
judgments  of  subdivided  space,  350 — The  perception  of  distance,  354 — Binocular 
vision,  356 — Pseudoscopic  vision,  357 — Binocular  combination  of  colors,  358 — Corre- 
sponding points,  358 — Visual  illusions,  360. 

B.  THE  EAR  AND  HEARING  (By  HENRY  SEWALL) 362 

The  anatomy  and  histology  of  the  ear,  362— The  external  ear,  362— The  middle  ear, 

363 — Movements  of  the  ear-ossicles,  367 — The  Eustachian  tube,  369 — The  muscles  of 
the  middle  ear,  369 — The  vibrations  of  the  tympanic  membrane,  370 — The  structure 
of  the  internal  ear,  371 — The  general  anatomy  of  the  cochlea,  374 — The  transmission 
of  vibrations  through  the  labyrinth,  376 — The  membranous  cochlea  and  the  organ  of 
Corti,  376— The  theory  of  auditory  sensations,  380. 

C.  THE  RELATIONS  BETWEEN  PHYSICAL  AND  PHYSIOLOGICAL  SOUND 381 

Production  of  sound-waves,  381 — Loudness  and  musical  pitch,  381 — The  tympanic 

membrane  as  an  organ  of  pressure-sense,  382 — Overtones  and  quality  of  sound,  383 — 
Analysis  of  composite  tones  by  the  ear,  384 — Inharmonic  overtones,  386 — The  produc- 
tion of  beats,  386 — Harmony  and  discord,  387 — Combinational  tones,  387 — Auditory 
fatigue,  387 — Imperfections  of  the  ear,  388 — Perceptions  of  time-intervals,  388 — Musi- 
cal tones  and  noises,  388 — Functions  of  different  parts  of  the  ear,  388 — The  judgment 
of  direction  and  distance,  389. 

D.  CUTANEOUS  AND  MUSCULAR  SENSATIONS 390 

General   importance  of  the  cutaneous  and  muscular  sensations,  390 — Ending  of 

sensory  nerve-fibres  in  the  skin,  391 — Relations  of  stimulus  to  the  touch-sensations, 
392 — The  localization  of  touch-sensations,  394 — Pressure-points,  396 — The  importance 
of  the  end-organ,  396 — Touch-illusions,  396 — The  temperature-sense,  397 — Cold  and 
warm  points,  398 — Common  sensation  and  pain,  399 — Transferred  or  sympathetic 
pains ;  allochiria,  400 — Muscular  sensation,  401 — Hunger  and  thirst,  404. 

E.  THE  EQUILIBRIUM  OF  THE  BODY  ;  THE  FUNCTION  OF  THE  SEMICIRCULAR  CANALS    404 
The  sense  of  equilibrium,  404 — Disturbances  of  the  sense  of  equilibrium,  405 — 

Theory  of  the  relation  of  the  semicircular  canals  to  equilibrium,  406 — The  relation  of 
the  vestibular  sacs  to  equilibrium,  407. 

F.  SMELL  . 408 

Structure  of  the  olfactorv  epithelium,  408 — The  production  of  olfactory  sensations, 

409. 

G.  TASTE 410 

Structure  of  the  taste-buds,  410 — The  production  of  taste-sensations,  411 — Classifica- 
tion of  taste-sensations,  412 — Specific  energy  of  taste-nerves,  413. 

PHYSIOLOGY  OF  SPECIAL  MUSCULAR  MECHANISMS 414 

A.  THE  ACTION  OF  LOCOMOTOR  MECHANISMS  (By  WARREN  P.  LOMBARD) 414 

The  articulations,    414— Sutures,   414— Symphyses,   414— Syndesmoses,   414— Diar- 

throses,  415— The  lever-action  of  muscles  on  bones,  417— The  act  of  standing,  418— 
The  act  of  locomotion,  420— Walking,  420— Running,  421. 

B.  VOICE  AND  SPEECH  (By  HENRY  SEWALL) 421 

Voice-production,  421— Functions  of  the  epiglottis,  422— Ventricular  bands   and 

ventricles  of  Morgagni,  422— The  true  vocal  cords,  423— The  cartilages  of  the  larynx, 
425— The  muscles  of  the  larynx,  425— Specific  actions  of  the  laryngeal  muscles,  427— 
The  nerve-supply  of  the  larynx,  428 — The  laryngoscopic  appearance  of  the  larynx, 
429— The  production  of  voice,  430— Loudness  and  pitch  of  voice,  430— Quality  of  voice, 
430— Arrangements  for  changing  the  pitch  of  the  voice,  432— The  vocal  registers,  432 
—A  whistling  register,  433— Speech,  433— The  classification  of  vowel  sounds,  434— 
Whispering,  436— The  production  and  classification  of  consonants,  436. 

REPRODUCTION  (By  FREDERIC  S.  LEE) \   ...  439 

A.  REPRODUCTION  IN  GENERAL 439 

Asexual  reproduction,  439 — Sexual  reproduction,  440 — Origin  of  sex  and  theory  of 

reproduction,  441 — Primary  and  secondary  sexual  characters,  442 — The  sexual  organs, 
443. 

B.  THE  MALE  REPRODUCTIVE  ORGANS •   .    443 

Structure  and  properties  of  the  spermatozoon,  443 — Maturation  of  the  spermatozoon, 

445 — The  composition  of  semen,  445— The  testis,  446— The  urethra,  448— The  prostate 
gland,  448— Cowper's  glands,  448— The  penis,  448. 


CONTENTS.  15 

PAGE 

C.  THE  FEMALE  REPRODUCTIVE  ORGANS 449 

The  ovum,  449 — Maturation  of  the  ovum,  451 — The  ovary  and  ovulation,  454 — The 

Fallopian  tuhes,  456 — The  uterus,  456 — The  act  of  menstruation.  r>7  -Comparative 
physiology  of  menstruation,  459 — Theories  of  menstruation,  460— The  vagina,  462 — 
The  vulva  and  its  parts,  462 — The  mammary  glands,  462 — The  internal  secretion  of 
the  ovaries,  1  )>•_>. 

D.  THE  REPRODUCTIVE  PROCESS 463 

The  act  of  copulation,  463 — Locomotion  of  the  spermatozoa,  465 — Fertilization  of 

the  ovum,  466 — Segmentation  of  the  ovum,  467 — Polysperiny,  471 — The  decidua 
graviditatis,  471 — The  fetal  membranes,  472 — The  placenta,  474 — Nutrition  of  the 
embryo,  475 — Physiological  effects  of  pregnancy  on  the  mother,  477 — The  duration  of 
gestation,  l?s— Parturition,  479 — First  stage  of  labor,  479 — Second  stage  of  labor,  480 
— Third  stage  of  labor,  480 — Physiology  of  labor,  481 — Multiple  conceptions,  482 — 
The  determination  of  sex,  483. 

E.  EPOCHS  IN  THE  PHYSIOLOGICAL  LIFE  OF  THE  INDIVIDUAL 486 

Growth  of  the  cells,  the  tissues,  and  the  organs,  486 — Growth  of  the  body  before 

birth,  486— Growth  of  the  body  after  birth,  487— The  condition  of  puberty,  489— The 
Climacteric,  490— Senescence,  490— Death,  491— Theory  of  death,  492. 

F.  HEREDITY 494 

The  facts  of  inheritance,  494 — Latent  characters,  atavism  or  reversion,  495 — Re- 
generation, 496 — The  inheritance  of  acquired  characters,  496 — The  inheritance  of  dis- 
ease, 498 — Theories  of  heredity,  498 — Germ-plasm,  499 — Variation,  500 — Darwin's  theory 

of  pangeuesis,  501 — Weismann's  theory  of  heredity,  502 — Theory  of  epigenesis,  504. 


I.   GENERAL  PHYSIOLOGY  OF  MUSCLE  AND 

NERVE. 


A.  INTRODUCTION. 

IT  is  seldom  that  the  physical  and  chemical  structure  of  a  tissue,  as  revealed 
by  the  microscope  and  the  most  careful  analysis,  gives  even  a  suggestion  as  to 
its  function.  No  one  would  conclude  from  looking  at  a  piece  of  beef,  or  even 
microscopically  examining  a  muscle,  that  it  had  once  been  capable  of  motion, 
nor  would  the  most  exact  statement  of  its  chemical  composition  give  indication 
of  such  a  form  of  activity.  The  most  thorough  histological  and  chemical 
examination  of  the  bundle  of  fibres  which  compose  a  nerve  would  fail  to  sug- 
gest that  a  blow  upon  one  end  of  it  would  cause  to  be  transmitted  to  the  other 
end  an  invisible  change  capable  of  exciting  to  action  the  cell  with  which  the 
nerve  communicated.  To  understand  such  a  structure  we  must  first  learn  the 
forms  of  activity  of  which  the  tissue  is  capable,  the  influences  which  excite 
it  to  action,  and  the  conditions  essential  to  its  activity,  and  then  seek  an  expla- 
nation of  these  facts  in  its  physical  and  chemical  constitution. 

Contractility. — One  of  the  most  striking  properties  of  living  matter  is 
its  power  to  move  and  to  change  its  form.  At  times  the  movements  occur 
apparently  spontaneously,  the  exciting  cause  seeming  to  originate  within  the 
living  substance,  but  more  often  the  motions  are  developed  in  response  to  some 
external  influence.  This  power  finds  its  best  expression  in  muscle-substance. 
In  its  resting  form  a  muscle,  such  as  the  biceps,  is  elongated,  and  when  it  is 
excited  to  action  it  assumes  a  more  spherical  shape,  i.  e.  shortens  and  thickens, 
whence  it  is  said  to  have  the  property  of  contractility.  It  is  the  shortening, 
the  contraction,  of  the  muscle  which  enables  it  to  perform  its  function  of 
moving  the  parts  to  which  it  is  attached,  as  the  bones  of  the  arm  or  leg,  and 
of  altering  the  size  of  the  structures  of  which  it  forms  a  part,  as  the  walls 
of  the  heart,  intestine,  or  bladder.  Ordinary  muscle-substance  is  arranged 
in  fine  threads,  each  one  of  which  is  enveloped  in  a  delicate  membrane,  the 
sarcolemma ;  these  muscle-fibres  can  be  compared  to  long  sausages  of  micro- 
scopic proportions.  A  muscle  is  composed  of  a  vast  number  of  fibres 
arranged  side  by  side  in  bundles,  the  whole  being  firmly  bound  together  by 
connective  tissue.  Since  isolated  muscle-fibres  have  been  seen  under  the 
microscope  to  contract,  each  fibre  can  be  looked  upon  as  containing  true  muscle- 
substance  and  being  endowed  with  contractility.  The  movements  of  muscles 
are  the  resultant  of  the  combined  activity  of  the  many  microscopic  fibres  of 
which  the  muscles  are  composed. 

The  rate,  extent,  strength,  and  duration  of  muscular  contractions  are  adapted 

VOL.  II.— 2  17 


18 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


to  the  needs  of  the  parts  to  be  influenced,  and  it  is  found  that  the  structure  of 
the  muscles  differs  according  to  the  work  which  they  have  to  perform.  Thus 
we  find  two  large  classes_pf  muscles :  the  one,  like  the  muscles  which  move  the 
bones,  remarkable  for  the  rapidity  with  which  they  change  their  form,  but 
unsuited  to  long-continued  action ;  the  other,  occurring  in  the  walls  of  the 
intestine,  blood-vessels,  bladder,  etc.,  sluggish  of  movement,  but  possessing  great 
endurance.  The  first  of  these,  when  examined  with  the  microscope,  is  seen 
to  be  composed  of  bundles  of  fibres,  which  are  transversely  marked  by  alter- 
nating dark  and  light  bands,  and  hence  are  called  striated  or  striped  muscles ; 
the  other,  though  composed  of  fibres,  shows  no  such  cross  markings,  and 
therefore  is  known  as  smooth  or  non-striated  muscle. "  Striated  muscles  are 


FIG.  1.— Amoeba  proteus,  magnified  200  times :  a,  endosarc ;  b,  simple  pseudopodium ;  c,  ectosarc ;  d, 
first  stage  in  the  growth  of  a  pseudopodium ;  e,  pseudopodium  a  little  older  than  d ;  /,  branched  pseudo- 
podium  ;  g,  food-vacuole ;  h,  food-ball ;  i,  endoplast ;  k,  contractile  vesicle  (after  Brooks :  Handbook  of 
Invertebrate  Zoology). 

often  called  voluntary,  because  most  of  them  can  be  excited  to  action  by  the 
will,  whereas  non-striated  muscles  are  termed  involuntary,  because  in  most 
cases  they  cannot  be  so  controlled.  Within  these  two  large  classes  of  muscles 
we  find  special  forms  presenting  other,  though  lesser,  differences  in  function 
and  structure.  The  muscle  of  the  heart,  though  striated,  differs  so  much  from 
other  forms  of  striped  muscle  as  almost  to  belong  in  a  special  class. 


GENERAL    PHYSIOLOGY    OF  MUSCLE    AND    NERVE.       19 


Since  contractility  is  possessed  by  all  forms  of  muscle-tissue,  it  is  evident  that 
it  is  independent  of  superficial  structural  differences.  Nor  is  muscle  the  only 
substance  possessing  this  property.  Even  isolated  microscopic  particles  of  liv- 
ing mutter  are  capable  of  making  movements,  both  spontaneously  and  when 
excited  by  external  influences.  As  far  back  as  1755,  Rosel  von  Rosenhof 
described  the  apparently  spontaneous  changes  in  form  of  a  living  organism 
composed  of  a  single  cell,  a  fresh-water  amoeba.  Moreover,  he  noted  that,  if 
quiet,  it  could  be  excited  to  action  by  mechanical  shocks. 

The  amoeba  (Fig.  1)  is  a  little  animal,  of  microscopic  size,  which  is  found 
in  the  ooze  at  the  bottom  of  pools,  or  in  the  slime  which  clings  to  some  of  our 
fresh-water  plants.  Under  the  microscope  it  is  seen  to  be  composed  of  jelly- 
like,  almost  transparent  matter,  in  which  are  a  vast  number  of  fine  granules,  a 
delicate  tracery  of  finest  fibrils,  a  small  round  body,  called  the  nucleus  or 
endoplast,  a  round  hollow  space  termed  the  contractile  vesicle,  which  is  seen  to 
change  in  size,  appearing  or  disappearing  from  time  to  time,  and  small  parti- 
cles, which  are  bits  of  food  or  foreign  bodies.  In  the  resting  state  the  body 
has  a  somewhat  flattened,  irregular  form,  which,  if  the  slide  on  which  it  rests  be 
kept  warm,  is  found  to  alter  from  minute  to  minute.  Little  tongue-like  projec- 
tions, pseudopods  (false  feet),  are  protruded  from  the  surface  like  feelers,  and 
are  then  withdrawn,  while  others  appear  in  new  places.  Evidently  the  little 
creature,  though  composed  of  a  single  cell,  is  endowed  with  life  and  has  the 
power  of  making  movements.  Moreover,  it  may  be  seen  to  change  its  place, 
the  method  of  locomotion  being  a  peculiar 
one.  One  of  the  processes,  or  pseudopods, 
may  be  extended  a  considerable  distance,  and 
then,  instead  of  being  withdrawn,  grow  in  size, 
while  the  body  of  the  animal  becomes  corre- 
spondingly smaller ;  thus  a  transfer  of  material 
takes  place,  and  this  continues  until  the  whole 
of  the  material  of  the  cell  has  flowed  over  to 
the  new  place.  This  power  of  movement  per- 
mits the  animal  to  eat.  If  when  moving  over 
the  slide  it  encounters  suitable  food  material,  a 
diatom  for  instance,  it  flows  round  it,  engulf- 
ing it  in  its  semifluid  mass ;  and  in  a  similar 
manner  the  animal  gets  rid  of  the  useless  sub- 
stances which  it  may  have  surrounded,  by  flow- 
ing away  from  them.  These  movements  may 

!^  p  "         ,  I'll  j      •-!  •  FIG.  2.— Vorticella  nebulifera,  X  600: 

result  from  changes  which  have  occurred  within    a  cilia  of  ciliated  disk .  6>  ciliated  disk . 
its  own  substance,  and  apparentlv  independently    c,  peristome  -,  d,  vestibule ;  e,  oesophagus  ; 

„  ,  .     ,  f,  contractile  vesicle ;   g,  food-vacuoles ; 

of  any  external  influence.     On  the  other  hand,    ht  endopiast;  i,  endosarc;  k,  ectosarc;  i, 

if  its  body  be  disturbed  bv  being  touched,  by     cuticle;  m,  axis  of  stem  (after  Brooks: 
*  .      ,         Handbook  of  Invertebrate  Zoology). 

an  unusual  temperature,  by  certain  chemicals, 

or  by  an  electric  shock,  it  replies  by  drawing  in  all  of  its  pseudopods  and 

assuming  a  contracted,  ball  form. 

The  movements  of  the  leucocytes  of  the  blood  resemble  in  many  respects 


20  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

those  of  ^  the  amoeba.1  The  property  of  contractility  is  possessed  by  a  vast 
variety  of  unicellular  structures  in  lower  forms  of  animal  life.  Another 
example  is  the  Vorticella  (Fig.  2). 

The  vorticella,  like  the  amoeba,  is  a  little  animal  which,  although  consisting 
of  a  single  cell,  possesses  within  its  microscopic  form  all  the  physiological  prop- 
erties essential  to  life  and  the  perpetuation  of  its  species.  It  consists  of  a  bell, 
with  ciliated  margin,  borne  upon  a  contractile  stalk.  If  touched  with  a 
hair,  or  jarred,  the  cell  rapidly  contracts ;  the  edge  of  the  bell  is  drawn  in  so 
as  to  make  the  body  nearly  spherical,  and  the  stalk  is  thrown  into  a  spiral 
and  drags  the  body  back  toward  the  point  of  attachment.  The  contraction  is 
rapid;  the  relaxation,  which  comes  when  the  irritation  ceases,  is  gradual.  An 
interesting  account  of  the  movements  of  Vorticella  gracilis  is  given  by  Hodge 
and  Aikins 2  under  the  title  of  "  The  Daily  Life  of  a  Protozoan." 

Other  examples  of  contractile  power  possessed  by  apparently  simple  organ- 
isms are  to  be  found  in  the  tentacles  of  Actiniae,  the  surface  sarcode  of  sponges, 
the  chromatoblasts  of  Pleuronectidse,3  which  are  controlled  by  nerves  and 
under  the  influence  of  light  and  darkness  change  their  size  and  so  alter  the 
color  of  the  skin,  and  the  vast  variety  of  ciliated-  forms,  including  spermatozoa, 
and  some  of  the  cells  of  mucous  membranes.4 

Irritability. — We  have  thus  far  referred  to  but  one  of  the  vital  properties 
of  protoplasm,  viz.  contractility.  Another  property  intimately  associated  with 
it  is  irritability.  Irritability  is  the  property  of  living  protoplasm  which  causes 
it  to  undergo  characteristic  chemical  and  physical  changes  when  subjected  to 
certain  external  influences  called  irritants.  Muscle  protoplasm  is  very  irri- 
table, and  is  easily  excited  to  contraction  by  such  irritants  as  electric  shocks, 
mechanical  blows,  etc.  The  muscles  which  move  the  bones  rarely,  if  ever,  in 
a  normal  condition,  exhibit  spontaneous  alterations  in  form,  and  cannot  be  said 
to  possess  automatic  power.  By  automatism  is  meant  that  property  of  cell- 
protoplasm  which  enables  it  to  become  active  as  a  result  of  changes  which 
originate  within  itself,  and  independently  of  any  external  irritant.  Examples 
of  this  power  may  perhaps  be  found  in  the  movements  of  ciliated  organisms 
and  the  infusoria.  Possibly  the  rhythmic  movements  of  heart  muscle  are  of 
this  nature.  Still  another  property  of  protoplasm,  closely  allied  to  contractility 
and  irritability,  and  possessed  by  muscle-substance,  is  conductivity. 

Conductivity  is  the  property  which  enables  a  substance,  when  excited  in 
one  part,  to  transmit  the  condition  of  activity  throughout  the  irritable  mate- 
rial. For  example,  an  external  influence  capable  of  exciting  an  irritable 
muscle-fibre  to  contraction,  although  it  may  directly  affect  only  a  small  part  of 
the  fibre,  may  indirectly  influence  the  whole,  because  the  condition  of  activity 
which  it  excites  at  the  point  of  application  is  transmitted  by  the  muscle-sub- 
stance throughout  the  extent  of  the  fibre. 

1  An  excellent  description  of  these  movements,  accompanied  by  illustrations,  is  given  in 
Quain's  Anatomy,  vol.  i.,  pt.  2,  pp.  174-179. 

2  Hodge  and  Aikins :  American  Journal  of  Psychology,  1895,  vol.  vi.,  No.  4,  p.  524. 

3  Krukenberg  :    Vergleichend-physiologische  Vortrdge,  1886,  Bd.  i.  S.  274. 

4  A  careful  study  of  the  different  forms  of  movement  exhibited  by  simple  organisms  has 
been  made  by  Engelmann:  Hermann'    Handbnch  der  Physiologic,  1879,  Bd   i.,  Th.  1,  S,  344. 


L   nrvsio: 

Irritability  and  conductivity  are  not  confined  to  contractile  mechan- 
isms.    T 

a^  tli  not  tlr  n  found  to  have  -or  of 

rnov  it  \\ 'hieli  with  the  growth  of  a  cell, 

near  !  one  or  more  branches.     The  bodies  of  the 

nerv  chiefly  in  the  spinal  cord  ami  brain,  a  smaller  nu 

being  found  in  the  spinal  ganglia  and  in  the  ganglia  of  the  so-called  sympa- 
thet;  .     The  branches  of  a  neurone  are  of  two  kinds,  an  axis 

T  axone,  which  frequent!  it  its  extremity  a  specially  formed 

liieli  it  is  able  to  excite  to  action  the  cells  with  which  it 
comes  in  contact,  and  protoplasmic  processes,  or  dendritos,  which  have  no 
such  and  are  destined  to  receive  excitation  and  trai; 

it  to  the  body  of  the  nerve-cell.  Outside  the  central  nervous  system,  at 
•  >ne  and  the  dendrite  acquire  a  delicate  membranous  sheath, 
;na,  which  invests  it  as  the  sarcolemma  does  the  muscle-fibre. 
The  branches  of  nerve-cells  together  with  their  sheaths  form  the  nerve- 
- .  There  are  two  classes  of  nerve-fibres,  medullated  and  non-medullated, 
which  are  distinguished  by  the  fact  that  the  former  has  between  the  axis- 
cylinder  and  the  neurilemma  another  covering  composed  of  fatty  material, 
called  the  medullary  sheath,  while  in  the  latter  this  is  absent.  Just  as 
it  is  the  special  function  of  the  muscle-fibre  to  change  its  form  when  it 
is  excited,  so  it  is  the  special  function  of  the  nerve-fibre  to  transmit  the 
condition  of  activity  excited  at  one  end  throughout  its  length,  and  to 
•» \\aken  to  action  the  cell  with  which  it  communicates.  Nerve-fibres  are 
the  paths  of  communication  between  nerve-cells  in  the  central  nervous  sys- 
tem, between  sense-organs  at  the  surface  of  the  body  and  the  nerve-cells, 
and  between  the  nerve-cells  and  the  muscle-  and  gland-cells.  Nerve- 
fibres  are  distinguished  as  afferent  and  efferent,  or  centripetal  and  centrifugal, 
according  as  they  carry  impulses  from  the  surface  of  the  body  inward  or  from 
the  central  nervous  system  outward.  Further,  they  receive  names  according 
to  the  diameter  of  the  activity  which  they  excite:  those  which  excite  muscle- 
to  contract  are  called  motor  nerves;  those  distributed  to  the  muscles 
in  the  walls  of  blood-vessels,  vaso-motor ;  those  which  stimulate  gland-cells  to 
action,  secretory ;  those  which  influence  certain  nerve-cells  in  the  brain  and  so 
cause  sensation*,  sensory.  Still  other  names  are  given,  as  "  trophic  "  to  fibres 
which  are  supposed  to  have  a  nutritive  function,  and  "  inhibitory  "  to  those 
which  check  the  activities  of  various  organs.  The  method  of  conduction  is  the 
same  in  all  tin  dt  depending  wholly  on  the  organ  stimulated. 

ve-fibres  do  not  run  for  any  distance  separately,  but  always  in  company 
with  others.     Thus  large  nerve-trunks  may  be  formed,  as  in  the  case  of  the 
s  to  the  limbs,  in  which  afferent  and  efferent  fibres  run  side  by  side,  the 
whole  being  bound  together  into  a  compact  bundle  by  connective  tissue.     The 
separate  fibres,  though  thus  grouped  together,  are  anatomically  and  physiologi- 
cally as  distinct  as  the  wires  of  an  ocean  cable;  that  tit  are 
bound  together  is  of  anatomical  interest,  but  has  little  physiological  signifi< 
The  aci               'ance  of  the  nerve-fibre  does  not  show  contractility,  but  this 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

does  not  prevent  it  from  being  classed  with  other  irritable  forms  of.  living  cell- 
substance  as  protoplasm.  In  spite  of  differences  in  structure  and  composition, 
nerve  protoplasm  and  muscle  protoplasm  are  found  to  have  many  points  of 
resemblance.  An  explanation  of  the  physiological  resemblances  may  be  found 
in  their  common  ancestry.  All  the  cells  of  the  many  structures  of  the  animal 
body  are  descended  from  the  two  parent  cells  from  which  the  animal  is  developed. 
The  fertilized  ovurn  divides,  and  two  cells  are  formed,  these  new  cells  divide, 
and  so  the  process  continues,  the  developing  cells  through  unknown  causes  be- 
coming arranged  to  form  more  or  less  definite  layers  and  groups,  which  by  means 
of  foldings  and  unequal  growths  develop  into  the  .various  structures  and  organs 
of  the  fetus.  At  the  same  time  that  the  division  is  going  on,  the  total  amount 
of  material  is  increasing.  Each  of  the  cells  absorbs  and  assimilates  dead  food- 
material,  and  this  dead  material  is  built  into  living  substance.  Daring  this 
process  -of  development  and  growth  the  cells  of  special  tissues  and  organs 
acquire  special  anatomical  and  chemical  characters.  This  development  of 
specialized  cells  is  termed  cell-differentiation.  Hand  in  hand  with  the  ana- 
tomical and  chemical  differentiation  goes  a  physiological  differentiation.  The 
protoplasm  of  each  type  of  cell,  while  retaining  the  general  characteristics  of 
protoplasm,  has  certain  physiological  properties  developed  to  a  marked  degree 
and  other  properties  but  little  developed,  or  altogether  lacking.  The  fertilized 
ovum  does  not  have  all  the  anatomical  and  chemical  characteristics  of  all  the 
cells  which  are  descended  from  it,  not  at  least  in  just  the  form  in  which  they 
are  possessed  by  these  cells,  and  it  cannot  be  assumed  that  its  living  sub- 
stance possesses  all  the  physiological  properties  which  are  owned  by  its 
descendants.  Many  of  these  properties  it  must  have,  for  many  of  them  are 
essential  to  the  continuance  of  life  of  all  active  cells, — such  as  the  power  to  take 
in,  alter,  and  utilize  materials  which  are  suitable  for  the  building  up  and  r< 
of  the  cell-substance,  the  power  of  chemically  changing  materials  possessing 
potential  energy  so  that  the  form  of  actual  energy  which  is  essential  to  the  per- 
formance of  the  work  of  the  cell  shall  be  liberated,  and  the  power  to  give 
off  the  waste  materials  which  result  from  chemical  changes.  The  protoplasm 
of  the  ovum,  to  have  these  powers,  has  properties  closely  allied  to  absorption, 
digestion,  assimilation,  respiration,  excretion  ;  and,  in  consideration  of  the  special 
function  of  the  ovum,  we  may  add  that  it  possesses  the  property  of  reproduc- 
tion. The  question  of  its  possessing  the  characteristic  properties  of  muscle  and 
nerve  protoplasm  cannot  be  answered  off-hand.  Careful  study,  however,  has 
shown  the  ovum  of  Hydra  to  possess  irritability,  conductivity,  and  contractility. 
It  undergoes  amoeboid  movements,  as  was  first  shown  by  Kleinenberg. 
Balfour,1  in  writing  of  the  development  of  the  ova  of  Tubularidse,  whi 
of  a  type  similar  to  Hydra,  says:  "  The  mode  of  nutrition  of  the  ovum  may 
be  very  instructively  studied  in  this  type.  The  process  is  one  of  actual  feed- 
ing, much  as  an  amoeba  might  feed  on  other  organisms."  Something  similar 
seems  to  be  true  of  the  ova  of  echinodermata.  During  impregnation  various 
movements  are  described  implying  the  properties  of  irritability,  conductivity, 
and  contractility.  Thus  in  the  case  of  Asterias  glacialis,  when  the  head  of  the 
1  Comparative  Embryology •,  pp.  17,  29. 


/'//  }  ' 


•  murl!;i" 

matozoon  n«  uper- 

i  layer  of  pro  until  it  coraes  in  contact  with 

u  At  •  »ntnct  between  the  sperma- 

fi  and  the  egg%,  the  outermost  layer  of  protoplasm  of  the  lair  itself 

up  as  a  distinct  membrane,  which  separates  from  the  egg  and  prevents  the 
entrance  of  other  spermatozoa."  Some  of  the  eggs  of  arthropods  and  other 
forms  have  likewi  observed  to  undergo  amoeboid  movements  as  a  result 

of  the  phy--  -timulus  given  by  the  Spermatozoon,1 

Although  irritability  and  contractility  of  the  ovum  have  thus  far  been  made 
out  in  but  few  forms,  it  is  probable  that  they  play  an  important  part  in  all 
during  fertilization  and  division.     It  would  seem,  then,  that  the  ovum  has  all 
the  principal  properties  which  we  ascribe  to  cell-protoplasm,  and  that 
pro}).  >      inherited  more  or  less  completely  developed  by  the  many  forms 

of  cells  descended  from  it.  The  protoplasm  of  specialized  cells,  in  spite  of 
their  differences  in  structure,  still  retains  its  protoplasmic  nature.  Undoubtedly 
structural  peculiarities  are  intimately  related  to  specialized  functions,  —  the 
striped  muscle,  for  example,  is  especially  adapted  for  rapid  movements,  and 
the  nerve-fibre  is  remarkable  for  its  power  of  conduction. 

Physiological  methods  for  the  examination  of  individual  cells  are  as  yet  in 
their  infancy,  and  we  must  still  seek  for  exact  knowledge  of  the  functional 
activity  of  cells  by  observing  the  combined  action  of  many  cells  of  the  same 
kind.2 

B.    IRRITABILITY  OF  MUSCLE  AND  NERVE. 

Irritability  is  the  property  of  living  protoplasm  which  causes  it  to  undergo 
characteristic  physical  and  chemical  changes  when  it  is  subjected  to  certain 
influences,  called  irritants,  or  stimuli/  By  an  irritant  is  meant  an  external  influ- 
ence which,  when  applied  to  living  protoplasm,  as  of  a  nerve  or  muscle,  excites 
it  to  action.  Irritants  may  be  roughly  classed  as  mechanical,  chemical,  thermal, 
and  electrical.  The  normal  physiological  stimulus  is.  developed  within  some 
of  the  nervous  mechanisms  of  the  body  as  the  result  of  the  activity  of  the 
nerve-protoplasm,  this  having  been  excited  as  a  rule  by  some  form  of  irritant. 
The  degree  of  irritability  of  a  given  form  of  protoplasm  is  measured  by  the 
amount  of  activity  which  it  displays  in  response  to  a  definite  irritant,  or  by  fhe 
minimal  amount  of  irritation  required  to  excite  it  to  action.  If  the  irritant  be 
applied  directly  to  a  muscle,  the  height  to  which  the  muscle  contracts  and  r 
a  given  weight  may  be  taken  as  an  indication  of  its  activity.  A^  the  nerve 
gives  no  visible  evidence  of  activity,  the  effect  of  the  irritant  upon  it  is  usually 
uted  by  the  extent  to  which  the  organ  stimulated  by  the  nerve  reacts  :  in 
the  case  of  motor  nerves  the  strength  of  the  contraction  of  the  corresponding 
•le  is  taken  as  an  imi 

To  determine  the  exact  relation  of  an  irritant  to  its  irritating  effect  we  should 

1  Korschelt:  Zoologiseht-.  1801,  Anat,   Urtheil.,  Bd.  iv.,  -vig: 

e  Centralbl'i 

OS  of  animal  life,  see  <. 
-lation  by  !  w  York,  ]  - 


24  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

be  able  to  accurately  measure  them.  This' we  cannot  do.  AW  are  unable  to 
state  in  irritation-units  the  relative  value  of  different  kinds  of  irritants,  i 
if  we  could  accurately  estimate  the  amount  of  energy  which  each  form  of  irri- 
tant can  expend  in  iritation,  we  should  have  only  one  of  the  many  factors 
which  determine  its  efficiency.  It  is  equally  difficult  to  compare  the  irritating 
effect  of  irritants  upon  different  forms  of  protoplasm ;  e.  g.  we  cannot  state 
what  degree  of  activity  of  a  nerve-fibre  corresponds  to  a  certain  amount  of 
activity  in  a  muscle-fibre.  In  spite  of  the  lack  of  exact  quantitative  measure- 
ments, we  have  gained  a  clear  idea  of  the  way  different  forms  of  irritants 
act  when  applied  to  nerves  and  muscles  in  certain  ways,  and  have  learned  to 
control  the  methods  of  excitation  sufficiently  to  permit  the  influences  which 
alter  the  irritability  of  nerves  and  muscles  to  show  themselves.  The  effect  of 
irritants  can  best  be  studied  upon  the  nerves  and  muscles  of  cold-blooded  ani- 
mals, because  these  retain  their  vitality  and  irritability  for  a  considerable  time 
after  they  have  been  separated  from  the  rest  of  the  body.  It  is  a  common 
observation  of  country  folk  that  the  body  of  a  snake  remains  alive  for  a  long 
time  after  the  head  has  been  crushed,  while  the  body  of  a  chicken  loses  all  signs 
of  life  in  a  comparatively  short  time  after  it  has  been  decapitated.  More  care- 
ful examination  would  show  that  in  neither  case  do  all  parts  of  the  body  die'J 
simultaneously.  Each  of  the  myriad  cells  has  a  life  of  its  own,  which  it 
loses  sooner  or  later  according  to  its  nature  and  to  the  alterations  to  which  it 
is  subjected  by  the  fatal  change.  The  cells  of  cold-blooded  animals,  as  the 
snake  and  frog,  are  much  more  resistant  than  those  of  warm-blooded  animals, 
because  the  vital  processes  within  the  cells  are  less  active,  and  the  chemical 
changes  which  precede  and  lead  to  the  death  of  the  part  occur  more  slowly. 
For  instance,  the  nerves  and  muscles  of  a  frog  remain  irritable  for  many  hours, 
or  even  days,  after  the  animal  has  been  killed  and  they  have  been  removed 
from  the  body.  This  fact  is  of  the  greatest  use  to  the  student.  It  enables  him 
to  study  the  nerve  or  muscle  by  itself,  and  under  such  artificial  conditions  as 
he  cares  to  employ.  Experience  shows  that  the  facts  learned  from  the  study  of 
the  isolated  nerve  and  muscle  hold  good,  with  but  slight  modification,  for  the 
nerves  and  muscles  when  in  the  normal  body.  Moreover,  it  has  been  found 
that  the  nerves  and  muscles  of  warm-blooded  animals,  and  even  man,  resemble 
physiologically  as  well  as  anatomically  those  of  the  frog.  The  correspondence 
is  by  no  means  complete,  but  it  is  so  great  as  to  make  the  facts  discovered  by 
a  study  of  the  nerves  and  muscles  of  the  frog  of  the  utmost  importance  to  us. 
We  are  driven  to  such  sources  of  information  because  of  the  great  difficulty  of 
keeping  the  muscles  of  warm-blooded  animals  alive  and  in  a  normal  condition 
•after  removal  from  the  circulation. 

Irritability  of  Nerves. — The  following  preparation  suffices  to  illustrate 
the  more  striking  effects  of  irritants  upon  a  nerve.  A  frog  is  rapidly  killed. 
and  then  the  sciatic  nerve  is  cut  high  up  in  the  thigh  and  dissected  out  from 
its  groove,  the  branches  going  to  the  thigh-muscles  being  divided.  The  leg  is 
then  cut  through  just  above  the  knee.  This  gives  a  preparation  consisting  of 
the  uninjured  lower  leg  and  foot,  and  the  carefully  prepared  nerve  supplying 
the  muscles  of  these  parts.  The  leg  may  be  placed  foot  upward,  and  fastened 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE. 

in  this  position  by  a  clamp  which  grasps  the  bones  at  the  knee,  tli.    c 
being  supported  by  an  upright  (see  Fig.  3).     This  preparation  can   then   !*> 
subjected  to  a  variety  of  tests. 

Mechanical  Irritation. — If  the  nerve  be  cut,  pinched,  suddenly  stretched,  or 
subjected  to  a  blow,  the  muscles  of  the  leg  will 
contract  and  the  foot  will  be  quickly  moved. 

Chemical  Irritation. — If  acid,  alkalies,  vari- 
ous salts,  glycerin,  or  some  other  chemical  sub- 
stances be  placed  upon  the  nerve,  the  muscles 
of  the  leg  begin  to  twitch  irregularly,  and  as 
the  chemical  enters  more  and  more  deeply  into 
the  nerve  the  movements  will  become  more 
and  more  marked,  until  finally  all  the  muscles 
are  actively  contracted  and  the  foot  is  held 
straight  up. 

Thermal   Irritation.— If    hot   glass,   or  the    *»•  ^-Experiment  for  dete-- 

the  irritability  of  nerves. 

name  of   a  match,  be  applied  to  the   nerve,  a 

condition  of  activity  will  be  developed  in  the  rapidly  heated  nerve-fibres,  and 

lr  responded  to  by  more  or  less  vigorous  muscular  contractions. 

Electrical  Irritation. — If    the  wires  connected   with  the  two  poles  .of  a 
galvanic  cell,  static  machine,  or  induction  apparatus  be  brought  in  contact 
with  the  nerve,  the  muscles  will  twitch  each  time  there  is  a  sudden  chan 
potential. 

Physiological  Irritation. — By  all  these  methods  the  nerve  was  excited  by 
irritants  applied  to  it  from  without,  and  the  muscle  was  excited  to  action  by 
the  physiological  stimulus  coming  to  it  from  the  excited  nerve.  The  irritant 
produced  no  visible  change  in  the  nerve,  but  the  movement  of  the  mil 
was  an  evidence  that  the  nerve  had  undergone  a  change  at  the  point  of  stim- 
ulation, and  thatthe  active,  state  thus  produced  had  been  transmitted  tin  mgh  the 
length  of  the  nerve,  and  had  been  sufficiently  marked  to  stimulate  the  mnx-lo 
to  contraction.  This  condition  of  activity  which  was  transmitted  along  the 
nerve  is  called  the  nerve-impulse.  The  same  condition  is  excited  in  the 
nerve-fibre  when  the  body  of  the  cell  becomes  active. 

Independent  Irritability  of  Muscle. — In  the  above  instances  the  irritant 
were  applied  to  the  nerve,  and  the  muscle  was  indirectly  stimulated.     Miisd. 
protoplasm,  like  nerve  protoplasm,  may  be  directly  excited  to  action  by  variou 
forms  of  irritants.     A  nerve  after  entering  a  muscle  branches  freely,  and  the 
nerve-fibres  are  distributed  quite  generally  through  the  muscle.     An  irritant, 
if  directly  applied  to  muscle,  would  probably  excite  the  nerve-fibres  present  a> 
well  as  the  muscle-fibres,  and  to  obtain  proof  of  independent  irritability  of 
muscle-substance  it  would  be  necessary  to  prevent  the  nerves  from  stimulating 
the  muscle.     This  can  be  done  by  paralyzing  the  nerve-endings  with  cur. 

Curare,  the  South^merican  arrow-poison,  \?  used  by  the  Indians  in  hunt- 
ing.    The  bird  shot  by  these  poisoned  arrows  gradually  becomes  para' 
and,  losing  power  to  move  its   muscles,  is  easily  captured.     The  following 
experiment  reveals  the  method  of  the  action  of  this  drug,  and  at  the  - 


26  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

time  shows,  first,  that  the  muscle  protoplasm  can  be  irritated  directly,  and 
secondly,  that  the  nerves  do  not  communicate  directly  with  the  muscles,  but 
stimulate  them  through  the  agency  of  terminal  end-organs,  called  motor  end- 
plates} 

Curare  Experiment. — Rapidly  destroy  the  brain  of  a  frog  with  a  slightly 
curved,  blunt  needle,  and,  to  prevent  hemorrhage,  plug  the  wound  by  thrust- 
ing a  pointed  match  through  the  foramen  magnum  into  the  brain-cavity. 
Expose  the  sciatic  nerve  of  the  left  thigh,  carefully  pass  a  ligature  under  it,  and 
tie  the  ligature  tightly  about  all  the  tissues  of  the  thigh  excepting  the  nerve, 
thus  cutting  off  the  circulation  from  all  the  leg  below  the  ligature  without  in- 
jury to  the  nerve.  Inject  into  the  dorsal  lymph-sac  or  the  abdominal  cavity  a 
few  drops  of  a  2  per  cent,  solution  of  curare.  In  from  twenty  to  forty  minutes 
the  drug  will  have  reached  the  general  circulation  and  produced  its  effect. 

Although  the  brain  has  been  destroyed  and  the  frog  is  incapable  of  having 
sensation,  it  will  be  found  that  muscular  movements  will  be  made  if  the  skin 
be  pinched  soon  after  the  drug  has  been  given.  These  are  reflex  movements, 
and  are  due  to  excitation  of  the  spinal  cord  by  the  nerves  connected  with  the 
skin.  As  the  paralyzing  action  of  the  drug  progresses,  these  reflex  actions  be- 
come feebler  and  feebler  until  altogether  lost  in  the  parts  exposed  to  the  drug, 
although  they  may  still  be  shown  by  the  parts  from  which  the  drug  has  been 
excluded.  The  condition  of  the  nerves  and  muscles  can  be  examined  as  soon 
as  reflex  movements  of  the  poisoned  parts  cease. 

To  ascertain  the  action  of  the  poison,  expose  the  nerves  of  the  two  legs, 
either  high  up  in  the  thigh  or  inside  the  abdominal  cavity,  where  they  have 
been  subjected  to  the  poison,  and  test  their  irritability  by  exciting  them  with 
electric  shocks.  Stimulation  of  the  motor  nerve  of  the  right  leg  (a,  Fig.  4) 
causes  no  contraction  of  the  muscles  of  that  leg,  while  stimulation  of  the  motor 
nerve  of  the  left  leg  (6),  results  in  active  movements  of  the  muscles  of  that 
leg.  The  response  of  the  left  leg  shows  that  nerve-trunks  are  not  injured  by 
\  the  poison,  and  that  the  paralysis  of  the  right  leg  must  find  some  other  expla- 
nation. -  On  testing  the  muscles  it  is  found  that  they  are  irritable  and  contract 
when  directly  stimulated.  Since  neither  nerve-trunks  nor  muscles  are  poisoned, 
t  is  necessary  to  assume  that  the  cause  of  the  paralysis  is  something  which  pre- 
s  the  nerve-impulse  from  passing  from  the  nerve  to  the  muscle.  Micro- 
opic  examination  shows  that  the  nerve-fibre  does  not  communicate  directly 
ith  the  muscle-fibre,  but  ends  inside  the  sarcolemma  in  an  organ  which  is 
^P* called  the  motor  end-plate  (see  Fig.  31).  It  appears  that  the  nerve  acts  on  the 
muscle  through  this  organ,  and  its  failure  to  act  on  the  side  which  was  exposed 
to  the  curare  was  because  the  end-plate  had  been  paralyzed  by  the  drug.  By 
the  use  of  curare,  therefore,  we  are  enabled  to  prevent  the  nerve-impulse  from 
reaching  the  muscles,  and,  when  we  have  done  this,  we  find  that  the  muscle 
is  still  able  to  respond  to  direct  excitation  with  all  forms  of  irritants,  viz., 

1  Ch.  Bernard :  "Analyse  physiolo  iqne  des  Prnprietes  des  Systemes  muscnlaires  et  nerveux 
au  moyen  du  Curare,"  Comptes-rendus,  1856,  p.  825.  Kolliker:  "  Physiologische  Untersuch- 
ungen  iiber  den  Wirkungen  einiger  Gifte,"  Archiv  fur  pathologische  Anatomie,  1856. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.        27 

electrical,  mechanical,  thermal,  and  ehemieal.  Kvidently  the  muscle-proto- 
plasm is  irritable  and  is  capable  of  developing  a  contraction  independently  of 
the  nerves.  There  are  a  number  of  natural  plant  bases  that  have  a  "curare- 
like"  action  —  c.  </.,  brucin,  strychnin,  leucin,  nicotin,  conin,  etc.1  I!'  a 
nerve-muscle  preparation  be  dissected  out  and  placed  in  a  0.7  per  cent,  solu- 
tion of  sodium  chloride  containing  one  of  these  drugs,  sooner  or  later  the  nerve- 
ends  will  be  poisoned,  and  it  will  be  found  that  excitation  of  the  nerve  has  no 
effect  on  the  muscle,  although  the  muscle  responds  well  to  direct  excitation. 

Other  7  Voo/x  Ihnt  tlie  Muscle-protoplasm  can  be  Directly  Irritated.  —  Mus- 
cles with  long  parallel  fibres,  such  as  the  sartorius  of  the  frog,  contain  no 
nerves  at  their  extremities,  the  nerve-fibres  joining  the 
muscle-fibres  at  some  little  distance  from  their  ends. 
The  tip  of  such  a  muscle,  where  no  nerve-fibres  can 
be  discovered  by  the  most  careful  microscopical  exam- 
ination, is  found  to  be  irritable.  The  fact  that  in  some 
of  the  lower  animals  there  are  simple  forms  of  contrac- 
tile tissue  in  which  nerves  cannot  be  discovered,  and 
which  are  irritable,  is  interesting  as  corroborative  evi- 
dence, although  it  is  not  a  proof,  of  the  independent 
irritability  of  a  highly  differentiated  tissue  such  as 
striated  muscle.  Another  similar  piece  of  evidence  is 
to  be  found  in  the  fact  that  the  heart  of  the  embryo 
beats  rhythmically  before  nerve  appears  to  have  been 
developed.  A  proof  can  be  found  in  the  observation 
that  if  a  nerve  be  cut  it  begins  to  undergo  degenera-  FIG.  4  -curare  experiment: 

the  shaded  parts  show  the  re- 

tion  and  loses  its  irritability  and  conductivity  in  four  gion  of  the  body  to  which  the 
or  five  days,  and  the  excitation  of  such  a  nerve  has  ^^SKSS^ 
no  effect  upon  the  muscle  although  direct  stimulation  protected  by  the  ligature 

P    ,1  i      *L     11*    •     .*  11          j    i_  x-  A        from  tne  action  of  the  drug. 

Of  the   muscle   itself.  IS   followed    by    contraction.      As    The  unbroken  lines  represent 

degeneration  involves  not  onlv  the  whole  course  of  the   tlie  sensory   nerves  which 

,  ,          .  "  ,      ,  .  .          carry  sensory  impulses  from 

nerve,  but  also  the  nerve  end-plates,  the  contraction  the  skin  to  the  central  nerv- 
must  be  attributed  to  the  irritability  of  the  muscle-  °us  system  ;  the  broken  lines 

•  .  .          indicate   the   motor    nerves, 

substance.    Another  point  of  interest  in  this  connection   which  carry  motor  impulses 


is  the  behavior  of  a  dying  muscle.     If  it  be  struck, 

instead  of  contracting  as  a  whole  it  contracts  at  the  Lauder  Brunton: 

place  where  it  was  irritated,  the  drawing  together  of   SioT^ 

the  fibres  at  the  part  forming  a  local  swelling,  or  welt. 

If  such  a  muscle  be  stroked,  a  wave  of  contraction  spreads  over  it,  following 

the  instrument,  instead  of  extending,  as  under  normal  conditions,  by  means  of 

the  excited  nerve-fibres  to  other  parts.     Under  these  circumstances  the  circum- 

scribed contraction  would  seem  to  show  that  the  nerves  had  lost  their  irrita- 

bility, or  that  the  nerve-ends  no  longer  transmitted  the  stimulus  to  the  muscle, 

and  the  response  was  due  to  the  direct  excitation  of  the  dying  muscle-fibres. 

This  phenomenon  is  known  as  an  idiomuscular  contraction. 

1  Santesson  :   Archivfiir  experimentdle  Pathologic  und  Pharmakologie,  l>95j  Bd.  35,  S.  23. 


28  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

CONDITIONS  WHICH  DETERMINE  THE  EFFECT  OF  EXCITATION. 

The  result  of  the  irritation  of  nerve  and  muscle  is  dependent  on  two  sets 
of  conditions — namely,  conditions  which  determine  the  irritability ;  condi- 
tions which  determine  the  efficiency  of  the  irritant. 

It  will  be  necessary  for  us  to  study  the  second  set  of  conditions  first — for, 
before  we  can  judge  of  the  irritability  and  the  effect  of  various  influences  upon 
it,  we  must  consider  how  far  the  activity  of  the  nerve  and  muscle  is  depend- 
ent on  the  character,  strength,  and  method  of  application  of  the  irritant. 

Conditions  which  Determine  the  Efficiency  of  Irritants. — Some  of 
these  conditions  can  be  best  studied  on  nerves,  while  others  are  more  ap- 
parent in  their  effects  on  muscles.  The  most  useful  irritant  for  purposes  of 
study  is  the  electric  current.  Mechanical,  thermal,  and  chemical  irritants  are 
likely  to  injure  the  tissue,  and  are  not  manageable,  whereas  electricity,  if  not 
too  strong,  can  be  applied  again  and  again  without  producing  any  permanent 
alteration,  and  can  be  accurately  graded  as  to  strength,  place,  time,  duration 
of  application,  etc.  Of  course,  the  results  obtained  by  the  use  of  a  given 
irritant  cannot  be  accepted  for  others  until  verified.  The  conditions  which 
determine  the  effectiveness  of  the  electric  current  as  an  irritant  may  be 
classed  as  follows :  (a)  The  rate  at  which  the  intensity  changes,  (b)  The 
strength  of  current,  (c)  The  density  of  current,  (d)  The  duration  of 
application,  (e)  The  angle  of  application.  (/)  The  direction  of  flow. 

Irritating  Effect  of  the  Electric  Current — Luigi  Galvani,  Professor  of 
Physics  at  Bologna,  1791  (or,  according  to  some,  his  wife  Lucia),  observed  the 
legs  of  frogs  which  had  been  prepared  for  the  kitchen,  and  had  been  suspended 
by  brass  hooks  from  an  iron  balcony,  make  convulsive  movements  every  time 
the  wind  blew  them  against  the  iron.  He  repeated  the  experiment  in  his 
laboratory,  and  decided  that  the  frogs  had  been  excited  to  action  by  electric 
currents  developed  within  themselves ;  he  looked  upon  the  metals  which  he 
had  used  merely  as  conductors  for  this  current.  Yolta,  Professor  of  Natural 
Philosophy  at  Pavia,  repeated  Galvani' s  experiment,  and  concluded  that 
there  had  been  an  electric  current  developed  from  the  contact  of  the  dissimilar 
metals  with  the  moist  tissues  of  the  frog.  In  accordance  with  this  idea  he  con- 
structed the  voltaic  pile,  and  this  was  the  starting-point  of  the  science  of 
electricity  of  to-day. 

Although  it  is  true  that,  under  certain  conditions,  differences  in  electric 
potential  sufficient  to  excite  muscles  to  contraction  can  be  developed  in  the 
animal  body,  the  contractions  of  the  frog's  leg  which  Galvani  observed  were 
due  to  the  metals  which  he  employed.  The  experiment  can  be  easily  per- 
formed by  connecting  a  bit  of  zinc  to  a  piece  of  curved  copper  wire,  and  bring- 
ing the  two  ends  of  the  arc  against  the  moist  nerve  and  muscle  of  a  frog.  A 
stronger  and  more  efficient  shock  can  be  obtained  from  a  Daniell  or  some  other 
voltaic  cell. 

A  Daniell  cell  (Fig.  5)  is  composed  of  a  zinc  and  copper  plate,  the  former  dipping 
into  dilute  sulphuric  acid,  the  latter  into  a  strong  copper-sulphate  solution.  Although 
gravity  will  keep  these  liquids  separated,  if  the  cell  is  to  be  moved  about  it  is  better 


GENERAL    PHYSIOLOGY    OF 


AND    NERVI-:.        29 


to  enclose  one  of  them  in  a  porous  cup.  A  common  form  of  cell  consists  of  a  glass  jar, 
in  the  middle  of  which  is  a  porous  cup;  outside  the  rup  is  the  sulphuric  arid  and  tlie 
zinc  plate,  and  inside  the  cup  is  the  copper  sulphate  solution  and  the  copper  plan-. 
The  zinc  plate  is  acted  upon  by  the  sulphuric  acid,  and,  as  a  result  of  the  chemical 
change,  a  difference  of  electric  potential  is  set  up  between  the 
metals,  so  that  if  the  zinc  and  copper  be  connected  by  a  piece  of 
metal,  what  we  call  an  electric  current  flows  from  the  zinc  to  the 
copper  inside  the  cell,  and  from  the  copper  to  the  zinc  outside  the 
cell.  The  zinc  plate,  being  the  seat  of  the  chemical  change,  is 
called  the  positive  plate,  and  the  copper  the  negative  plate. 
Several  such  cells  may  be  connected  together  to  form  a  battery, 
each  cell  adding  to  the  electro-motive  force,  and  hence  to  the 
strength  of  the  current.  As  the  current  is  alwaj's  considered  to 
flow  from  +  to  — ,  we  call  the  end  of  the  wire  connected  with  the 
copper  (negative  plate)  the  positive  pole,  or  anode,  and  the  end 
of  the  wire  connected  with  the  zinc  (positive  plate)  the  negative 
pole,  or  Ivctthode.  If  one  of  these  wires  be  touched  to  a  nerve, 
under  ordinary  circumstances  no  effect  is  produced ;  but  when  the 
other  wire  is  likewise  brought  in  contact  with  the  nerve,  the 
moist  tissues  of  the  nerve  form  a  conductor,  complete  the  cir- 
cuit, and  an  electric  current  at  once  flows  through  the  nerve  from 

the  anode  to  the  kathode.     The  effect  of  tha  sudden  flow  of 
,        .  .  .  FIG.  5.— Darnell  cell, 

electricity  into  the  nerve  is  to  give   it  a  shock — as  we  say,  it 

irritates  the  nerve — and  the  muscle  which  the  nerve  controls  is  seen  to  contract. 

In  the  place  of  using  ordinary  wires  for  applying  the  electricity,  we  use  electrodes. 
These  are  practically  the  same  thing,  but  have  insulated  handles,  and  have  a  form  better 
suited  to  stimulate  nerves  or  other  tissues.  The  two  wires  may  be  held  in  two  different 


FIG.  6.— a,  Ordinary  electrode  for  exciting  exposed  nerves  and  muscles,  consisting  of  two  wires 
enclosed,  except  at  their  extremities,  in  a  handle  of  non-conducting  material ;  6,  c,  non-polarizable  elec- 
trodes. When  metals  come  in  contact  with  moist  tissues  a  galvanic  action  is  likely  to  occur  and  polariz- 
ing currents  to  be  formed.  These  extra  currents  would  complicate  or  interfere  with  the  results  of  many 
forms  of  experiment,  and  they  are  avoided  by  the  use  of  non-polarizable  electrodes.  A  simple  form  con- 
sists of  a  short  glass  tube,  at  one  end  of  which  is  a  plug  of  china  clay  mixed  with  a  0.6  per  cent,  solution 
of  sodium  chloride,  and  at  the  other  end  a  cork  through  which  an  amalgamated  zinc  rod  is  thrust.  The 
zinc  rod  dips  into  a  saturated  solution  of  zinc  sulphate,  which  is  in  contact  with  the  clay.  The  clay  plugs 
touch  the  tissue  to  be  excited,  and  the  current  passes  from  the  zinc  rods  through  the  zinc-sulphate  and 
sodium-chloride  solutions  in  the  clay  to  the  tissues ;  d-f,  electrodes  for  exciting  human  nerves  and  mus- 
cles through  the  skin  (after  Erb) :  these  may  be  of  various  forms  and  sizes,  and  are  arranged  to  screw 
into  handles  (g),  to  which  the  wires  are  attached ;  they  are  usually  made  of  brass  and  covered  with 
sponge  or  other  absorbent  material  wet  with  salt-solution.  The  smaller  electrodes  are  used  when  a  dense, 
well-localized  stream  is  required,  and  the  larger  electrodes  when  little  action  is  wished  and  it  is  of 
advantage  to  have  the  stream  diffuse. 


handles,  in  which  case  we  speak  of  the  positive  and  negative  electrodes,  or  the  anode 
and  the  kathode,  or  they  may  be  held  in  the  same  handle  (Fig.  6). 


30  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

v  Keys. — It  is  not  as  convenient  to  stimulate  a  nerve  by  touching  it  with  the  electrodes  as 
it  is  to  place  it  upon  the  electrodes  and  close  the  connection  between  the  zinc  and  copper  at 
some  other  part  of  the  circuit ;  this  may  be  done  by  what  is  called  a  key.  Any  mechanism 

which  can  be  used  to  complete  the  circuit  could 
receive  this  name,  and  there  are  a  number 
of  convenient  forms.  The  one  most  used  by 
physiologists  is  that  devised  by  Du  Bois-Rey- 
mond,  and  which  bears  his  name  (see  Fig.  7). 
This  has  the  advantage  of  being  capable  of 
being  used  in  two  different  ways — one  simply 
as  a  means  to  close  the  circuit,  and  the  other 
to  short-circuit  the  current.  These  two  meth- 
ods are  shown  in  Figure  8. 

By  the  former  method  the  key  supplies  a 
movable  piece  of  metal  by  which  contact  be- 
tween the  two  ends  of  the  wires  may  be  made 
as  in  a  (Fig.  8),  or  broken  as  in  6,  and  the 
current  be  sent  through  the  nerve,  or  prevented 
from  entering  it.  By  the  latter  method  the 
battery  is  all  the  time  connected  with  the 
electrodes.,  and  the  key  acts  as  a  movable 
bridge  between  the  wires,  and  when  closed  gives  a  path  of  slight  resistance  by  which 
the  current  can  return  to  the  battery  without  passing  through  the  nerve.  The  current 
always  takes  the  path  of  least  resistance,  and  so,  if  the  key  be  closed  as  in  c,  all  the  cur- 
rent will  pass  through  the  key  and  none  will  go  to  the  nerve,  which  has  a  high  resistance, 
whereas  if  the  key  be  opened  as  in  d,  the  bridge  being  removed,  all  the  current  will  go 
through  the  nerve.  It  is  often  better  to  let  the  cell  or  battery  work  a  short  time  and  to 
get  its  full  strength  before  letting  the  current  enter  the  nerve,  and  the  short-circuiting  key 
permits  of  this.  Moreover,  there  are  times  when  a  nerve  may  be  stimulated  if  connected 


FIG.  7.— Electric  key. 


FIG.  8.— Electric  circuiting. 

with  the  source  of  electricity  by  only  one  wire  ;  when  the  nerve  is  so  excited,  it  is  called 
unipolar  stimulation  ;  this  may  be  prevented  by  the  short-circuiting  key. 

As  has  been  said,  a  nerve  is  irritated  if  it  be  connected  with  a  battery  and 
an  electric  current  suddenly  passes  through  it.  Unless  the  current  be  very 
strong  the  irritation  is  transient,  however ;  the  muscle  connected  with  the 


GENERAL    rilYNTOLOOY    OF   MUSCLE   AND    NERVE.       31 

uerve  gives  a  single  twitch  at  the  moment  that  the  current  enters  the 
nerve,  and  then  remains  quiet;  and  thus  we  meet  with  the  remarkable  faet 
that  an  electric  current,  though  irritating  a  nerve  at  the  moment  that  it 
enters  it,  can  flow  through  the  nerve  continuously  without  exciting  it.  Fur- 
ther, although  the  current  while  flowing  through  the  nerve  does  not  excite 
it,  a  sudden  withdrawal  of  the  current  from  the  nerve  irritates  it,  and  causes 
the  muscle  connected  with  it  to  contract.  It  is  our  custom  to  speak  of 
closing,  or  making,  the  circuit  when  we  complete  the  circuit  and  let  the 
current  flow  through  the  nerve,  and  of  opening,  or  breaking,  the  circuit 
when  we  withdraw  the  current  from  the  nerve.  Since  the  closing  of  the 
circuit  acts  as  a  sudden  irritant  to  the  nerve,  we  speak  of  this  irritant  as 
a  "making"  or  " closing "  shock,  and  the  corresponding  contraction  of  the 
muscles  as  a  making  or  closing  contraction ;  similarly  we  speak  of  the  effect 
of  opening  the  circuit  as  an  "  opening"  or  "  breaking"  shock,  and  the  result- 
ing contraction  as  an  opening  or  breaking  contraction.  As  we  shall  see  later, 
the  making  contraction  excited  by  the  direct  battery  current  is  stronger  than  the 
breaking  contraction  :  the  explanation  of  this  must  be  deferred  (see  page  38). 

(a)  Effect  of  the  Rate  at  which  an  Irritant  is  Applied,  Illustrated  by  the  Elec- 
tric Current. — As  has  been  said,  an  electric  current  of  constant  medium  strength 


FIG.  9.— Rheonome. 

does  not  irritate  a  nerve  while  flowing  through  it,  but  the  nerve  is  irritated  at 
the  instant  that  the  current  enters  it,  and  at  the  instant  that  the  current  leaves 
it.  Is  it  the  change  of  condition  to  which  the  nerve  is  subjected,  or  is  it  the 
suddenness  of  the  change,  which  produces  the  excitation  ?  Would  it  be  possi- 
ble to  turn  an  electric  current  into  a  nerve  and  remove  it  from  a  nerve  so 
slowly  that  it  would  not  act  as  an  irritant  ? 

The  experiment  has  been  tried,  and  it  has  been  found  that  if  the  nerve  be 
subjected  to  an  electric  current  the  strength  of  which  is  increased  or  decreased 
very  gradually,  no  change  occurs  in  the  nerve  sufficient  to  cause  a  contraction 
of  the  muscle.  In  this  experiment,  instead  of  using  the  ordinary  key,  we  close 
and  open  the  circuit  by  means  of  a  rheonome  (see  Fig.  9). 

This  instrument  contains  a  fluid  resistance,  which  can  be  altered  at  will,  thereby  per- 
mitting a  greater  or  less  strength  of  current  to   pass  from   the   battery  into  the  circuit 


32  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

containing  the  nerve.  The  wires  from  the  battery  are  connected  with  binding-posts,  a,  b 
(Fig.  9),  at  opposite  sides  of  a  circular  groove  containing  a  saturated  solution  of  zinc  sul- 
phate. Strips  of  amalgamated  zinc  connect  the  binding-posts  with  the  fluid,  and  so  com- 
plete a  circuit  which  offers  much  resistance  to  the  passage  of  the  current.  From  the  centre 
of  the  block  containing  the  groove  rises  an  upright  bearing  a  movable  horizontal  bar,  from 
each  extremity  of  which  an  amalgamated  zinc  rod,  e  and/,  descends  and  dips  into  the  zinc- 
sulphate  solution.  The  zinc  rods  are  connected  with  binding-posts  on  the  movable  bar,  and 
from  these  wires  pass  to  the  electrodes  on  which  the  nerve  rests.  The  bar  revolves  on  a 
pivot  on  the  top  of  the  upright,  and  thus  the  zinc  rods  can  be  readily  approached  to 
or  removed  from  the  zinc  strips,  the  poles  of  the  battery.  When  the  zinc  rods  hold  a 
position  midway  between  these  poles,  the  current  all  passes  by  the  way  of  the  fluid.  As 
the  bar  is  turned,  so  as  to  bring  the  zinc  rods  nearer  and  nearer  the  two  poles  of  the  bat- 
tery, the  current  divides,  and  more  and  more  of  it  passes  through  the  path  of  lessening 
resistance  of  which  the  nerve  is  a  part.  When  the  zinc  rods  are  brought  directly  opposite 
the  poles  of  the  battery  nearly  all  the  current  passes  by  the  way  of  the  nerve.  If  the  bar  be 
turned  more  or  less  rapidly,  the  current  is  thrown  into,  or  withdrawn  from,  the  nerve  more 
or  less  quickly. 

By  this  arrangement  we  can  not  only  observe  that  the  nerve  fails  to  be  irritated 
when  the  current  is  made  to  enter  or  leave  it  gradually,  and  when  it  is  flowing 
continuously  through  it,  but  that  sudden  variations  in  the  density  of  the  cur- 
rent flowing  through  the  nerve,  such  as  are  caused  by  quick  movements  of  the 
bar,  although  they  do  not  make  or  break  the  circuit,  serve  to  excite.  This 
experiment  shows  that  electricity,  as  such,  does  not  irritate  a  nerve,  but  that  a 
sudden  change  in  the  density  of  the  current,  whether  it  be  an  increase  or 
decrease,  produces  an  alteration  in  the  nerve-protoplasm  which  excites  it  to 
action  and  causes  the  development  of  what  we  call  the  nerve-impulse. 

Du  Bois-Reymond's  Law. — Du  Bois-Reymond  formulated  the  following 
rule  for  the  irritation  of  nerves  by  the  electrical  current :  "  It  is  not  the  abso- 
lute value  of  the  current  at  each  instant  to  which  the  motor  nerve  replies  by  a 
contraction  of  its  muscle,  but  the  alteration  of  this  value  from  one  moment  to 
another;  and,  indeed,  the  excitation  to  movement  which  results  from  this  change 
is  greater  the  more  rapidly  it  occurs  by  equal  amounts,  or  the  greater  it  is  in 
a  given  time." 

We  shall  have  occasion  to  see  that  this  rule  has  exceptions,  or  rather  that 
there  is  an  upper  as  well  as  lower  limit  to  the  rate  of  change  of  density  of  the 
electric  current  which  is  favorable  to  irritation. 

Similar  observations  may  be  made  with  other  forms  of  irritants.  Pres- 
sure, if  brought  to  bear  on  a  nerve  gradually  enough,  may  be  increased  to  the 
point  of  crushing  it  without  causing  sufficient  irritation  to  excite  the  attached 
muscle  to  contract,  although,  as  has  been  said,  a  very  slight  tap  is  capable  of 
stimulating  a  nerve.  Temperature,  and  various  chemicals,  likewise,  must  be  so 
applied  as  to  produce  rapid  alterations  in  the  nerve-protoplasm  in  order  to  act 
as  irritants.  The  same  rule  would  seem  to  hold  good  for  the  nerve-cells  of  the 
central  nervous  system.  It  is  a  matter  of  daily  experience  that  the  nervous 
mechanisms  through  which  sensory  impressions  are  perceived  are  vigorously 
excited  by  sudden  alterations  in  the  intensity  of  stimuli  reaching  them,  and  but 
little  affected  by  their  continuous  application ;  the  withdrawal  of  light,  a  sudden 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND   NERVE.       33 


alteration  of  temperature,  an  unexpected  noise,  or  the  cessation  of  a  monotonous 
sound,  as  exemplified  by  the  common  experience  that  a  sleeper  is  awakened 


FIG.  10.— Induction  apparatus:  a,  primary  coil;  6,  secondary  coil ;  c,  the  automatic  interrupter. 

when  reading  aloud  abruptly  ceases,  attract  the  attention,  although  a  continu- 
ous sensory  irritation  may  be  unnoticed.  This  physiological  law  of  the  nervous 
system  would  seem  to  have  a  psychological  bearing  as  well. 


FIG.  11.— Schema  of  induction  apparatus. 

Irritating  Effect  of  Induced  Electric  Currents. — Within  certain  limits,  the 
more  rapid  the  change  in  intensity  of  an  electric  current  the  greater  its  power  to 
irritate.  This  probably  accounts  in  part 
for  the  fact  that  the  induced  current  is  a 
more  powerful  irritant  to  nerves  than  the 
direct  galvanic  current.  Induced  currents 
are  usually  obtained  by  means  of  an  induc- 
tion apparatus  (see  Fig.  10). 

The  ordinary  induction  apparatus  employed 
in  the  laboratory  (see  Fig.  11)  consists  of  a  coil  of 
wire,  p,  which  may  be  connected  with  the  ter- 
minals of  a  battery,  6,  and  a  second  coil,  s,  wholly 
independent  of  the  first,  which  is  connected  with 
electrodes,  e.  At  the  instant  that  the  key,  &,  in 
the  primary  circuit  is  closed,  and  the  battery  cur- 
rent enters  the  primary  coil,  an  induced  current 
s  developed  in  the  secondary  coil,  and  the  nerve 
resting  on  the  electrodes  is  irritated.  The  in- 
duced current  is  of  exceedingly  short  duration, 
suddenly  rising  to  full  intensity  and  falling  to 
zero.  As  long  as  the  battery  current  continues  to 
flow  constantly  through  the  primary  coil,  there  is 
no  change  in  the  electrical  condition  of  the  sec- 
ondary coil,  but  at  the  instant  the  primary  current  is  broken  another  induced  cuyent  of  short 
duration  is  set  up  in  the  secondary  coil,  and  again  the  nerve  receives  a  shock.  The  rise  and 
VOL.  II.— 3 


FIG.  12.— Schema  of  the  relative  intensity 
of  induction  currents  (after  Hermann,  Hand- 
buck  der  Physiologic,  Bd.  ii.  S.  37) :  P,  abscissa 
for  the  primary  current;  S,  abscissa  for  the 
secondary  current;  1,  curve  of  the  rise  of 
intensity  of  the  primary  current  when  made ; 
2,  curve  of  the  rise  and  fall  of  intensity  of 
the  corresponding  induced  current;  3,  curve 
of  fall  of  the  intensity  of  the  primary  cur- 
rent when  it  is  broken;  4,  curve  of  the  rise 
and  fall  of  intensity  of  the  corresponding  in- 
duced current. 


34 


AN  AMERICAN   TEXT-BOOK    OF  PHYS1OLCGY. 


fall  of  the  density  of  the  current  in  the  secondary  coil  is  very  rapid,  and  this  rapid  double  change 
in  density  of  the  current  causes  the  induction  shock  to  be  a  very  effective  irritant.  The  break- 
ing induction  shock,  as  we  call  that  which  is  produced  by  breaking  the  primary  current,  is 
^  found  to  act  more  vigorously  than  the  making  shock,  which  is  the  reverse  of  what  is  found 
with  direct  battery  currents.  The  cause  of  this  lies  iii  the  nature  of  'he  apparatus.  At  the 
moment  that  the  current  begins  to  flow  into  the  primary  coil,  it  induces  ftot  only  a  current 
in  the  secondary  coil,  but  also  currents  in  the  coils  of  wire  of  the  prmia'ry  coil.  These 
extra  induced  currents  in  the  primary  coil  have  the  opposite  direction  to  the  battery  cur- 
rent and  tend  to  oppose  its  entrance,  and  thereby  to  prevent  it  from  immediately  gain- 
ing its  full  intensity.  This  delay  affects  the  development  of  the  induced  current  in  the 
secondary  coil,  causing  it  to  be  weaker  and  to  have  a  slower  rise  and  fall  of  intensity  than 
would  otherwise  be  the  case.  When  the  primary  current  is  broken,  on  the  other  hand, 
there  is  no  opposition  to  its  cessation,  and  the  current  induced  in  the  secondary  coil  is 
intense  and  has  a  rapid  rise  and  fall.  These  differences  are  illustrated  in  Figure  12. 

Myogram. — To  accurately  test  the  effect  of  the  making  and  breaking 
induction  shocks,  it  is  necessary  to  record  the  reaction  of  the  nerve  ;  this  can 
be  done  by  recording  the  extent  to  which  the  corresponding  muscle  contracts 
in  response  to  the  stimulus  which  it  receives  from  the  nerve.  In  such  an 
experiment  it  is  customary  to  use  what  is  known  as  a  nerve-muscle  prepara- 
tion. The  gastrocnemius  muscle  and  sciatic  nerve  of  a  frog,  for  instance,  are 
carefully  dissected  out,  the  attachment  of  the  muscle  to  the  femur  being  pre- 
served, and  the  bone  being  cut  through  at  such  a  point  that  a  sufficiently  long 
piece  of  it  shall  be  left  to  fasten  in  a  clamp,  and  so  support  the  muscle  (see 
Fig.  13). 


FIG.  13.— Method  of  recording  muscular  contraction. 

The  simplest  method  of  recording  the  extent  of  the  muscular  contraction 
i*s  to  connect  the  muscle  by  means  of  a  fine  thread  with  a  light  lever,  and  let 
the  point  of  the  lever  rest  against  a  smooth  surface  covered  with  soot,  so  that 
when  the  muscle  contracts  it  shall  draw  up  the  lever  and  trace  a  line  of  cor- 
responding length  upon  the  blackened  surface.  The  combination  of  instru- 


GENERAL    PHYSIOLOGY    OF  MUtiCLE   AND    NERVE.       35 

raents  employed  to  record  the  contraction  of  a  muscle  is  called  a  myograph,  and 
the  record  of  the  contraction  is  termed  a  myogram.  If,  when  the  muscle  of  a 
nerve-muscle  preparation  is  thus  arranged  to  write  its 
contractions,  the  nerve  be  irritated  with  alternating  mak- 
ing and  breaking  induction  shocks  of  medium  strength, 
the  muscle  will  make  a  series  of  movements,  which,  if 
the  surface  be  moved  past  the  writing-point  a  short 
distance  after  each  contraction,  will  be  pictured  in  the 
record  as  a  row  of  alternating  long  and  short  lines,  the  FIG  14  _Effect  of  making 
records  of  the  breaking  contractions  being  higher  than  and  breaking  induction 
those  of  the  making  contractions  (Fig.  14).  Similar  sl 
results  are  obtained  if,  instead  of  irritating  the  nerve,  we  irritate  the  curarized 
muscle  directly. 

Stimulating  Effects  of  Making  and  Breaking  the  Direct  Battery  Current. — 
On  account  of  the  construction  of  the  induction  apparatus,  breaking  induction 
shocks  are  more  effective  stimuli  than  making  induction  shocks.  The  reverse 
is  true  of  the  stimulating  effects  which  come  from  making  and  breaking  the 
direct  battery  current.  The  excitation  which  results  from  sending  a  galvanic 
current  into  a  nerve  or  muscle  is  stronger  than  that  which  is  caused  by  the 
withdrawal  of  the  current.  This  difference  is  due  to  the  physiological  altera- 
tions produced  by  the  current  as  it  flows  through  the  irritable  substance,  and 
is  without  doubt  closely  associated  with  changes  in  the  irritability  which  occur 
at  the  moment  of  the  entrance  and  exit  of  the  current. 

The  making  contraction  starts  from  the  kathode,  and  the  breaking  contraction 
from  the  anode.  The  irritation  process  which  results  from  making  the  current 
is  developed  at  the  kathode,  and  that  which  results  from  breaking  the  current 
is  developed  at  the  anode.  This  was  first  demonstrated  on  normal  muscles  by 
Yon  Bezold,1  and  has  since  been  substantiated  for  nerves  as  well  as  muscles 


FIG.  15.— Schema  of  Bering's  double  myograph :  C,  clamp  holding  middle  of  muscle ;  P,P,  pulleys  to 
the  axes  of  which  the  recording  levers  are  attached ;  p,p,  pulleys  for  the  light  weights  which  keep  the 
muscle  under  slight  tension ;  A,  positive  electrode ;  K,  negative  electrode ;  r,  commutator  for  reversing 
the  current ;  k,  key  ;  6,  battery. 

by  the  experiments  of  a  great  many  observers.  Perhaps  the  most  striking 
demonstration  is  to  be  obtained  by  Engelmann's  method.  The  positive  and 
negative  electrodes  are  applied  to  the  two  extremities  of  a  long  curarized  sarto- 

1  Untersuchungen  uber  die  elektrische  Emeguny  von  Muskdn  und  Nerven,  1861. 


36 


AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


rius  muscle,  which  is  clamped  in  the  middle  firmly  enough  to  prevent  the  con- 
tractions of  one  half  from  moving  the  other,  but  not  enough  to  interfere  with 
the  conduction-power  of  the  tissue.  The  record  of  the  contractions  is  best 
obtained  by  the  double  myograph  of  Hering  (Fig.  15),  which  permits  the 
recording  levers  attached  to  the  two  ends  of  the  muscle  to  write  directly  under 
each  other,  so  that  any  difference  in  the  beginning  of  the  contraction  of  the 
two  halves  of  the  muscle  is  immediately  recognizable  from  the  relative  posi- 
tions of  the  records  of  their  contractions.  * 

The  current  is  applied  to  the  two  extremities  of  the  muscle  by  non-polarizable  electrodes. 
In  all  experiments  with  the  direct  battery  current  it  is  essential  to  employ  non-polarizable 
electrodes.  The  form  devised  by  Hering  is  very  useful  where  the  current  has  to  be  applied 
directly  to  the  muscle,  because  the  two  electrodes  are  hung  from  pivots  in  such  a  way  that 
they  move  with  the  movements  of  the  muscle,  and  hence  do  not  shift  their  position  when 
the  muscle  contracts.  Some  kind  of  apparatus  has  to  be  employed  for  quickly  reversing 
the  direction  of  the  current.  A  convenient  in- 
strument for  this  purpose  is  Pohl's  mercury  com- 
mutator (Fig.  16).  This  instrument  consists  of 
a  block  of  insulating  material  in  which  are  six 
little  cups  containing  mercury,  which  is  in  con- 
nection with  binding-posts  on  the  sides  of  the 
block.  Two  of  the  mercury  cups  on  the  opposite 


FIGS.  16, 17.— Pohl's  mercury  commutator. 

sides  of  the  block  a  and  b  (Fig.  17,  A),  are  connected  by  wires  with  the  battery ;  two  others, 
c  and  d,  are  connected  with  wires  which  pass  to  the  electrodes ;  the  remaining  two  on  the 
opposite  side  of  the  block,  e  and  /,  are  joined  by  movable  good  conducting  wires  with  the 
cups  c  and  d  in  such  a  way  that  c  connects  with  /,  and  d  with  e.  Two  anchor-like  pieces  of 
metal  are  connected  by  an  insulated  handle,  and  are  so  placed  that  the  stocks  of  the  anchors 
dip  into  the  mercury  cups  a  and  b  (Fig.  16).  The  anchors  can  be  rocked  to  one  side  or  the 
other,  so  that  the  ends  of  the  curved  arms  shall  dip  into  the  cups  c  and  d  (in  which  case 
cup  a  will  be  connected  with  cup  c,  and  cup  b  with  cup  d ),  or  so  that  the  other  ends  of  the 
arms  shall  dip  into  cups  e  and  /  (in  which  case  cup  a  will  be  connected  with  cup  e,  and  by 
means  of  the  cross  wire  with  cup  d,  and  cup  b  will  be  connected  with  cup/,  and  by  means 
of  the  cross  wire  with  cup  c).  By  the  arrangement  shown  in  Fig.  17,  A  the  current  can 
pass  from  the  battery  by  way  of  a  and  c  down  the  nerve,  and  by  way  of  d  and  b  back  to 
the  battery ;  or  it  can  pass  from  the  battery  by  way  of  a,  e,  d,  and  in  the  reverse  direction, 
up  the  nerve  and  back  to  the  battery,  by  way  of  c,  /,  b.  There  are  many  other  forms  of 
apparatus,  generally  known  as  pole-changers,  which  may  be  employed  to  reverse  the 
current. 

The  commutator  can  be  used  in  another  way  (see  Fig.  17,  J5).  If  the  battery  be  con- 
nected with  it  as  before,  and  the  cross  wires  be  removed,  the  current  can  be  sent  at  will 
into  either  one  of  two  separate  circuits.  For  instance,  if  the  cups  c,  d  be  connected  with 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AMD   NERVE.       37 

the  electrodes  on  one  part  of  the  nerve,  and  the  cups  e,  f  with  the  electrodes  on  another 
part,  the  anchors  have  only  to  be  rocked  to  one  side  or  the  other  to  complete  the  commu- 
nication between  the  battery  and  one  or  the  other  of  these  pairs  of  electrodes. 

In  experiments  with  the  double  myograph,  in  which  the  making  of  the 
current  is  used  to  irritate,  records  are  obtained  such  as  are  shown  in  Figure  18. 


FIG.  18.— The  making  contraction  starts  at  the  kathode  (after  Biedermann). 

In  these  records  the  beginning  of  the  tuning-fork  waves  shows  the  moment 
that  the  current  was  made  and  the  irritation  given.  In  the  experiment  from 
which  record  a  was  taken  the  anode  was  at  the  knee-end  of  a  curarized  sartorius 
muscle  and  the  kathode  at  the  pelvic  end — i.  e.  the  current  was  ascending 
through  the  muscle.  The  lower  of  the  two  curves  was  that  got  from  the 
kathode  half,  the  arrangement  being  that  shown  in  Figure  15,  and  the  lower 
curve  began  before  that  got  from  the  anode  half;  i.  e.  the  contraction  originated 
at  the  kathode  and  spread  thence  over  the  muscle.  In  b  the  current  was 
reversed,  and  the  upper  curve  was  obtained  from  the  kathode  half  and  the 
lower  from  the  anode  half;  in  this  also  the  kathode  end  contracted  first. 
In  the  above  experiments  the  making  of  the  current  was  used  to  irritate, 
and  the  muscular  contraction  began  at  the  kathode ;  in  experiments  in  which 
the  breaking  of  the  current  was  employed  the  opposite  was  observed,  the 
anode  end  being  seen  to  contract  first,  regardless  of  the  direction  of  the  cur- 
rent. 

If  strong  currents  be  used,  the  fleeting  contractions  which  result  from 
opening  and  closing  the  current  are  followed  by  continued  contractions,  the 
closing,  Wundt's,  and  the  opening,  Hitter's  tetanus,  as  they  are  called.  These 
continued  contractions,  which  last  for  a  considerable  time,  remain  strictly 
located  at  the  region  where  they  originate,  and  Engelmann  proved  by  his 


38  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

experiments  that  the  tetanus  which  results  from  closing  a  strong  current 
remains  located  at  the  kathode,  and  the  tetanus  following  the  opening  of 
the  current  remains  located  at  the  anode. 

The  same  is  true  of  the  nerve  as  of  the  muscle;  the  irritating  process  which 
is  called  out  by  the  sudden  entrance  of  a  battery  current  into  a  nerve  starts 
from  the  negative  pole,  the  kathode,  and  spreads  thence  throughout  the  nerve, 
while  the  irritating  process  excited  by  the  cessation  of  the  flow  of  the  current 
starts  from  the  region  of  the  positive  pole,  the  anode,  and  spreads  from  that 
point  throughout  the  nerve.  A  proof  of  this  was  obtained  by  Von  Bezold, 
who  observed  the  difference  in  the  time  between  the  moment  of  excitation  and 
the  beginning  of  the  contraction  of  the  muscle,  when  the  nerve  was  excited 
by  opening  and  by  closing  the  current,  with  the  anode  next  to  the  muscle, 
and  with  the  kathode  next  to  the  muscle.  He  found  the  time  to  be  longer 
when  the  current  was  closed  if  the  kathode  was  the  more  distant,  and  to  be 
longer  when  the  current  was  opened  if  the  anode  was  farther  from  the  muscle. 
Evidently  in  the  case  of  the  nerve  as  of  the  muscle,  the  irritable  substance 
subjected  to  the  current  is*  not  all  affected  alike.  The  current  does  not 
set  free  the  irritating  process  at  every  part  of  the  nerve,  but  produces 
peculiar  and  different  effects  at  the  two  poles,  the  change  which  occurs  at  the 
kathode  when  the  current  is  closed  being  of  a  nature  to  cause  the  development 
of  the  excitatory  process  which  awakens  the  closing  contraction,  and  the 
change  which  occurs  at  the  anode  when  the  current  is  opened  being  such  a? 
to  cause  the  development  of  the  excitatory  process  which  calls  out  the  opening 
contraction. 

Closing  contractions  are  stronger  than  opening  contractions.  The  irritation 
developed  at  the  kathode  is  stronger  than  that  developed  at  the  anode.  It  is 
true  of  both  striated  and  unstriated  muscles  that  an  efficient  irritation  can  be. 
developed  at  the  kathode  with  a  weaker  irritant  than  at  the  anode.  Moreover, 
a  greater  strength  of  current  is  required  to  produce  opening  than  closing  con- 
tinued contractions. 

The  same  may  be  said  of  nerves.  If  one  applies  a  very  weak  battery  cur- 
rent to  the  nerve  of  a  nerve-muscle  preparation,  he  notices  when  he  closes  the 
key  a  single  slight  contraction  of  the  muscle,  and  when  he  opens  the  key,  no 
effect.  If  he  then  increases  the  strength  of  the  current  very  gradually,  and 
tests  the  effects  of  the  making  and  breaking  of  the  current  from  time  to  time, 
he  observes  that  each  time  the  strength  of  the  current  is  increased  the  closing 
contraction,  which  is  due  to  irritation  originating  in  the  part  of  the  nerve  sub- 
ject to  the  kathode,  grows  stronger,  and  finally  contractions  are  also  seen  when 
the  circuit  is  broken,  the  irritation  process  developed  at  the  anode  having 
become  strong  enough  to  excite  the  muscle.  These  opening  contractions  at 
first  are  weak,  but  gradually  increase  in  strength,  until  with  a  medium  strength 
of  current  vigorous  contractions  are  seen  to  follow  both  opening  and  closing  of 
the  current.  If  the  strength  of  the  current  be  still  further  increased,  it  is 
found  that  either  the  closing  or  opening  contraction  begins  to  decrease  in 
size,  and  if  a  very  strong  current  be  employed,  the  closing  or  opening  con- 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.       39 

traction  will  be  absent.  It  has  been  ascertained  that  the  direction  in  which 
the  current  is  flowing  through  the  nerve  determines  which  of  these  con- 
tractions shall  cease  to  appear.  The  cause  of  this  will  be  explained  a  little 
later. 

^  (6)  Effect  of  Strength  of  Irritant. — As  a  rule,  the  stronger  an  electric  current 
the  greater  its  irritating  effect.  This  can  be  readily  tested  upon  a  nerve  with 
the  induction  current,  the  strength  of  which  can  be  varied  at  pleasure.  The 
strength  of  the  induced  current  obtained  from  a  given  apparatus  depends 
upon  the  strength  of  the  current  in  the  primary  coil,  and  on  the  distance  of 
the  secondary  from  the  primary  coil.  In  ordinary  induction  machines  (see  Fig. 
10,  p.  33)  the  secondary  coil  is  arranged  to  slide  in  a  groove,  and  can  be  easily 
approached  to  or  removed  from  the  primary  coil,  thus  placing  the  coils  of  wire 
of  the  secondary  coil  more  or  less  under  the  influence  of  the  magnetic  field 
about  the  primary  coil.  This  permits  the  strength  of  the  current  to  be  graded 
at  will.  The  strength  of  the  induced  current  does  not  increase,  however,  in 
direct  proportion  to  the  nearness  of  the  coils.  As  the  secondary  approaches 
the  primary  coil,  the  induced  current  increases  in  strength  at  first  very  slowly, 
and  later  more  and  more  rapidly,  reaching  its  greatest  intensity  when  the 
secondary  coil  has  been  pushed  over  the  primary. 

The  relation  of  the  strength  of  a  current  to  the  irritating  effect  upon  a  nerve 
can  be  readily  tested  with  such  an  induction  apparatus.  The  secondary  coils 
can  be  connected  with  a  pair  of  electrodes  on  which  the  nerve  of  a  nerve- 
muscle  preparation  rests  (as  in  Fig.  11,  page  33),  and  the  muscle  can  be 
arranged  to  record  the  height  of  its  contractions  (as  in  Fig.  13,  p.  34). 
The  experiment  can  be  begun  by  placing  the  secondary  coil  at  such  a  dis- 
tance from  the  primary  that  the  making  and  breaking  shocks  are  too  feeble  to 
have  any  effect  upon  the  nerve.  Then  the  secondary  coil  can  be  gradually 
approached  to  the  primary,  the  primary  current  being  made  and  broken  at 
regular  intervals.  At  a  certain  point  the  breaking  shock  will  excite  a  very 
feeble  contraction,  the  making  shock  producing  no  effect.  If  this  contraction 
is  barely  sufficient  to  be  recognized,  we  call  it  the  minimal  breaking  contraction 
(see  Fig.  19,  a).  In  seeking  the  minimal  contraction  care  must  be  taken  not 
to  excite  the  preparation  at  too  short  intervals  of  time,  for,  as  we  shall  see, 
an  irritation  too  slight  to  excite  even  a  minimal  contraction  may,  if  repeated 
at  short  intervals,  increase  the  irritability  of  the  preparation  and  so  become 
effective.  By  using  a  short-circuiting  key  in  the  secondary  circuit  we  can 
cut  out  the  making,  shocks,  and  test  the  effect  of  a  further  increase  in  the 
strength  of  the  current  by  the  response  of  the  muscle  to  the  breaking  shocks. 
As  the  contractions  become  larger,  care  must  be  taken  not  to  irritate  the  muscle 
too  frequently,  lest  it  be  fatigued  and  so  fail  to  give  the  normal  response.  As 
the  current  is  strengthened  the  breaking  contractions  will  become  higher  and 
higher  until  a  point  is  reached  beyond  which  the  strength  of  the  current  may 
be  increased  to  a  considerable  extent  without  any  further  heightening  effect  (Fig. 
19,  6).  If  the  current  be  still  further  increased,  this  first  maximum  is  suc- 
ceeded by  a  still  further  growth  in  the  height  of  the  contractions,  until  finally 


40  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

a  second  maximum  (Fig.  19,  d)  is  reached,  beyond  which  no  further  increase 
is  to  be  obtained,  however  much  the  current  may  be  strengthened.1 


FIG.  19.— Effect  of  increase  of  strength  of  current  on  the  efficiency  of  breaking  induction  shocks  (after 
Fick) :  a,  minimal  contraction ;  6-c,  first  maximum ;  d-e,  second  maximum.  , 

If  both  the  making  and  breaking  contractions  be  recorded,  inasmuch  as 
the  making  shocks  are  weaker  stimuli  than  the  breaking  (see  p.  35),  the  mak- 
ing contractions  do  not  appear  until  after  the  breaking  contractions  have 
acquired  a  considerable  height.  After  the  making  minimal  contraction  has 
been  obtained,  the  making  contractions  rapidly  gain  in  height  as  the  current 
is  strengthened,  and  finally  acquire  the  same  height  as  the  maximal  breaking 
shocks. 

The  relation  of  the  strength  of  the  electric  current  to  its  irritating  power 
can  be  demonstrated  equally  well  by  using  the  direct  galvanic  current.  The 
strength  of  the  galvanic  current  depends  upon  the  character  and  number  of  the 
cells  employed,  and  the  total  resistance  in  the  circuit.  The  strength  of  the 
current  can  be  easily  varied  by  altering  the  resistance,  and  there  are  a  number 
of  forms  of  apparatus  for  this  purpose. 


FIG.  20.— Rheostat. 

A  convenient  instrument  is  the  rheostat  (Fig.  20).  This  is  a  box  containing  coils  of 
wire  of  known  resistance.  These  coils  are  connected  with  a  series  of  heavy  brass  blocks  on  top 
of  the  box.  The  current  enters  the  box  by  a  binding-post  attached  to  the  first  of  the  brass 
blocks  and  passes  thence  from  block  to  block,  by  going  through  the  coils  of  wire  connecting 
them,  until  it  reaches  the  binding-post  at  the  other  end  of  the  series.  The  blocks  can  be 
also  connected  by  good  conducting  brass  plugs,  which  can  be  pushed  in  between  them,  and 
when  this  is  done,  as  the  current  passes  directly  from  block  to  block  instead  of  going  through 
the  resistance  coils  beneath,  the  resistance  is  reduced  to  a  corresponding  amount. 

Another  method  of  altering  the  strength  of  the  current  flowing  through  the 
nerve  is  to  employ  some  form  of  shunt  to  split  the  current  so  that  only  a  part 
of  it  shall  pass  by  way  of  the  nerve.  A  current  takes  the  path  of  least  resist- 

1  Fick :  Untersuchungen  uber  elektrische  Nervenreizung,  Braunschweig,  1864. 


GENERAL   PHYSIOLOGY   OF  MUSCLE   AND   NERVE.       41 

ance,  and  if  two  paths  are  opened  to  it,  more  or  less  can  be  sent  through  one 
of  them  by  decreasing  or  increasing  the  resistance  in  the  other. 

A  useful  instrument  for  dividing  the  current  is  the  rheocord.     The  schema  given  in 
Figure  21  illustrates  the  way  in 
which  it  is  used.     The  amount 

of  current  passing  to  the  nerve      *>,.  * "»     *•    • 

will  vary  with  the  relative  re-      (&)  <^me    3 

sistance  in  «,  b,  c,  d,  e,  f,  and         ^ 

in  a,  ft,  <?,  A,  e,  /     The  bridge 

c,  d  can  be  slid  along  the  fine 

German -silver    wires  6,  i  and 

e,  j,  and   thus    the    resistance 

a,  &,  c,  d,  e.  /,  and  the  amount  FIG.  21.— Rheocord. 

of  current  passing  through  the  nerve,  can  be  varied  at  pleasure. 

With  such  an  arrangement  we  should  find  that  the  irritating  effect  of  the 
current  is  largely  dependent  upon  its  strength.  In  the  case  of  strong  currents, 
however,  the  results  may  be  complicated  by  alterations  in  the  irritability  and 
conductivity,  which  we  will  consider  later.  It  is  true  also  of  other  forms  of 
irritants,  and  of  muscles  as  of  nerves,  that  the  effect  of  stimulation,  up  to  a 
certain  limit,  increases  with  the  strength  of  the  irritant. 

(c)  Effect  of  Density  of  the  Current. — Although  the  strength  of  the  current 
is  an  all-important  factor  in  its  excitatory  action,  the  effectiveness  of  the  cur- 
rent as  an  irritant  depends  very  largely  on  the  density  of  the  stream.  When 
the  current  enters  into  a  conductor,  it  spreads  widely  through  the  conducting 
substance,  and  though  the  larger  part  of  it  takes  the  path  of  least  resistance, 
which  is  usually  the  shortest  path  to  the  point  of  exit,  many  of  the  threads 
of  current  make  a  comparatively  wide  circuit  to  reach  the  outlet.  If  the  con- 
ductor is  equally  good  at  all  points,  but  is  irregularly  shaped,  the  density  of 
the  stream  will  be  greatest  where  the  diameter  of  the  conductor  is  least.  Thus 
it  happens  that  if  a  current  be  made  to  flow  from  end  to  end  of  a  muscle,  like 
the  sartorius  of  the  frog,  which  is  smaller  at  the  knee  end  than  at  the  pelvic 
end,  the  density  of  the  current  will  be  greater  at  the  lower  than  at  the  upper 
end,  and  the  irritating  power  of  the  current  will  be  greater  at  the  lower  end.1 

This  question  of  the  effect  of  the  density  of  the  current  is  important,  as  it 
helps  to  explain  the  peculiar  reactions  to  the  electric  stream  obtained  when  a 
current  is  applied  under  normal  conditions  through  the  skin  to  the  human 
nerve  (see  p.  51). 

Spread  of  Electric  Current. — The  tendency  of  electric  currents  to  spread 
widely  through  moist  conductors  is  a  common  source  of  error  in  electrical 
excitation,  and  should  be  always  guarded  against.  For  example,  if  it  is 
necessary  to  excite  a  nerve  at  the  bottom  of  a  deep  wound,  shielded  elec- 
trodes should  be  used — i.  e.,  electrodes  in  which  the  metal  terminals  are  insu- 
lated by  vulcanite,  except  at  the  part  which  the  nerve  is  to  touch.  More- 
over, care  should  be  taken  that  there  is  no  fluid  communication  between  the 
electrodes  and  the  surrounding  tissues.  If  these  precautions  are  not  observed, 
1  Biedermann :  Elektrophysiologie,  1895,  Bd.  i.  S.  185. 


42  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  current  may  excite  other  parts  than  those  which  it  is  intended  to  excite 
and  false  conclusions  may  be  reached. 

In  case  currents  of  high  potential  are  employed,  another  source  of  error 
may  arise  through  electrostatic  charging  of  distant  parts. 

Spread  of  Electrostatic  Charges. — If  the  primary  coil  of  an  induction 
apparatus  be  connected  with  a  battery  by  the  closure  of  a  key  in  the  primary 
circuit,  the  sudden  flow  of  current  through  the  coil  is  accompanied  by  a 
transient  change  in  the  stress  of  the  magnetic  field  about  the  coil.  This  change 
in  the  magnetic  field  induces  an  alteration  in  the  electrical  condition  of  the 
wire  of  the  secondary  coil  of  the  apparatus,  and  the  terminals  of  this  coil 
undergo  a  rapid  change  of  electrical  potential,  the  one  becoming  positive,  the 
other  negative.  If  two  electrodes  be  connected  with  the  binding  posts  of  the 
secondary  coil,  they  become  the  terminals  of  the  coil  and  are  given,  one  a 
positive,  the  other  a  negative  charge.  The  same  thing  happens  when  the  key 
in  the  primary  circuit  is  opened.  In  both  cases  the  change  of  potential  is  only 
momentary  in  its  duration.  The  effect  of  opening  the  primary  circuit  is  con- 
siderably stronger  than  that  of  closing  the  circuit,  for  reasons  stated  on  page  33. 

If  the  two  electrodes  are  connected  by  a  conducting  material,  an  electric 
current  will  flow  from  one  to  the  other  at  the  instant  the  change  of  potential 
takes  place.  If  the  electrodes  be  connected  by  the  nerve  of  a  nerve-muscle 
preparation,  an  electrical  current  will  flow  through  the  nerve ;  the  nerve  will 
be  excited,  a  nerve-impulse  will  be  developed  and  be  transmitted  along  the 
nerve  to  the  muscle  and  cause  it  to  contract.  It  not  infrequently  happens, 
if  the  current  entering  the  primary  coil  is  strong  and  a  large  electromotive 
force  is  developed  in  the  secondary  coil,  that  the  exciting  effect  of  the  sudden 
electrical  change  is  not  confined  to  the  part  of  the  nerve  directly  connecting 
the  electrodes,  but  spreads  to  distant  parts  of  the  nerve,  and  even  to  the 
muscle.  This  is  shown  by  the  fact  that  the  muscle  will  contract  even  after 
a  moist  ligature,  tied  tightly  about  the  nerve,  has  broken  the  continuity  of 
its  protoplasm  and  so  prevented  the  nerve  impulse  from  reaching  the  muscle. 
In  such  a  case  the  contraction  of  the  muscle  is  due  to  an  irritation  of  the 
nerve  beyond  the  point  to  which  the  ligature  was  applied  or  to  the  direct 
excitation  of  the  muscle  itself.1 

If  it  is  found  that  the  muscle  will  contract  after  the  nerve  has  been 
crushed  by  the  ligature,  it  will  also  be  found  that  it  will  contract  in  case  one 
electrode  be  removed  from  the  nerve,  so  that  it  remains  connected  with  only 
one  pole  of  the  induction  apparatus.  To  understand  this,  we  must  look  upon 
the  muscle  as  the  terminal  of  the  pole  of  the  secondary  coil  with  which  it  is 
in  connection.  When  the  potential  of  the  poles  of  the  secondary  coils  is 
suddenly  changed,  the  change  of  potential  spreads  through  all  conducting 
bodies  connected  with  these  poles,  and  in  the  case  in  question  it  passes,  by 
way  of  the  wire,  electrode,  and  nerve,  to  the  muscle.  In  short,  the  muscle, 
like  any  conductor,  is  charged  up,  and  in  the  process  of  charging  there  is  a 
flow  of  current  which  excites  the  nerve  and  muscle. 

1  Du  Bois-Keymond  :    Untersuchungen  ilber  thierische  Electricitat,  Bd.  i.  S.  423. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.       43 

A  much  stronger  contraction  is  obtained  if  the  muscle  be  connected  with 
a  large  conductor,  such  as  the  human  body,  a  large  surface  of  tin-foil,  a  con- 
denser, or  the  earth,  for  in  the  process  of  charging  and  discharging  these 
bodies  there  is  a  large  flow  of  current  through  the  preparation.  Further 
uniting  the  free  pole  of  the  secondary  coil  with  the  earth,  because  increasing 
the  difference  in  the  potential  of  the  two  poles,  increases  the  effect.  In  case 
the  free  pole  of  the  secondary  coil  be  united  to  a  large  insulated  conductor, 
and  this  be  brought  near  the  nerve-muscle  preparation  without  touching  it, 
the  amount  of  excitation  will  be  increased  through  what  is  known  as  "  influ- 
ence" action.  For  example,  if  the  observer  touch  the  free  pole  of  the 
secondary  coil  with  one  hand  and  approaches  the  other  to  the  nerve  prepara- 
tion a  larger  contraction  will  be  seen  when  the  primary  current  is  made  or 
broken.  The  effect  produced  on  the  preparation  by  the  presence  of  a  con- 
ductor, which  is  suddenly  given  an  electrostatic  charge  of  opposite  sign,  as  in 
the  case  just  mentioned,  cannot  be  discussed  here ;  suffice  it  to  say,  it  is 
analogous  to  the  influence  exerted  by  the  primary  coil  of  an  induction  appa- 
ratus on  the  secondary  coil  at  the  time  that  the  battery  current  is  made  and 
broken.1 

In  all  cases  which  we  have  cited  the  excitation  of  the  nerve  and  muscle 
was  caused  not  by  the  change  of  electric  potential,  but  by  the  sudden  flow  of 
current  accompanying  the  change.  The  exciting  effect  of  the  current  depends 
not  only  on  the  quantity  of  current,  but  also  on  the  density  of  the  stream.  In 
the  unipolar  experiments  thus  far  described  the  nerve-muscle  was  brought 
into  connection  with  the  secondary  coil  only  at  the  point  where  the  nerve 
touched  the  electrode  ;  the  electrical  charge  had  to  pass  the  length  of  the 
nerve  to  reach  the  muscle,  and  all  the  charging  current  had  to  flow  in  a 
dense  stream  the  length  of  the  nerve.  This  can  be  obviated  by  greatly 
enlarging  the  electrode  and  letting  it  come  in  contact  with  a  large  part  of 
the  nerve  and  muscle — e.  g.y  by  using  for  an  electrode  a  piece  of  thin  tin-foil, 
or  better  gold-foil,  and  applying  this  to  a  large  part  of  the  surface  of  the 
nerve  and  muscle  (see  Fig.  22).  By  this  arrangement  the  change  in  electric 
potential  will  be  transmitted  practically  instantaneously  throughout  the  good 
conducting  foil,  and  the  nerve  and  muscle  will  be  charged  from  a  vast 
number  of  points  of  contact  and  will  at  no  part  be  subjected  to  a  large  quan- 
tity of  electricity  flowing  in  a  dense  stream.  The  whole  of  the  nerve-muscle 
preparation  will  be  charged,  as  before,  to  the  potential  of  the  pole  with  which 
it  is  connected,  but  it  will  not  be  stimulated.  That  the  nerve-muscle  receives 
an  electrostatic  charge  under  the  above  conditions  can  be  readily  observed  by 
approaching  the  finger  to  the  muscle  at  the  time  that  the  primary  circuit  is 
closed  or  opened.  If  the  body  of  the  observer  has  a  large  capacity,  a  large 
amount  of  current  will  flow  through  the  nerve  and  muscle  to  the  finger.  This 
current  will  pass  in  a  dense  stream  from  the  muscle  at  the  point  of  contact 
with  the  finger,  and  the  muscle-fibres  at  this  part,  because  subjected  suddenly 

1  Hermann :  Handbuch  der  Physiologic,  Bd.  ii.  Thl.  1,  S.  87  ;  Biedermann :  Electro-physiology 
(translated  by  F.  A.  Welby),  1897,  vol.  ii.  p.  219. 


44 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


to  a  dense  flow  of  current,  will  be  excited  and  the  muscle  will  contract. 
Unless  the  primary  current  is  very  strong  the  electromotive  force  developed  in 
the  secondary  coil  on  the  closing  of  the  primary  circuit  may  be  too  weak  to 
cause  contraction,  and  only  the  effect  of  opening  the  circuit  may  be  observed ; 
in  any  case,  the  effect  of  breaking  the  primary  circuit  will  be  the  stronger. 
More  striking  results  will  be  obtained  if  the  primary  current  be  rapidly 
made  and  broken  by  an  automatic  interrupter  introduced  into  the  primary 
circuit;  the  muscle  will  then  be  excited  by  a  series  of  rapidly  following 
shocks. 


FIG.  22.— Unipolar,  localized  excitation  of  nerve.  By  this  arrangement  a  large  part  of  the  surface 
of  the  nerve  and  muscle  is  brought  into  immediate  connection  with  the  secondary  coil  through  the  sheet 
of  gold-foil.  The  nerve  is  locally  excited  at  the  point  that  is  touched  by  the  needle,  because  the  current 
going  to  charge  the  tin-foil  conductor  passes  out  of  the  nerve  at  this  point  as  a  dense  stream.  The 
muscle  (a)  is  supported  by  an  insulating  clamp  of  lead-glass  and  vulcanite  (6),  and  is  connected  to  the 
writing  lever  by  a  dry  lead-glass  hook  (c) ;  the  nerve  (d)  lies  on  a  sheet  of  gold-foil  («),  which  is  also 
wrapped  about  the  muscle,  and  which  rests  on  a  block  of  vulcanite  (/)  supported  by  a  glass  rod  (g) ;  the 
gold-foil  is  in  close  contact  with  the  binding-post  (h),  and  this  is  connected  with  one  terminal  of  the 
secondary  coil  (i)  of  an  induction  apparatus,  the  other  terminal  being  connected  with  a  gas  pipe  (j),  and 
so  with  the  earth  ;  in  the  primary  induction  circuit  there  are  a  battery  (k)  and  a  key  (I) ;  the  needle  (ra)  is 
connected  with  a  large  conductor  (n),  which  is  composed  of  a  board  covered  with  tin-foil,  and  is  sus- 
pended from  glass  hooks. 


For  the  sake  of  simplicity  we  have  thus  far  only  spoken  of  the  charging 
of  the  preparation  from  the  secondary  coil.  It  must  be  borne  in  mind,  how- 
ever, that  the  change  in  the  electrical  condition  of  the  secondary  coil  lasts 
only  an  instant,  and  the  terminals  of  the  coil  and  the  tissues  connected  with 
them  immediately  return  to  their  original  potential,  this  change  being  accom- 
panied by  a  backward  surge  of  the  electrical  wave  from  the  muscle  through 
the  nerve,  electrodes,  and  wire  to  the  coil,  and  this  reverse  current  acts  like 
the  charging  current  to  cause  excitation.  The  charging  and  discharging 
processes  follow  each  other  with  such  rapidity,  however,  that  they  act  upon 
the  tissues  as  a  single  excitation. 

To  Prevent  the  Spread  of  Current. — As  we  have  seen  when  the  nerve  of  a 
nerve-muscle  preparation  is  connected  by  two  electrodes  with  the  poles  of 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.        45 

the  secondary  coil  of  an  induction  apparatus,  if  a  large  electromotive  force  is 
developed  in  the  secondary  coil,  a  current  not  only  passes  through  the  part 
of  the  nerve  bridging  the  electrodes,  but  through  the  part  of  the  nerve 
between  the  electrodes  and  the  muscle.  This  spread  of  current  may  be  in 
part  prevented  by  connecting  the  electrode  nearest  to  the  muscle  with  a  gas- 
pipe  and  leading  the  charge  through  this  to  the  earth  (Hermann).  Another 
method  which  has  been  suggested  is  to  connect  the  two  sides  of  the  nerve 
beyond  the  electrodes  by  a  loop  of  good  conducting  metal,  so  that  the  spread- 
ing currents  shall  be  short-circuited  (Hering). 

Application  of  the  Unipolar-excitation  Method  to  the  Localization  of  Excita- 
tion.— The  principle  that  a  flow  of  current  will  excite  at  the  point  where  the 
current  is  dense  can  be  employed  to  obtain  definitely  localized  excitations  by 
the  unipolar  method  of  irritation.  For  example,  Kiihne  employs  the  follow- 
ing arrangement  to  show  isolated  contraction  of  muscle-fibres  by  localized 
excitation.  A  thin  parallel-fibred  sartorius  muscle  of  a  frog  is  curarized  to 
shut  out  the  effect  of  excitation  of  the  nerve  so  that  only  the  muscle-fibres 
which  are  directly  excited  will  contract.  The  preparation  is  then  placed 
on  an  insulated  copper  plate,  which  is  connected  with  one  pole  of  the  sec- 
ondary coil  of  an  induction  apparatus,'  the  other  pole  being  connected  with 
the  earth  (see  Fig.  23).  The  muscle  makes  no  contractions  when  the  key  in 


FIG.  23— Unipolar  localized  excitation  of  the  sartorius  muscle.  The  muscle  (a)  rests  on  a  sheet  of 
copper  (6),  which  is  on  a  plate  (c),  resting  on  a  sheet  of  vulcanite  (c/) ;  the  binding-post  (e)  on  the  copper 
plate  is  connected  with  one  terminal  of  the  secondary  coil  of  the  apparatus  (/),  the  other  terminal  being 
connected  with  a  gas  pipe  (g),  and  so  with  the  earth  ;  the  primary  coil  (h)  is  connected  with  a  battery  (i) 
and  a  key  (j);  the  muscle  is  locally  excited  by  the  current,  which  passes  in  a  dense  stream  through  it 
to  the  needle  (k)  held  in  the  hand. 

the  primary  circuit  is  closed  or  opened.  Its  potential  is  undoubtedly  changed, 
but  its  capacity  is  small,  and  it  is  charged  from  many  points ;  the  charging 
current  is  at  no  place  sufficient  in  quantity  or  density  to  excite.  If  now  the 
experimenter  touch  the  top  of  the  muscle  near  one  side  with  the  point  of  a 
needle  held  in  the  hand,  the  muscle  twitches  on  the  side  touched  each  time 
the  current  is  opened,  and  if  the  current  is  strong  each  time  it  is  closed. 
The  contraction  is  limited  to  the  fibres  just  below  the  needle,  because  this  is 
the  point  where  the  current  charging  the  body  of  the  observer  passes  through 
the  muscle  in  a  dense  stream.  The  effect  is  more  striking  if,  instead  of 
using  single  shocks,  the  primary  current  be  frequently  interrupted.  The 
fibres  on  the  side  stimulated  will  then  be  continuously  contracted  and  the 
muscle  will  curl  toward  the  stimulated  side. 


46  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

In  a  like  manner  if  a  nerve-muscle  preparation  be  isolated,  as  shown  in 
Fig.  22,  and  a  needle,  held  in  the  hand  or  connected  with  a  large  metallic 
conductor  or  a  condenser,  be  brought  in  contact  with  some  point  of  the  nerve, 
the  excitation  which  occurs  on  the  opening  and,  with  a  strong  current,  on  the 
closing  of  the  primary  circuit  will  be  strictly  limited  to  the  part  of  the  nerve 
touched  by  the  needle.  This  method  can  be  used  to  advantage  in  studying 
the  rate  of  conduction  in  nerves  or  any  problem  which  requires  strict  local- 
ization of  electric  excitation. 

(d)  Effect  of  the  Duration  of  the  Electric  Current  on  its  Power  to  Irritate 
Nerves  and  Muscles. — As  we  have  seen,  a  constant  battery  current,  when  flow- 
ing uninterruptedly  through  a  motor  nerve,  does  not  ordinarily  excite  it;  very 
slow  variations  in  the  strength  of  the  current  also  fail  to  irritate ;  but  rapid 
alterations  in  the  strength,  whether  in  the  direction  of  increase  or  decrease,  act 
as  vigorous  stimuli.  For  example,  medullated  nerves  are  irritated  more  vigor- 
ously by  the  rapid  changes  of  intensity  of  induced  currents  than  by  the  some- 
what slower  changes  occurring  at  the  make  and  break  of  battery  currents. 
Within  certain  limits,  at  least,  the  more  rapidly  the  intensity  of  the  current 
changes,  the  greater  the  irritating  effect  upon  nerves.  That  there  is  a  limit 
even  for  the  rapidly  reacting  protoplasm  of  medullated  nerves  is  shown  by 
the  fact  that  by  unipolar  excitation  the  charging  and  discharging  of  the  con- 
densers through  a  nerve  is  the  more  effective  the  greater  the  capacity  of  the 
condensers.  The  process  is  more  prolonged  if  the  condenser  is  large,  and 
the  effect  is  greater.1  Not  all  nerves  are  equally  susceptible  to  rapid  altera- 
tions of  the  intensity  of  the  current.  Non-medullated  nerves  do  not  appear 
to  react  as  readily  as  medullated  to  electric  currents  of  short  duration.  For 
instance,  the  nerves  of  the  claw  muscles  of  the  crab  are  not  readily  excited 
by  induced  currents,  and  respond  better  to  the  more  prolonged  influence  of 
the  closing  and  opening  of  battery  currents.2 

The  question  now  arises,  Is  the  reaction  of  muscle  to  electric  currents  the 
same  as  that  of  nerves  ?  Experiment  shows  that  muscles  which  have  been 
removed  from  the  action  of  nerves,  by  means  of  curare,  differ  from  medullated 
nerves  in  that  they  are  excited  more  vigorously  by  the  opening  and  closing  of 
battery  currents ;  less  vigorously  by  making  and  breaking  induction  currents. 
This  latter  fact  is  well  seen  in  experiments  in  which  two  gastrocnemius 
muscles  from  the  same  frog,  one  of  which  has  been  curarized  and  the  other 
not,  are  connected  with  an  induction  apparatus  in  series,  so  that  the  cur- 
rent shall  flow  through  them  both  in  the  same  direction.  If  the  primary 
current  be  made  and  broken,  the  non-curarized  muscle  will  respond  to  a 
weaker  induction  shock  than  the  curarized.  By  the  curarized  muscles  the  max- 
imal contraction  got  on  opening  and  closing  a  battery  current  is  both  higher 
and  more  prolonged  than  that  to  be  obtained  with  a  single  induction  shock. 
Unstriated  muscles  exhibit  this  difference  to  a  still  greater  degree  than 
striated  muscle ;  they  react  well  to  the  closing  of  battery  currents  of  medium 

1  Hermann :  Handbuch  der  Physiologic,  Bd.  ii.  Theil  1,  S.  88. 

2  Biedermann :  Elektrophysiologie,  1895,  Bd.  ii.  S.  546. 


GENERAL   PMY8IOLOQY    OF  Ml'SClJ-:   AND  NERVE.       47 


strength,  provided  these  last  some  little  time,  but  respond  to  induced  currents 
only  when  they  are  very  strong.  Thus  the  unstriated  muscle  which  closes  the 
shell  of  some  of  the  fresh-water  mussels,  as  the  Auodonta,  gives  larger  and 
larger  contractions  as  the  duration  of  the  current  is  increased  from  one-quarter 
of  a  second  to  three  seconds.  Much  the  same  is  true  of  the  uustriated  muscles 
of  the  ureters;1  the  battery  current  must  remain  closed  quite  a  while  for  the 
closing  contraction  to  be  called  out,  the  length  of  time  depending  upon  the 
strength  of  the  current  ;  and  induction  shocks  have  little  or  no  effect  unless 
very  strong.  Such  a  comparison  makes  it  evident  that  the  duration  of  the 
current  is  an  important  element  in  the  influence  exerted  by  electric  currents 
on  various  forms  of  protoplasm.  Unstriated  muscles  require  that  the  current 
shall  last  from  one-quarter  of  a  second  to  three  seconds  to  produce  maximum 
contractions.  Striated  muscles  require  that  a  current  shall  last  0.001  second 
(Fick),  and  even  medullated  nerves  fail  to  react  if  the  current  lasts  too  short 
a  time.  Various  forms  of  irritable  tissue  can  be  arranged  in  series  according 
to  their  ability  to  respond  to  electric  currents  of  short  duration,  viz.  medul- 
lated nerves,  non-medullated  nerves,  striated  muscles,  non-striated  muscles, 
and  the  little-differentiated  forms  of  protoplasm  of  many  of  the  protozoa. 
On  the  other  hand  these  tissues  are  found  to  respond  in  the  reverse  order  to 
currents  which  are  more  prolonged  and  which  change  their  intensity  slowly. 
It  would  seem  as  if  the  less  perfectly  differentiated  the  form  of  protoplasm, 
the  less  its  mobility  and  its  susceptibity  to  passing  influences. 

The  same  form  of  tissue  reacts  differently  in  different  animals.  For  instance, 
the  sluggish  striated  muscles  of  the  turtle  do  not  respond  as  well  to  induced 
currents  as  the  more  rapid  striated  muscles  of  the  frog.  Further,  the  condition 
of  the  tissue  at  the  time  is  found  to  have  an  influence  on  its  irritability  and  its 
power  to  respond  to  stimuli  of  short  duration.  Von  Kries  reports  that  nerves, 
if  cooled,  react  better  to  slow  variations  in  the  intensity  of  the  electric  current, 
and,  if  warmed,  to  rapid  variations.  Under  pathological  conditions  the  reac- 
tion of  nerve  and  muscle  to  electric  currents  may  become  blunted,  and,  as  the 
tissue  degenerates,  its  power  to  respond  to  rapid  changes  of  the  electric  current 
is  lessened.  If  a  nerve  be  cut,  the  part  which  is  separated  from  the  influence 
of  the  nerve-cells  degenerates.  The  irritability  at  first  increases  and  then 
very  rapidly  decreases,  in  from  three  to  four  days  being  wholly  lost.  As  the 
nerve  regenerates,  the  irritability  is  recovered  very  gradually,  and  the  power 
to  respond  to  the  relatively  prolonged  action  of  mechanical  stimuli  is  regained 
sooner  than  the  ability  to  reply  to  changes  as  rapid  as  those  of  induced  cur- 
rents. Howell  and  Huber  observed  that  regenerating  nerve-fibres  when  they 
have  reached  the  stage  resembling  embryonic  fibres,  i.  e.  are  strands  of  proto- 
plasm without  axis-cylinders,  fail  to  respond  to  induction  currents,  though  they 
can  be  excited  by  mechanical  stimuli.  It  was  found  that  it  is  not  until  the 
axis-cylinder  has  grown  down  into  the  regenerating  fibres  that  the  nerve  is 
capable  of  responding  to  induction  shocks. 

1  Engelmann  :   PflilgeSs  Archiv,  1870,  Bd.  iii.  S.  263. 


48  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

When  human  striated  muscle  undergoes  degeneration  as  a  result  of  an  in- 
jury to  its  nerve,  the  degenerating  muscle  comes  to  resemble  normal  unstriated 
muscle  in  its  reactions  to  electricity,  responding  feebly  to  induced  currents,  at 
a  time  when  irritability  to  mechanical  stimuli  and  to  direct  battery  currents 
is  even  increased.  This  is  used  by  clinicians  as  a  means  of  diagnosis  of  the 
condition  of  the  nerve  and  muscle. 

From  what  has  been  said  it  is  evident  that  the  rule  laid  down  by  Du  Bois- 
Reymond  (see  p.  32)  must  be  modified  in  so  far  that  there  is  for  each  tissue 
a  limit  to  the  rate  at  which  a  change  of  intensity  of  the  electric  current  acts 
as  an  irritant. 

(e)  Effect  of  the  Angle  at  which  the  Current  Enters  and  Leaves  the  Muscle 
and  Nerve. — The  angle  at  which  the  current  acts  on  the  muscle-fibre  has 
been  found  to  have  a  bearing  upon  its  power  to  stimulate.  Leicher  *  succeeded 
in  obtaining  definite  experimental  evidence  that  when  the  current  is  so  sent 
through  a  muscle  as  to  cross  it  at  right  angles  to  its  fibres  it  has  no  irritating 
effect,  and  that  its  power  to  stimulate  increases  as  the  angle  at  which  the 
threads  of  current  strike  the  muscle-fibres  decreases,  being  greatest  when  the 
current  passes  longitudinally  through  the  fibres. 

Similarly,  it  was  found  by  Albrecht  and  Meyer 2  that  the  irritating  effect 
of  the  electric  current  is  most  active  when  it  flows  longitudinally  through  the 
nerve,  and  that  it  is  altogether  absent  when  it  flows  transversely  through  it. 
This  view  is  doubted  by  some  observers,  who  would  attribute  the  difference 
observed  to  differences  in  the  electrical  resistance.  It  is  true  that  the  resist- 
ance to  cross  transmission  is  greater  than  to  longitudinal  transmission,  but  it 
is  not  likely  that  this  difference  suffices  to  explain  the  lack  of  response  to  cur- 
rents applied  at  right  angles  to  the  nerve-axis. 

Relative  Efficacy  of  the  above  Conditions  upon  the  Irritating  Power  of  the 
Electric  Current. — When  a  current  is  applied  to  an  irritable  part  of  a  nerve 
or  muscle  at  an  angle  suitable  to  excitation,  the  stimulating  effect  of  the  current 
depends  upon  the  rate  at  which  its  intensity  is  changed,  the  strength  and 
density  of  the  current,  i.  e.  its  intensity,  and  the  duration  of  the  current. 

Fick  3  gives  the  following  schema  (Fig.  24)  for  the  different  ways  in  which 
the  intensity  of  the  electric  current  may  be  varied,  and  compares  the  effects 
of  these  different  methods  of  application  of  the  current.  It  must  be  re- 
membered that  a  decrease  of  intensity  acts  no  less  than  an  increase  to  produce 
excitation.  In  the  above  schema  the  abscissa  represents  the  time,  and  the 
ordinates  the  strength,  of  the  current.  Suppose  the  rise  of  intensity  has  a 
form  such  as  is  represented  in  a,  Figure  24 — that  is,  that  the  strength  of  the 
current  increases  to  a  considerable  height,  but  very  slowly.  Such  a  rate  of 
change,  even  though  the  rise  of  intensity  were  continued  until  the  strength  of 
current  was  very  great,  would  have  no  exciting  effect  upon  a  nerve  and  might 

1  Untersuchungen  aus  dem  physiologischen  Institut  der  Universitdt  Halle,  Heft  i.  S.  5. 

2  Pfluger's  Archiv,  1880,  Bd.  xxi.  S.  462. 

3  Beitrdge  zur  vergleichende  Physiologic  der  irritablen  Substanzen,  Braunschweig,  1863. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.       49 

fail  to  irritate  a  striated  or  non-striated  muscle.  A  more  rapid  rise,  such  as 
is  shown  in  6,  might  irritate  a  non-striated  muscle,  but  fail  to  irritate  a  nerve 
or  a  striated  muscle.  With  currents  which  rapidly  gain  their  full  intensity 
and  then  return  again  to  zero,  the  following  cases  would  be  possible:  A 
rapid  rise  and  fall  of  intensity  (see  c),  such  as  occurs  by  an  induction  shock 
or  by  the  momentary  closure  of  a  battery  current,  might  suffice  to  excite 
a  nerve  but  not  be  an  effective  irritant  to  a  striated,  much  less  a  non-striated 
muscle,  unless  the  short  duration  of  the  current  were  compensated  for  by 
a  considerable  increase  in  the  intensity  (see  d).  On  the  other  hand  a^form 
of  variation  such  as  is  shown  in  e,  where  the  rate  of  change  is  very  rapid, 
although  the  intensity  is  not  great,  might  act. to  irritate  nerves,  and,  because 
of  the  longer  duration  of  the  current,  striated  muscles,  though  having  no  effect 
on  non-striated  muscles ;  and  the  slower  rate  of  change,  and  considerable  dura- 


10       20       SO      40 


100  110 


20 
10, 

o. 

i 
c 

A 

I 

«                  / 

r\ 

FIG. 24.. —Schema  of  relation  of  the  method  of  application  of  the  electric  current  to  the 

irritating  effect. 

tion,  illustrated  by/,  though  not  affecting  nerves,  might  suffice  for  striated 
muscles  and  be  favorable  to  the  excitation  of  non-striated  muscles. 

In  the  case  of  nerves,  duration  of  current  is  less  important  than  a  rapid 
change  of  intensity.  In  the  case  of  striated  muscles  the  advantage  to  be 
gained  by  rapid  variations  can  be  easily  overstepped,  and  the  importance  of 
the  duration  of  the  current  is  greater ;  while  in  the  case  of  non-striated  muscles 
duration  of  current  is  of  the  first  importance  and  rapid  variation  may  fail  to 
excite.  In  the  case  of  all  tissues,  strength  and  density  of  current,  what  we 
may  call  intensity  of  current,  is  favorable  to  excitation. 

(/)  Effect  of  the  Direction  in  which  the  Current  flows  along  the  Nerve. — 
The  result  of  the  irritating  change  produced  in  a  nerve  by  a  battery  current 
has  been  found  to  depend  upon  whether  the  current  flows  toward  or  away 
from  the  organ  stimulated  by  the  nerve.  This  fact  can  be  most  readily  ob- 
served in  the  case  of  isolated  motor  nerves.  In  the  case  of  these  nerves,  the 
effects  produced  by  opening  and  closing  the  current  are  different  according  as 
the  current  is  descending,  i.  e.  flows  through  the  nerve  in  the  direction  of  the 
muscle,  or  ascending,  i.  e.  flows  through  the  nerve  in  the  opposite  direction. 

VOL.  II.— 4 


50 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


Moreover,  by  a  given  rate  of  change  of  intensity,  the  stimulating  effect  varies 
with  the  strength  of  the  current  employed.  Pfliiger  in  his  celebrated  mono- 
graph, Untersuchungen  uber  die  Physiologic  des  Elektrotonus,  published  in 
Berlin,  1859,  p.  454,  formulated  the  following  rule  for  the  result  of  excitations 
under  varying  conditions : 


Pfliiger's  Law  of  Contraction. 

Ascending  Current. 
Closing.          Opening. 

Weak  current Contr.  Kest. 

Medium   "        Contr.    '         Contr. 

Strong      "       Best.  Contr. 


Descending  Current. 
Closing.          Opening. 

Contr.  Eest. 

Contr.  Contr. 

Contr.  Eest. 


To  understand  this  so-called  "  law  of  contraction  "  we  must  bear  in  mind 
certain  fundamental  facts,  namely  : 

a.  When  a  nerve  is  subjected  to  a  battery  current,  an  excitatory  process  is 
developed  in  the  part  of  the  nerve  near  the  kathode  when  the  current  is 
closed,  and  in  the  part  of  the  nerve  near  the  anode  when  the  current  is  opened 
(see  p.  38). 

6.  The  excitatory  process  developed  at  the  kathode  is  stronger  than  that 
developed  at  the  anode  (see  p.  38). 

c.  A  third  fact  which  is  of  no  less  importance,  and  which  will  be  considered 
in  detail  when  we  study  the  effects  of  the  constant  current  on  the  irritability 
and  conductivity  of  nerve  and  muscle  (see  p.  95),  is  the  following :  During 
the  time  that  a  strong  constant  current  is  flowing  through  a  nerve,  the  conduct- 
ing power  is  somewhat  lessened  in  the  part  to  which  the  kathode  is  applied,  and 
is  greatly  decreased,  or  altogether  lost,  in  the  region  of  the  anode ;  moreover, 
at  the  instant  that  the  current  is  withdrawn  from  the  nerve  the  conducting 
power  is  suddenly  restored  in  the  region  of  the  anode,  and  greatly  lessened,  or 
lost,  in  the  region  of  the  kathode. 


Ascending  Current. 
K  A 


Descending  Current. 
A  K 


Weak  current. 


Medium  current. 


Strong  current. 


FIG.  25.— Diagram  illustrating  Pfluger's  law. 


The  twelve  cases  included  in  the  above  table  can  be  represented  in  the  fol- 
lowing diagram  (Fig.  25),  in  which  a  cross  is  marked  at  the  part  of  the  nerve 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND    NERVE.        :>1 


from  which  the  irritation  that  is  effective  in  producing  a  contraction  takes 
its  rise. 

In  the  case  of  fresh  motor  nerves  of  the  frog,  when  the  current  is  weak, 
only  closing  excitations,  /.  e.,  those  originating  at  the  kathode,  are  effective  by 
both  directions  of  the  current.  As  the  strength  of  the  current  is  increased,  at 
the  same  time  that  the  closing  kathodic  contractions  grow  stronger,  opening 
anodic  contractions  begin  to  appear ;  and  with  currents  of  medium  strength 
both  closing  and  opening  contractions  are  obtained  with  both  directions  of  the 
current*  If  the  strength  of  the  current  be  still  further  increased,  a  change  is 
observed;  with  a  strong  current,  the  closing  of  the  ascending  and  the  opening 
of  the  descending  current  fails  to  excite  a 
muscular  contraction.  This  fact  is  demon- 
strated most  clearly  if  we  employ  two 
nerve-muscle  preparations,  and  lay  the  nerves 
in  opposite  directions  across  the  non-polar- 
izable  electrodes,  so  that  the  current  from 
the  battery  shall  flow  through  one  of  the 
nerves  in  an  ascending  direction  and  through 
the  other  in  the  descending  direction  (see 
Fig.  26).  If  under  these  conditions  a  strong 
battery  current  be  employed,  muscle  a  (through 
the  nerve  of  which  the  current  is  descending) 
will  contract  only  when  the  circuit  is  closed, 
and  muscle  b  (through  the  nerve  of  which  the  current  is  ascending)  will  con- 
tract only  when  the  circuit  is  opened. 

Since  in  the  case  of  currents  of  medium  strength,  both  opening  and  clos- 
ing the  circuit,  when  the  current  is  ascending  and  when  it  is  descending, 
develops  a  condition  of  excitation  in  the  nerve  sufficient  to  cause  contractions, 
the  failure  of  the  contraction  by  the  closing  of  the  strong  ascending  current, 
and  by  the  opening  of  the  strong  descending  current,  can  scarcely  be  supposed 
to  be  due  to  a  failure  of  the  exciting  process  to  be  developed  in  the  nerve ;  and 
it  would  seem  more  likely  that  the  nerve-impulse  is  for  some  reason  prevented 
from  reaching  the  muscle — which,  as  has  been  said,  is  the  fact,  the  region  of  the 
anode  being  incapable  of  conducting  during  the  flow  of  a  strong  current,  and 
the  region  of  the  kathode  losing  its  power  to  conduct  at  the  instant  such 
a  current  is  opened. 

Effect-  of  Battery  Currents  upon  Normal  Human  Nerves. — In  experi- 
ments upon  normal  human  nerves,  the  current  cannot  be  applied  directly  to  the 
nerve,  but  has  to  be  applied  to  the  skin  over  the  nerve.  As  it  passes  from  the 
anode,  the  positive  electrode,  through  the  skin,  the  threads  of  current  spread 
through  the  fluids  and  tissues  beneath,  somewhat  as  the  bristles  of  a  brush 
spread  out,  and  the  current  flows  in  a  more  or  less  diffuse  stream  toward  the 
point  of  exit,  where  the  threads  of  current  concentrate  again  to  enter  the 
kathode,  the  negative  electrode.  This  spread  of  the  current  is  illustrated  in 
Figure  27. 


V  \) 

A   *—>  K 

FIG.  26.— Effect  of  direction  of  current 
as  shown  by  simultaneous  excitation  of 
two  nerve-muscle  preparations. 


52 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


FIG.  27.— Rough  schema  of  active  threads  of  current  by 
the  ordinary  application  of  electrodes  to  the  skin  over  a 
nerve  (ulnar  nerve  in  the  upper  arm).  The  inactive  threads 
are  given  in  dotted  lines  (after  Erb :  Ziemssen's  Pathologic  und 
Therapie,  Bd.  iii.  S.  76). 


The  density  of  the  current  entering  any  structure  beneath  the  skin  will 
depend  in  part  upon  the  size  of  the  electrode  directly  over  it — that  is,  the 

amount  to  which  the  current  is 
concentrated  at  its  point  of  en- 
trance or  exit — in  part  on  the 
nearness  of  the  structure  to  the 
skin,  and  in  part  on  the  con- 
ductivity of  the  tissues  of  the 
organ  in  question  as  compared 
with  the  tissues  and  fluids 
about  it.  If  the  conditions  be 
such  as  are  given  in  Figure  27, 
the  current  will  not,  as  in  the 
case  of  the  isolated  nerve,  enter 
the  nerve  at  a  given  point,  flow 
longitudinally  through  it,  and 
then  leave  it  at  a  given  point ; 
most  of  the  threads  of  current 
will  pass  at  varying  angles  di- 
agonally through  the  part  of 
the  nerve  beneath  the  positive 
pole,  then  flow  through  the  fluids  and  tissues  about  the  nerve,  until,  at  a  point 
beneath  the  negative  pole,  the  concentrating  threads  of  current  again  pass 
through  the  nerve.  A  distinction  is  to  be  drawn  between  the  physical  and 
physiological  anode  and  kathode.  The  physical  anode  is  the  extremity  of  the 
positive  electrode,  and  the  physical  kathode  is  the  extremity  of  the  negative 
electrode ;  the  physiological  anode  is  the  point  at  which  the  current  enters  the 
tissue  under  consideration,  and  the  physiological  kathode  is  the  point  where  it 
leaves  it.  There  is  a  physiological  anode  at  every  point  where  the  current 
enters  the  nerve,  and  a  physiological  kathode  at  every  point  where  it  leaves  the 
nerve;  therefore  there  is  a  physiological  anode  and  kathode,  or  groups  of 
anodes  and  kathodes,  for  the  part  of  the  nerve  beneath  the  positive  electrode, 
and  another  physiological  anode  and  kathode,  or  collection  of  anodes  and 
kathodes,  for  the  part  of  the  nerve  beneath  the  negative  electrode. 

To  understand  the  eifect  upon  the  normal  human  nerve  of  opening  and 
closing  the  battery  current,  it  is  necessary  to  bear  in  mind  three  facts,  viz.: 

1.  At  the  moment  that  a  battery  current  is  closed,  an  irritating  process  is 
developed  at  the  physiological  kathode,  and  when  it  is  opened,  at  the  physio- 
logical anode. 

2.  The  irritating  process  developed  at  the  kathode  on  the  closing  of  the 
current  is  stronger  than  that  developed  at  the  anode  on  the  opening  of  the 
current. 

3.  The  effect  of  the  current  is  greatest  where  its  density  is  greatest. 

The  amount  of  the  irritation  process  developed  in  a  motor  nerve  is  esti- 
mated from  the  amount  of  the  contraction  of  the  muscle.     The  contraction 


GENERAL   PHYSIOLOGY  OF  MUSCLE   AND   NERVE.       53 

which  results  from  closing  the  current,  the  closing  contraction  as  it  is  called, 
represents  the  irritating  change  which  occurs  at  the  physiological  kathode,  while 
the  contraction  which  results  from  opening  the  current,  the  opening  contrac- 
tion, represents  the  irritating  change  developed  at  the  physiological  anode. 
Since  there  are  physiological  anodes  and  kathodes  under  each  of  the  two  elec- 
trodes— the  physical  anode  and  physical  kathode  (see  Fig.  28) — four  possible 

may  arise,  namely: 
1.  Anodic  dosing  contraction — i.  e.  the  effect  of  the  change  developed  at 


FIG.  28.— Diagram  showing  physical  and  physiological  anodes  and  kathodes :  A,  the  physical  anode, 
or  positive  electrode ;  K,  the  physical  kathode,  or  negative  electrode ;  a,  a,  a,  physiological  anodes ;  k,  k,  k, 
physiological  kathodes. 

the  physiological   kathode,  the  place  where  the  current  leaves  the  nerve, 
beneath  the  physical  anode  (the  positive  pole). 

2.  Anodic  opening  contraction — i.  e.,  the  effect  of  the  change  developed  at 
the  physiological  anode,  where  the  current  enters  the  nerve,  beneath  the 
physical  anode  (the  positive  pole). 

3.  Kathodic  closing  contraction. — i.  e.  the  effect  of  the  change  developed  at 
the  physiological  kathode,  where  the  current  leaves  the  nerve,  beneath  the 
physical  kathode  (the  negative  pole). 

4.  Kathodic  opening  contraction — i.  e.,  the  effect  of  the  change  developed  at 
the  physiological  anode,  where  the  current  enters    the  nerve,  beneath  the 
physical  kathode  (the  negative  pole). 

For  convenience  these  four  cases  are  represented  by  the  abbreviations  ACC, 
AOC,  KCC,  and  KOC. 

Since  the  irritation  process  developed  at  a  physiological  kathode  by 
closing  a  current,  is,  other  things  being  equal,  stronger  than  that  developed 
at  a  physiological  anode  by  opening  the  current,  we  should  expect  that  the 
two  closing  contractions,  KCC  and  ACC,  would  be  stronger  than  the  two 
opening  contractions,  KOC  and  AOC.  This  is  the  case,  and  as  the  current  is 
more  dense  in  the  region  of  the  physiological  kathode,  beneath  the  physical 
kathode,  than  at  the  physiological  kathode,  beneath  the  physical  anode,  KCC 
is  stronger  than  ACC. 

Of  the  two  opening  contractions,  AOC  is  stronger  than  KOC  because 
of  the  greater  density  of  the  current  in  the  region  of  the  physiological  anode, 


54  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

beneath  the  physical  anode,  than  in  the  region  of  the  physiological  anode, 
beneath  the  physical  kathode. 

These  differences  in  the  strength  of  the  irritation  process  developed  in  these 
different  regions  is  well  shown  by  examining  the  reaction  of  nerves  to  cur- 
rents of  gradually  increasing  strength.  The  effect  of  the  opening  and  closing 
irritation  is  seen  to  be  as  follows  : 

Weak  currents.  Medium  currents.  Strong  currents. 

KCC  KCC  KCC 

ACC  AGO 

AOC  AOC 

KOC 

The  natural  order,  therefore,  would  be  KCC,  ACC,  AOC,  KOC.  Some- 
times, however,  AOC  is  stronger  than  ACC ;  this  happens  when  on  account 
of  the  relation  of  the  surrounding  tissues  to  the  nerve  the  density  of  the  cur- 
rent at  the  physiological  anode  is  great  as  compared  with  the  density  at  the 
physiological  kathode.  Borclier l  tested  the  strength  of  battery  current  neces- 
sary to  awaken  minimal  sensations  by  unipolar  excitations,  and  found  the 
effect  to  be  greatest  by  KC,  then  AC,  then  AO  ;  and  that  it  was  least  by  KO— 
i.  e.j  sensory  behave  like  motor  nerves. 

In  testing  the  effect  of  the  battery  current  on  the  nerves  and  muscles  of 
man,  it  is  customary  to  use  one  small  and  one  large  electrode  (Fig.  6,  d,  e,f). 
The  small  electrode  is  placed  over  the  part  to  be  stimulated,  while  the  large 
electrode  is  put  over  some  distant  portion  of  the  body.  This  arrangement 
causes  the  current  to  be  condensed,  and  hence  efficient,  when  it  enters  or 
leaves  the  small  exciting  electrode,  and  to  be  diffused,  and  hence  ineffective, 
at  the  large  indifferent  electrode.  For  example,  the  indifferent  electrode 
may  be  placed  on  the  sternum  or  over  the  back  of  the  neck,  while  the  excit- 
ing electrode  may  be  put  over  the  ulnar  nerve  at  the  elbow.  The  two  poles 
may  be  connected  with  the  battery,  a  pole-changer,  rheostat,  milliamperemeter, 
and  exciting-key  being  introduced  in  the  circuit.  The  pole-changer  permits 
the  exciting  pole  to  be  made  A  or  K  at  the  wish  of  the  operator,  the  rheostat 
allows  the  strength  of  current  to  be  raised  gradually,  and  the  milliampere- 
meter shows  the  strength  of  the  current  employed.  With  this  arrangement 
the  reaction  of  the  nerve  can  be  readily  tested. 

When  the  currents  employed  are  strong,  it  occasionally  happens  in  the 
case  of  men  that  not  only  are  the  make  and  break  followed  by  the  usual  rapid 
contractions  of  short  duration,  but  during  the  closure  of  the  current  there  is 
a  continued  contraction — galvanotonous,  as  it  is  sometimes  called.  This  is 
especially  seen  under  certain  pathological  conditions. 

When  the  nerve  or  muscle  is  diseased  we  may  have  the  above  order 
changed,  and  ACC  obtained  with  weaker  currents  than  KCC,  and  KOC  than 
AOC  (Babinski) 2.  This  is  known  as  the  reaction  of  degeneration.  Under 

1  Bordier  :  Archives  de  Physiologic  normals  et  Pathologique,  1897,  pp  543-553. 

2  Babinski  :   Comptes  rendus  de  la  Sociele  de  Bioloyie,  1899,  p.  343. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND   NERVE.       55 

such  circumstances  the  nerve  might  respond  well  to  the  direct  battery  current 
and  yet  fail  to  respond  to  the  induced  current.  This  would  be  still  more 
markedly  the  case  with  the  muscle,  which  at  the  same  time  that  it  gave  no 
response  to  induction  shocks  would  react  better  than  normally  to  battery 
currents.  At  such  times  galvanotonus  is  easily  excited.  Thus  during 
degeneration  the  irritability  of  the  nerve  and  muscle  approaches  that  of 
slowly  reacting  forms  of  protoplasm  (see  p.  70). 

Conditions  which  Determine  the  Irritability  of  Nerves  and  Muscles. 
— We  have  thus  far  considered  the  conditions  which  determine  the  efficiency 
of  such  an  irritant  as  the  electric  current.  Other  irritants  are  subject  to  like 
conditions,  their  activity  being  controlled  to  a  considerable  extent  by  the  sud- 
denness, strength,  density,  duration,  and,  possibly,  direction  of  application.  It 
is  not  necessary  for  us  to  consider  how  each  special  form  of  irritant  is  affected 
by  these  conditions ;  it  will  be  more  instructive  for  us  to  study  how  different 
irritants  alter  the  irritability  of  nerve  and  muscle,  and  the  relation  of  irri- 
tability to  the  state  of  excitation. 

The  power  to  irritate  is  intimately  connected  with  the  power  to  heighten 
irritability — for  a  condition  of  heightened  irritability  is  difficult  to  distin- 
guish from  a  state  of  excitation.  The  irritability  of  cell-protoplasm  is  very 
dependent  upon  its  physical  and  chemical  constitution,  and  even  slight  altera- 
tions of  this  constitution,  such  as  may  be  induced  by  various  irritants, 
will  modify  the  finely  adjusted  molecular  structure  upon  which  the  normal 
response  to  irritants  depends.  If  this  change  be  in  the  direction  of  increased 
irritability,  the  result  may  be  irritation.  But  we  must  defer  the  discussion  of 
the  relation  of  irritability  to  irritation  until  we  have  considered  the  conditions 
upon  which  the  irritability  of  nerve  and  muscle  depends.  These  conditions 
can  be  best  studied  in  connection  with  the  influences  which  modify  them — 
namely  : 

(a)  Irritants. 

(6)  Influences  which  favor  the  maintenance  of  the  normal  physiological 
condition. 

(c)  The  effects  of  functional  activity. 

(a)  The  Influence  of  Irritants  upon  the  Irritability  of  Nerve  and  Muscle. — 
Effect  of  Mechanical  Agencies. — A  sudden  blow,  pinch,  twitch,  or  cut  excites 
a  nerve  or  muscle.  All  have  experienced  the  effect  of  a  mechanical  stimulation 
of  a  sensory  nerve,  through  accidental  blows  on  the  ulnar  nerve  where  it  passes 
over  the  elbow,  "  the  crazy  bone."  The  amount  of  mechanical  energy  required 
to  cause  a  maximal  excitation  of  an  exposed  motor  nerve  of  a  frog  is  estimated 
by  Tigerstedt1  to  be  7000  to  8000  milligrammillimeters,  which  would  corre- 
spond roughly  to  a  weight  of  0.500  gram  falling  fifteen  millimeters — at  least 
a  hundred  times  less  energy  than  that  given  out  by  the  muscles  in  response  to 
the  nerve-impulse  developed.  Such  stimuli  can  be  repeated  a  great  many 
times,  if  not  given  at  too  short  intervals,  without  interfering  with  the  activity 

1  Studien  iiber  mechanische  Nervenreizung,"  Acta  Societatis  Scientiarum  Fennictz,  1880, 
Bd.  xi.  S.  32. 


56  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

of  the  nerve.  A  nerve  can  be  irritated  thirty  to  forty  times,  at  intervals  of 
three  to  four  minutes,  by  blows  from  a  weight  of  0.485  gram,  falling  1  to  20 
millimeters,  the  contractions  of  the  muscle,  weighted  with  30  to  50  grams, 
varying  from  minimal  to  from  3  to  4  millimeters  in  height.  Rapidly  following 
light  blows  or  twitches  applied  to  a  motor  nerve,  by  the  tetanomotor  of  Heiden- 
hain  or  Tigerstedt,  excite  a  series  of  contractions  in  the  corresponding  muscles 
which  fuse  more  or  less  into  a  form  of  continuous  contraction,  known  as 
tetanus. 

Not  only  may  a  nerve  be  excited  by  bringing  sudden  pressure  to  bear  on  it, 
but  the  sudden  removal  of  weights  or  a  sudden  lessening  of  tension  irritates.1 
Kiihne  long  ago  called  attention  to  the  excitation  of  sensory  fibres  of  the 
ulnar  nerve  of  man  on  the  removal  of  pressure.  The  cause  is  probably  the 
irregular  return  of  the  semi-fluid  parts  of  the  nerve  to  their  normal  relations. 

Mechanical  applications  to  nerve  and  muscle  first  increase  and  later  lessen 
and  destroy  the  irritability.  Thus  pressure  gradually  applied  first  increases 
and  later  reduces  the  power  to  respond  to  irritants.  Stretching  a  nerve  acts  in 
a  similar  way,  for  this  also  is  a  form  of  pressure ;  as  Valentin  said,  the  stretch- 
ing causes  the  outer  sheath  of  the  nerve  to  compress  the  myelin,  and  this  in 
turn  to  compress  the  axis-cylinder.  Tigerstedt  states  :2  "  From  a  tension  of 
0  up  to  20  grams  the  irritability  of  the  nerve  is  continually  increased,  but 
it  lessens  as  soon  as  the  weight  is  further  increased." 

Surgically  the  stretching  of  nerves  is  sometimes  employed  to  destroy  their 
excitability.  Slight  stretching  heightens  the  excitability  and  even  quite  vigor- 
ous stretching  has  only  a  temporary  depressing  effect  unless  it  be  carried  to 
the  point  of  doing  positive  injury  to  the  axis-cylinder,  and  of  causing  degen- 
eration. As  nerves  have  the  power  to  regenerate,  they  may  recover  from  even 
such  an  injury. 

The  irritability  of  muscles  is  likewise  increased  by  moderate  stretching  and 
destroyed  if  it  be  excessive.  Thus  slight  stretching  produced  by  a  weight 
causes  a  muscle  to  respond  more  vigorously  to  irritants.  Similarly  tension  of 
the  muscles  of  the  leg,  produced  by  slight  over-flexion  or  extension,  makes 
them  more  irritable  to  reflex  stimuli,  as  in  the  case  of  the  knee-jerk  and  ankle- 
clonus.  Tension  must  be  very  marked  to  permanently  alter  the  irritability  of 
the  muscles. 

Effect  of  Temperature. — Changes  in  temperature,  if  sudden  and  extreme, 
irritate  nferves  and  muscles.  If  the  nerve  or  muscle  be  quickly  frozen  or 
plunged  into  a  hot  fluid  it  will  be  excited  and  the  muscle  be  seen  to  contract 
The  cause  of  the  irritation  has  been  attributed  to  mechanical  or  chemical 
alterations  produced  by  the  change  of  temperature.  The  ulnar  nerve  at  the 
elbow  is  excited  if  the  part  be  dipped  into  ice-water  and  allowed  to  remain 
there  until  the  cold  has  had  time  to  penetrate ;  as  is  proved  by  the  fact  that  in 
addition  to  the  sensations  from  the  skin,  pain  is  felt  which  is  attributed  by  the 
subject  of  the  experiment  to  the  region  supplied  by  the  nerve.  As  the  effect 

1  v.  Uxhull:  Zeitschrift  fur  Biologic,  1894,  Bd.  xxxi.  S.  148  ;  1895,  Bd.  xxxii.  S.  438. 

2  Op.  tit.,  S.  43. 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND   NERVE.       57 

of  the  cold  becomes  greater  the  pain  is  replaced  by  numbness,  both  the  irrita- 
bility and  power  of  conduction  of  the  nerve  being  reduced.  Gradual  cooling 
of  motor  nerves  or  muscles,  and  gradual  heating,  even  to  the  point  of  death 
of  the  tissue,  fails  to  excite  contractions.  It  is  stated  that  if  a  frog  whose 
brain  has  been  destroyed  is  placed  in  a  bath  the  temperature  of  which  is  very 
gradually  increased,  the  heating  may  be  carried  so  far  as  to  boil  the  frog  without 
active  movements  having  been  called  out.  If  a  muscle  be  heated  to  45°  C. 
for  frogs  and  50°  C.  for  mammals,  it  undergoes  a  chemical  change,  which  is 
accompanied  by  a  form  of  shortening  different  from  the  contraction  induced  by 
irritants.  This  form  of  contraction,  though  extensive,  is  feeble  and  is  asso- 
ciated with  a  stiffening  of  the  muscle,  known  as  rigor  caloris  (see  p.  164). 

In  general  it  may  be  said  that  raising  the  temperature  above  the  usual  tem- 
perature of  the  animal  increases,  while  cooling  decreases,  the  irritability  of  the 
nerves  and  muscles.  This  statement  requires  to  be  amplified,  because  the 
character  of  the  stimulus  has  a  marked  effect  upon  the  result.  Cooling 
the  nerve  increases  its  irritability  for  mechanical  and  chemical  stimuli,  for 
the  constant  current  if  it  lasts  at  least  0.005  sec.,  for  condenser  discharges, 
and  for  sine  currents  of  at  least  0.005-0.01  sec.  duration  :  heating  the  nerve 
increases  its  irritability  for  these  forms  of  electrical  excitation  when  of 
shorter  duration,  and  also  for  induced  currents.1  If  a  nerve  be  excited  by 
charging  or  discharging  a  condenser  through  it,  the  size  of  the  condenser 
plays  an  important  part,  because  it  determines  the  duration  of  the  stimulus ; 
for  example  a  slow,  prolonged  rate  of  discharge  may  excite  a  nerve  at  4° 
C.  and  fail  to  excite  one  at  30°  C.,  while  a  rapid,  brief  fall  of  energy  will 
excite  a  nerve  at  30°  C.  and  fail  to  excite  one  at  4°  C.2  Not  only  does 
temperature  influence  the  ability  of  the  nerve  to  take  on  the  change  which  is 
associated  with  the  development  of  what  we  call  the  nerve  impulse,  but  it 
alters  its  power  of  recovery.  This  appears  in  experiments  in  which  the 
ability  of  the  nerve  to  respond  to  two  rapidly  following  stimuli  is  tested  by 
different  temperatures.  A  nerve,  like  the  heart-muscle,  shows  a  "refrac- 
tory period  "  for  a  short  interval  after  excitation,  and  during  this  period  it  is 
incapable  of  responding  to  stimulation.  The  length  of  the  interval  varies 
with  the  temperature.  If  the  two  stimuli  are  separated  by  an  interval  of 
0.001  sec.,  the  second  stimulus  will  be  effective  at  15°  C.,  but  it  will  fail  at 
3°  C. ;  at  this  temperature,  even  with  an  interval  of  0.006  sec.,  the  second 
stimulus  will  be  without  effect,  as  much  as  0.01-0.02  sec.  being  needed  for  the 
recovery  of  nerve  at  this  low  temperature.3 

Cold,  unless  excessive  and  long  continued,  though  it  temporarily  suspends, 
does  not  destroy  the  irritability ;  while  heat,  if  at  all  great,  so  alters  the 
chemical  constitution  of  the  cell-protoplasm  as  to  destroy  its  life. 

The  higher  the  temperature  the  more  rapid  the  chemical  changes  of  the 
body  and  the  less  its  power  of  resistance  ;  low  temperature,  on  the  other  hand, 

1  Gotch  and  Macdonald  :  Journal  of  Physiology,  1896,  xx.  p.  247. 

a  Waller:  Ibid.,  1899,  xxiv.  p.  1. 

3  Gotch  and  Burch:  Ibid.,  1899,  xxiii.  p.  22 ;  Boycott:  Ibiff.,  1899,  xxiv.  p.  144. 


58  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

slows  chemical  processes  and  increases  the  endurance.  It  is  noticeable  that 
nerves  and  muscles  remain  irritable  much  longer  than  ordinarily  in  case  the 
body  be  cooled  before  their  removal.  In  the  case  of  a  mammal,  the  irritability 
may  last  from  six  to  eight  hours  instead  of  two  and  a  half,  while  in  the  case 
of  frogs  it  may  be  preserved  at  0°  for  ten  days,  although  at  summer  heat  it  lasts 
only  twenty-four  hours.  In  the  case  of  frogs  which  have  been  kept  at  a  low 
temperature  the  irritability  becomes  abnormally  high  when  they  are  warmed 
to  ordinary  room-temperature. 

Effect  of  Chemicals  and  Drugs. — The  irritability  of  nerve  and  muscle  proto- 
plasm is  markedly  influenced  by  even  slight  changes  in  its  constitution.  If 
a  nerve  or  muscle  be  allowed  to  lie  in  a  liquid  of  a  different  composition  from 
its  own  fluid,  and  especially  if  such  a  liquid  be  injected  into  its  blood-vessels, 
an  interchange  of  materials  takes  place  which  results  in  an  alteration  of  the 
constitution  of  the  tissue  and  a  change  in  its  irritability.  Indeed,  the  only 
solutions  which  fail  to  alter  the  irritability  are  those  which  closely  resemble 
serum  and  lymph.  Fluids  having  other  than  the  normal  percentage  of  salts 
have  a  marked  effect,  while  even  the  absence  of  proteids  appears  to  have  little 
influence  unless  continued  for  a  considerable  time. 

Pure  water  acts  as  a  poison  to  protoplasm,  soon  destroying  its  life. 
Through  diffusion  and  osmosis  it  is  imbibed  into  the  cells  at  the  same  time 
that  the  salts  pass  out,  and  the  resulting  change  in  the  physical  and  chemical 
condition  of  the  tissue  cause  if  rapid,  first  an  increase,  and  in  any  case 
later  a  decrease,  and  finally  a  total  loss  of  irritability.  Thus  water  injected 
into  the  blood-vessels  of  muscles  first  excites  contraction  and  later  destroys 
the  irritability,  and  results  in  the  condition  known  as  water  rigor.  These 
effects  are  prevented  by  the  presence  of  small  amounts  of  salt.  A  sodium 
chloride  solution,  of  a  strength  of  6  parts  per  1000  of  distilled  water,  has 
been  called  the  physiological  solution,  because  it  was  supposed  to  have  no 
effect  on  the  irritability  of  nerves  and  muscles  of  cold-blooded  animals ;  even 
this  solution,  if  long  continued,  gradually  increases  and  later  decreases  the 
irritability.  A  solution  containing  7  parts  of  sodium  chloride  per  1000  is 
more  nearly  isotonic  to  the  fluids  of  cells  of  the  frog,  and  one  containing  9 
parts  per  1000  is  approximately  in  osmotic  equilibrium  with  the  fluids  of  the 
cells  of  the  mammal.  Such  fluids  cannot  be  properly  regarded  as  physio- 
logical solutions,  however,  for  this  would  mean  that  they  would  cause  no 
change  in  constitution  of  the  cells.  They  contain  only  one  of  the  salts  essen- 
tial to  the  normal  activity  of  the  tissues,  and  the  difference  in  the  partial 
pressure  of  the  other  salts  of  the  muscle  would  cause  the  muscle  cells  to  lose 
some  of  each  of  these,  and,  as  a  result,  to  have  their  irritability  altered.  The 
importance  of  the  individual  salts  present  in  the  fluids  normally  surrounding 
the  tissues,  and  the  need  that  they  should  be  present  in  definite  proportions, 
were  most  strikingly  demonstrated  by  experiments,  by  Ringer  and  others,  on 
the  nature  of  the  fluid  which  is  essential  to  the  maintenance  of  the  activity 
of  the  isolated  heart  of  the  frog.  These  experiments  have  shown  that  not 
only  Na,  but  Ca  and  K  are  essential.  The  heart  of  the  terrapin  can  be  kept 


GENERAL    PHYSIOLOGY   OF   MUSCLE   AND   NERVE.       59 

beating  for  more  than  forty-eight  hours  in  a  solution  containing  NaCl,  CaCl, 
and  KC1,  even  when  there  are  no  energy-giving  substances  present  in  the 
fluid.  Howell l  says  that  NaCl  is  needed  in  the  proportion  in  which  it  occurs 
in  the  blood,  to  preserve  the  osmotic  relations  of  the  tissues,  while  Ca  and  K 
are  essential  to  the  development  of  the  rhythmic  movements  of  the  heart- 
muscle.  Loeb 2  made  a  careful  study  of  the  relation  of  salts  of  the  blood  to 
the  activity  of  striated  and  non-striated  muscles.  He  found,  as  others  had 
done  before,  that  a  striated  muscle  if  left  in  0.7  per  cent.  NaCl  solution  in 
time  develops  more  or  less  rhythmic  automatic  contractions.  He  considers 
that  Na,  Ca,  and  K  are  held  in  the  muscle,  not  only  as  salts,  but  in  combi- 
nation with  the  proteids,  and  that  all  of  these  are  necessary  to  the  normal 
functional  activity  of  the  protoplasm.  A  fluid  to  deserve  the  name  of  physio- 
logical must  contain  all  these  ions.  The  muscle  contracts  rhythmically  in  a 
solution  of  pure  NaCl  because  the  Na  drives  some  of  the  Ca  and  K  out  of 
their  ion-proteid  combinations.  If  Ca  and  K  are  present  in  the  solution, 
this  cannot  occur.  He  goes  so  far  as  to  say  that  were  it  not  for  the  Ca  and 
K  in  the  blood,  the  human  skeletal  muscles  would  show  rhythmic  contraction. 

A  truly  physiological  solution  would  contain  all  the  constituents  of  the 
fluid  of  the  blood,  and  a  physiological  salt  solution  would  contain  all  the  salts 
of  the  blood,  in  the  proportion  in  which  they  exist  in  the  blood.  The  salts 
would  appear  to  have  a  twofold  function  :  they  would  maintain  the  normal 
imbibition  relations  of  the  cells,  and  they  would  supply  the  Na,  Ca,  and  K 
ions  which  are  required  for  the  ion-proteid  compounds  in  the  muscle.  The 
quantities  of  inorganic  salts  are  different  in  different  tissues  of  the  same 
animal,  which  shows  that  the  presence  of  these  inorganic  substances  is 
dependent  not  merely  on  the  amount  presented  to  them  by  the  fluids  in 
which  they  are  bathed,  but  also  on  the  chemical  conditions  within  the  cells, 
each  type  of  cell  requiring  a  definite  supply  for  its  normal  functional  activity. 
Howell  reports  a  fact  of  interest  in  this  connection  :  the  muscle  of  the  ven- 
tricle of  the  heart  of  the  terrapin  does  not  make  automatic  rhythmic  move- 
ments in  a  Ringer's  solution  containing  Na,  Ca,  and  K  in  amounts  equal  to 
those  occurring  in  the  blood,  but  the  large  veins  at  the  base  of  the  heart  do 
make  such  contractions  and  supply  the  excitation  necessary  to  rhythmic  con- 
traction of  the  whole  heart. 

The  presence  of  inorganic  salts  is  essential  to  the  normal  functional 
activity  of  nerves,  as  it  is  of  muscles.  If  the  nerve  be  subjected  to  distilled 
water,  it  gradually  loses  its  salts  through  osmosis,  and  imbibes  water,  and 
the  resulting  chemical  and  physical  change  in  its  constitution  is  accompanied 
by  a  loss  of  irritability.  Likewise  the  withdrawal  of  water  from  a  motor 
nerve  by  drying,  or  by  strong  solutions  of  urea,  glycerin,  etc.,  causes  a  change 
of  irritability.  The  irritability  is  first  increased,  due  to  a  concentration  of 
the  salts  within  the  nerve  and  to  the  mechanical  excitation  resulting  from 
the  shrinkage  of  the  tissue.  If  the  change  is  a  rapid  one,  it  is  frequently 
accompanied  by  an  active  irritation,  and  the  muscle  connected  with  the  nerve 

1  American  Journal  of  Physiology,  1898,  ii.  p.  47.  z  Ibid.,  1900,  iii.  p.  383. 


60  AN   AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

shows  irregular  contractions,  as  the  different  fibres  of  the  nerve  are  one  after 
the  other  affected.  If  the  drying  has  not  been  continued  too  long,  the  normal 
irritability  may  be  restored  by  supplying  water.  Muscles  behave  like  nerves 
in  these  respects. 

Most  drugs  and  chemicals  capable  of  altering  the  irritability  of  nerves  and 
muscles  first  increase  and  later  destroy  the  irritability.  If  the  change  in  the 
chemical  constitution  of  the  nerve  is  sufficiently  rapid,  it  may  be  accompanied 
by  the  phenomena  of  excitation.  For  example,  veratria,  eserin,  digitalis, 
most  mineral  acids,  and  many  organic  acids,  free  alkalies,  most  salts  of  heavy 
metals,  destroy  the  irritability  of  nerves  and  muscles,  as  a  rule  after  first  pro- 
ducing increased  excitability.  Potash  salts,  if  concentrated,  rapidly  kill,  but 
excite  less  than  soda  compounds.  Verworn  says :  Acids,  alkalies,  and  salts 
have  a  similar  effect  on  the  protoplasm  of  a  thick  pseudopod  of  one  of  the 
rhizopods  of  the  Red  Sea ;  they  first  excite  and  later  paralyze,  acting  like 
narcotics  on  the  central  nervous  system. 

Ammonia,  carbon  disulphide,  and  ethereal  oils  may  destroy  the  irritability 
of  nerves  without  causing  excitations,  at  least  not  in  sufficient  amount  to 
produce  visible  muscular  contractions.  If  applied  directly  to  the  muscle, 
however,  these  substances  excite  contractions. 

The  attempt  to  ascertain  some  exact  relation  between  the  molecular 
weight  of  different  salts  and  acids  and  their  destructive  power  has  encoun- 
tered too  many  exceptions  for  the  establishment  of  any  definite  rule ;  in 
general,  however,  the  higher  the  molecular  weight  the  stronger  the  effect  on 
the  muscle.1  Many  gases  and  vapors  have  a  marked  effect  on  the  irri- 
tability and  activity  of  protoplasm.55  Carbonic-acid  gas,  tobacco-smoke,  the 
fumes  of  ether,  alcohol,  and  chloroform,  applied  directly  to  exposed  nerves, 
first  stimulate,  later  anaesthetize,  and  finally  kill.  CO2  has  a  very  powerful 
effect,  even  a  fiftieth  of  a  milligram  sufficing  to  influence  profoundly  the 
activity  of  the  protoplasm  of  the  nerve,  a  fact  of  considerable  importance  if 
we  recall  that  this  gas  is  produced  by  the  normal  oxidation  of  carbon  within 
the  tissues  of  the  body.  Tobacco-smoke  acts  like  CO2  and  probably  because 
of  the  CO2  which  it  contains.  Alcohol  first  excites  and  then  paralyzes  the 
nerve.  If  the  fumes  of  alcohol  have  not  acted  for  too  long  a  time,  the  para- 
lyzed nerve  may  recover  its  function,  and  the  same  is  true  for  ether  and 
chloroform.  These  vapors,  if  present  in  considerable  quantities  act  rapidly 
upon  exposed  nerves ;  thus  ether  (diethyl  oxide)  will  anaesthetize  a  nerve  in 
three  minutes  ;  if  the  drug  be  then  removed,  the  nerve  can  completely  recover 
in  five  minutes.  Chloroform  would  appear  to  be  a  more  dangerous  anaes- 
thetic than  ether,  as  recovery  of  the  nerve  is  less  likely  to  occur  in  case  the 
anaesthetic  action  is  somewhat  prolonged.  Many  other  gases  and  fumes 
chemically  irritate  and  kill  nerve-muscle  protoplasm. 

From  all  these  results  it  becomes  evident  that  the  normal  irritability  of 

»  Bluraenthal :  Pfluger's  Archiv,  1896,  Bd.  62,  S.  513. 

2  Waller:  Lectures  on  Physiology,  first  series,  "  On  Animal  Electricity,"  London,  1897,  pp. 
42-46. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.       61 

nerves  and  muscles  requires  that  a  certain  chemical  constitution  be  maintained, 
and  that  even  slight  variations  from  this  suffice  to  alter,  and,  if  continued,  to 
destroy,  the  irritability.  Further,  it  is  noticeable  that  in  most  cases  the  first 
step  toward  deterioration  is  a  rise  of  irritability,  which,  if  sudden  and  marked, 
is  accompanied  by  a  condition  of  irritation.  If  the  cause  of  the  increase  in 
irritability  and  excitation  be  continued,  sooner  or  later  exhaustion  supervenes, 
the  irritability  lessens,  and  finally  is  lost. 

Effect  of  the  Electric  Current  upon  Muscles. — If  a  constant-battery  current 
of  medium  strength  be  sent  through  a  muscle  for  a  short  time,  the  muscle  will 
give  a  single  short  contraction  at  the  moment  that  the  current  enters  it,  and 
again  when  the  current  leaves  it.  If  a  strong  current  be  used,  the  short 
closing  contraction  may  be  followed  by  a  prolonged  contraction  (Wundt's  closing 
tetanus),  which,  though  gradually  decreasing,  may  last  as  long  as  the  current 
is  closed ;  and  when  the  current  is  broken,  the  usual  opening  contraction  may 
be  likewise  followed  by  a  prolonged  contraction  (Hitter's  opening  tetanus), 
which  only  gradually  passes  off.  The  closing  contraction  originates  at,  and 
the  closing  continued  contraction  may  be  limited  to,  the  region  of  the  kathode; 
and  the  opening  contraction  originates  at,  and  the  opening  continued  contrac- 
tion may  be  limited  to,  the  region  of  the  anode. 

In  case  a  very  weak  current  is  used,  no  contraction  will  be  observed ; 
nevertheless,  while  the  current  is  flowing  through  the  muscle  it  modifies  its 
condition  ;  a  state  of  latent  excitation  is  produced  at  the  kathode,  which  shows 
itself  in  a  considerable  increase  of  irritability  of  that  part  of  the  muscle.  On 
the  other  hand,  the  irritability  of  the  muscle  at  the  kathode  will  be  found  to 
be  lessened  after  the  withdrawal  of  the  polarizing  current,  because  the  condi- 
tion of  excitation  which  it  causes  fatigues  that  part  of  the  muscle. 

The  effects  of  the  battery  current  at  the  region  of  the  anode  are  just  oppo- 
site to  those  produced  at  the  kathode.  While  the  current  is  flowing,  the  irri- 
tability at  the  anode  is  lessened,  and  when  the  polarizing  current  is  removed, 
irritability  at  the  anode  is  found  to  be  greater  than  it  was  before  the  battery 
current  was  applied. 

The  lessened  irritability  which  is  produced  at  the  anode  during  the  flow 
of  the  battery  current  may  be  shown  by  an  inhibition  of  a  condition  of  exci- 
tation which  may  be  present  at  the  time  that  the  current  is  applied  to  the 
muscle.  For  example,  in  the  case  of  unstriated  muscles,  not  only  does  closing 
the  battery  circuit  never  cause  a  contraction  at  the  anode,  but  if  the  part  of 
the  muscle  exposed  to  the  influence  of  the  anode  happens  to  be  at  the  time  in 
a  condition  of  tonic  contraction,  the  entrance  of  the  current  causes  that  part 
of  the  muscle  to  relax.  The  inhibitory  influence  exerted  by  the  anode,  as  a 
result  of  the  lowering  of  the  irritability,  is  seen  to  a  remarkable  degree  in  its 
effect  upon  the  heart.1  If  the  anode  rest  on  the  ventricle  of  the  frog's  heart, 
and  the  kathode  at  some  indifferent  point,  relaxation  is  seen  in  the  region  of  the 
anode  with  each  systole  of  the  ventricle.  Inasmuch  as  the  rest  of  the  ventricle 
contracts,  the  pressure  of  the  blood  causes  the  wall  of  the  ventricle  to  bulge 
1  Biedermann:  Elektrophyswlogie,  1895,  S.  195. 


62  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

out,  and  make  a  little  vesicle  at  the  region  of  the  anode.  A  similar  inhibitory 
influence  may  be  observed  upon  an  ordinary  striated  muscle  at  the  point  of 
application  of  the  anode,  if  it  be  in  a  condition  of  tonic  contraction  when  the 
battery  current  is  sent  into  it.  During  the  flow  of  the  constant  current  through 
a  muscle,  the  irritability  is  increased  in  the  region  of  the  kathode  and  decreased 
in  the  region  of  the  anode.  When  the  current  is  withdrawn  from  the  muscle, 
on  the  other  hand,  the  irritability  of  the  kathode  is  found  to  be  decreased,  and 
at  the  anode  to  be  increased. 

Effect  of  the  Electric  Current  upon  Nerves. — The  polarizing  effects  of  a  con- 
tinuous constant  current  are  the  same  upon  a  nerve  as  upon  a  muscle,  with  the 
exception  that  in  the  case  of  the  nerve  the  condition  of  altered  irritability  is 
not  so  strictly  limited  to  the  point  of  application  of  the  anode  and  kathode,  but 
spreads  thence  throughout  the  part  of  the  nerve  between  the  two  electrodes,  the 
intrapolar  region,  as  it  is  called,  and  for  a  considerable  distance  into  the  parts 
of  the  nerve  through  which  the  current  does  not  flow,  i.  e.  the  extrapolar  region. 
The  term  electrotonus  has  been  applied  to  the  effects  of  battery  currents  on 
nerves  and  muscles,  and  includes  two  sets  of  changes — (1)  manifested  by  the 
alterations  of  irritability  which  we  are  considering ;  (2)  exhibited  in  changes 
of  the  electrical  condition  of  the  tissue. 

There  can  be  little  doubt  that  both  of  these  sets  of  changes  are  the  result 
of  electrolytic  alterations  of  the  nerve  protoplasm,  caused  by  the  flow  of  the 
polarizing  current.  We  shall  consider  here  only  the  former  of  these  sets  of 
changes.  The  true  nature  of  the  electrotonic  changes  of  the  electrical  condi- 
tion of  the  nerve,  and  their  relation  to  the  nerve  impulse,  embrace  a  number 
of  difficult  problems,  which  are  still  under  discussion  and  cannot  be  profit- 
ably considered  here.1 

The  most  important  work  on  the  influence  of  the  constant  current  on  the 
irritability  of  nerves  was  done  by  Pfliiger.2  He  ascertained  the  electrotonic 
effects  of  the  polarizing  current  to  be  most  vigorous  in  the  immediate  vicinity 
of  the  anode  and  kathode,  and  to  spread  thence  in  both  directions  along  the 
nerve.  He  called  the  change  produced  in  the  nerve  in  the  region  of  the 
anode  "  anelectrotonic,"  and  the  condition  itself  "  anelectrotonus ;"  while  the 
change  at  the  kathode  was  termed  "  katelectrotouic,"  and  the  condition 
"  katelectrotonus."  The  same  names  are  given  to  the  effects  of  battery  cur- 
rents upon  muscles. 

To  test  the  effect  of  a  constant  battery  current  upon  the  irritability  of  a 
nerve,  put  the  nerve  of  a  nerve-muscle  preparation  upon  two  non-polarizable 
electrodes  (A,  K,  Fig.  29)  which  are  placed  at  some  little  distance  apart  and 
at  a  considerable  distance  from  the  muscle.  Connect  these  electrodes  with  a 
battery,  introducing  into  the  circuit  a  key  (&),  which  permits  the  current  to 
be  quickly  thrown  into  or  removed  from  the  nerve,  and  a  commutator  ((7), 
which  allows  the  current  to  be  reversed  and  to  be  sent  through  the  nerve  in 

1  Waller:  Lectures  on  Animal  Electricity,  London,  1897;  Biedermann:  Electrophysiology, 
translated  by  F.  A.  Welby,  1898,  vol.  ii. 

a  Pfliiger :    Untersuchungen  iiber  die  Physiologie  des  Electrotonus,  Berlin,  1859. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.        63 

either  the  ascending  or  descending  direction.  ( \nimvt  the  muscle  with  a  myo- 
graph  lever,  arranged  so  as  to  record  the  height  of  the  muscle  contractions. 
Then  apply  to  the  nerve  at  some  point  between  the  polarizing  electrodes  and 
the  muscle  a  pair  of  electrodes  (/)  connected  with  the  secondary  coil  of  an 
induction  apparatus,  which  is  placed  near  enough  to  the  primary  coil  to  cause 
excitations  of  medium  strength,  and  introduce  into  the  secondary  circuit  a 
short-circuiting  key  (S\  by  which  the  closing  shocks  can  be  prevented  from 
reaching  the  nerve. 

If,  with  this  arrangement,  a  breaking  induction  shock  of  medium  strength 
be  given,  the  nerve  will  be  excited,  and  the  height  of  the  muscular  contraction 
which  results  may  be  taken  as  a  test  of  the  irritability  of  the  nerve  at  /. 


Pis.  29, —Method  of  testing  anelectrotonic  and  katelectrotonic  alterations  of  irritability  in  nerves. 

Now  send  the  polarizing  current  through  the  nerve,  in  the  ascending  direction, 
that  is,  with  the  anode  nearer  the  muscle.  At  the  moment  the  current  is 
closed,  if  it  be  of  medium  strength,  a  closing  contraction  will  be  observed ; 
then  comes  a  period  during  which  the  muscle  is  not  contracting  and  the  polar- 
izing current  is  apparently  producing  no  effect  on  the  nerve ;  if,  however,  after 
the  current  has  acted  a  short  time,  the  irritability  of  the  nerve  at  the  point 
/  be  again  tested  with  a  breaking  induction  shock,  it  will  be  found  to  be  de- 
creased, on  account  of  the  condition  of  anelectrotonus  which  has  been  induced. 
If  the  key  in  the  polarizing  current  be  then  opened,  the  usual  opening  con- 
traction will  be  recorded.  After  the  polarizing  current  has  been  removed,  the 
condition  of  the  nerve  at  I  can  be  again  tested,  and  it  will  be  seen  that  the 
irritability  has  returned  to  the  normal,  or  is  even  greater  than  it  was  at  the 
start. 


64  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

The  effect  of  the  kathode  on  the  irritability  may  be  tested  in  a  similar  way, 
by  reversing  the  polarizing  current  and  again  sending  it  into  the  nerve.  This 
time  the  current  will  be  descending,  i.  e.  the  kathode  nearest  the  muscle.  As 
before,  a  closing  contraction  will  be  seen  when  the  circuit  is  made,  but  on  test- 
ing the  irritability  at  /with  an  induction  shock  of  the  same  strength  as  before, 
it  will  be  found  to  be  increased,  the  shock  causing  a  larger  contraction.  On 
opening  the  polarizing  current  the  usual  opening  contraction  will  be  seen,  and 
if  after  the  current  has  been  removed  the  irritability  be  again  tested,  it  will 
be  found  to  have  returned  to  the  normal,  or  to  be  decreased.  The  changes 
in  irritability  described  can  be  ascertained  by  using  mechanical  or  chemical 
stimuli  as  well  as  induction  shocks.  Alterations  of  the  irritability  induced  by 
anelectrotonic  and  katelectrotonic  changes  of  the  nerve-substance  are  to  be 
found  not  only  in  the  part  of  the  nerve  between  the  point  to  which  the  polar- 
izing current  is  applied  and  the  muscle,  but  in  the  extrapolar  region  at  the 
central  end  of  the  nerve,  and  in  the  intrapolar  region.  The  experimental 
evidence  of  this  is  not  so  readily  obtained,  but  there  is  no  doubt  of  the  fact. 

The  effect  of  the  polarizing  current  is  the  greater,  the  better  the  condition 
of  the  nerve ;  moreover,  the  stronger  the  current  employed,  the  more  of  the 
nerve  influenced  by  it.  Of  course,  in  the  intrapolar  region  there  is  a  point 
where  the  effect  of  the  anode  to  decrease  the  irritability  comes  into  conflict  with 
the  effect  of  the  kathode  to  increase  it,  and  where,  in  consequence,  the  irrita- 
bility remains  unchanged.  This  indifferent  point  may  be  observed  to  approach 
the  kathode  as  the  strength  of  the  current  is  increased.  The  following  schema 
is  given  by  Pfluger  to  illustrate  the  way  in  which  the  irritability  is  changed  in 
the  anelectrotonic  and  katelectrotonic  regions  as  the  strength  of  the  current  is 
increased : 


y» 


FIG.  30.— Electrotonic  alterations  of  irritability  caused  by  weak,  medium,  and  strong  battery 
currents :  A  and  B  indicate  the  points  of  application  of  the  electrodes  to  the  nerve,  A  being  the  anode, 
E  the  kathode.  The  horizontal  line  represents  the  nerve  at  normal  irritability  ;  the  curved  lines  illus- 
trate how  the  irritability  is  altered  at  different  parts  of  the  nerve  with  currents  of  different  strengths. 
Curve  yl  shows  the  effect  of  a  weak  current,  the  part  below  the  line  indicating  decreased,  and  that  above 
the  line  increased  irritability,  at  a:1  the  curve  crosses  the  line,  this  being  the  indifferent  point  at  which 
the  katelectrotonic  effects  are  compensated  for  by  anelectrotonic  effects ;  7/2  gives  the  effect  of  a  stronger 
current,  and  y3,  of  a  still  stronger  current.  As  the  strength  of  the  ciirrent  is  increased  the  effect  becomes 
greater  and  extends  farther  into  the  extrapolar  regions.  In  the  intrapolar  region  the  indifferent  point  is 
seen  to  advance  with  increasing  strengths  of  current  from  the  anode  toward  the  kathode. 

As  in  the  case  of  the  muscle,  so  of  the  nerve,  the  constant  current  leaves 
behind  it  important  after-effects.  In  general  it  may  be  stated  that  wherever 
during  the  flow  of  the  current  the  irritability  is  increased,  there  is  a  decrease 


GENERAL    PHYSIOLOGY    OF    MUSCLE   AND    NERVE. 


of  irritability  immediately  after  the  removal  of  the  current,  and   r'n-c 
When  the  current  is  withdrawn  from  the  nerve,  the  irritability  in  the  reo-io;- 
of  the  kathode  is  lowered,  and  in  the  region  of  the  anode  raised.     It  musi 
added,  however,  that  the  decrease  of  irritability  soon  at  the  kathode  gradual 
passes  over  into  a  second  increase  of  irritability,  while  the  increase  seen  at  the 
anode  upon  the  removal  of  the  current  continues  a  considerable  time  and  is 
not  reconverted  to  a  decrease  ;    therefore  the  total  after-effect  is  an  increase 
of  irritability.     The  effect  of  the  battery  current  to  alter  the  irritability  of 
the  human  nerve  can  be  made  out  by  the  following  experiment.1     Test  the 
irritability  of  the   nerve  by  giving  it  a  series  of  light  blows  through  the 
medium  of  the  electrode  itself,  which  is  made  anodic  or  kathodic.     The  elec- 
trode is  pressed  carefully  upon  the  nerve,  and  is  regularly  tapped  by  a  light 
mallet  just  hard  enough  to  give  distinct  twitches  of  the  fingers;  if  while  this 
is  going  on  the  electrode  is  made  kathodic,  the  twitches  become  stronger, 
and  if  it  be  made  anodic  they  are  abolished. 

The  fact  that  when  the  current  is  closed  the  irritation  starts  from  the  kathode, 
and  when  the  current  is  opened  from  the  anode,  may  well  be  associated  with  the 
changes  in  irritability  which  take  place  at  the  kathode  and  anode  upon  the  closing 
and  the  opening  of  the  current.  The  setting  free  of  an  irritation  appears  to  be 
associated  only  with  an  increase  of  irritability.  When  the  current  is  closed  the 
establishment  of  the  condition  of  katelectrotonus  is  accompanied  by  a  rise  of 
irritability  at  the  kathode,  and  when  the  current  is  opened  the  cessation  of  the 
condition  of  anelectrotonus  is  likewise  accompanied  by  a  rise  of  irritability.  In 
the  first  case  the  irritability  rises  from  the  normal  to  something  above  the 
normal,  and  in  the  second  case  thejirritability  rises  from  the  condition  of 
decreased  irritability  up  to  somethingpbove  the  normal  irritability.  The  change 
from  the  normal  to  the  anelectrotouic  condition  of  decreased  irritability,  or 
from  the  katelectrotonic  condition  of  increased  irritability  down  to  normal 
irritability,  does  not  irritate.  As  has  often  been  said,  it  is  hard  to  distinguish 
between  increase  of  irritability  and  irritation. 

Effect  of  Frequency  of  Application  of  the  Stimulus  on  Irritability.  —  We  have 
seen  that  influences  which  act  as  irritants  may  also  have  an  effect  upon  the  irri- 
tability of  the  nerve  or  muscle.  In  order  to  produce  this  change  they  must  be 
as  a  rule  powerful,  or  act  for  a  considerable  time.  Nevertheless,  in  the  case 
of  muscles,  at  least,  even  a  weak  irritant  of  short  duration,  if  repeated  fre- 
quently, tends  to  heighten  irritability.  For  example,  if  a  muscle  be  stimulated 
by  separate  weak  induction  shocks  at  long  intervals,  the  effect  of  each  shock  is 
slight,  and  the  change  produced  by  it  is  compensated  for  by  restorative  pro- 
cesses which  occur  within  the  living  protoplasm  during  the  following  interval 
of  rest,  and  each  of  the  succeeding  irritations  finds  the  mechanism  in  much  the 
same  condition  ;  if,  however,  the  shocks  follow  each  other  rapidly,  each  stimu- 
lation leaves  an  after-effect  which  may  have  an  influence  upon  the  effectiveness 
of  the  stimulus  following  it.  As  a  result  of  this,  induction  shocks  too  feeble  to 
excite  contractions  may,  if  frequently  repeated,  after  a  "Tittle  time  cause  a  visible 

1  Waller  and  de  Watteville:  Philosophical  Transactions  of  the  Royal  Society,  1882. 
VOL.  II.—  5 


66  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

movement,  and  shocks  of  medium  strength,  if  given  at  short  intervals,  may 
each  cause  a  larger  contraction  than  its  predecessor,  until  a  certain  height  of 
contraction  has  been  reached,  beyond  which  there  is  no  further  increase  pos- 
sible. We  shall  consider  these  so-called  "  staircase  contractions  "  more  care- 
fully later  (see  page  112).  When  irritations  follow  each  other  very  rapidly 
the  whole  character  of  the  contraction  is  changed,  and  the  muscle,  instead  of 
making  rapid  single  contractions,  enters  into  the  condition  of  apparently  con- 
tinuous contraction  known  as  tetanus,  during  which  it  shortens  considerably 
more  than  it  does  when  making  single  contractions.  Increase  in  irritability 
plays  only  a  comparatively  small  part  in  the  production  of  this  remarkable 
phenomenon,  which  we  shall  study  more  carefully  when  we  come  to  the 
mechanical  problems  involved  in  muscular  contractions. 

Rapidly  repeated  stimuli,  though  at  first  favorable  to  activity  of  a  muscle, 
soon  exert  an  unfavorable  influence  by  causing  the  lessened  irritability  which 
is  associated  with  fatigue. 

When  a  nerve  is  excited  there  is  a  change  in  its  electrical  condition,  and 
the  extent  of  the  change  is  generally  believed  to  be  an  indication  of  the 
extent  to  which  the  protoplasm  of  the  nerve  has  become  active  in  response 
to  excitation.  Waller,1  taking  the  amount  of  change  in  the  electrical 
condition  of  the  nerve  as  an  evidence  of  the  ability  of  the  protoplasm  to 
react  under  varying  conditions,  found  that  repeated  excitation  increases  the 
activity  of  the  nerve  as  it  does  of  the  muscle.  Repeated  excitation  of  a 
nerve  at  suitable,  regular  intervals  causes  a  staircase-like  increase  in  the 
strength  of  the  electrical  response,  the  record  resembling  that  got  by  stair- 
case contractions  of  muscles  (see  page  112).  Moreover,  if  the  electrical  con- 
dition of  the  nerve  is  tested  by  a  series  of  excitations  of  equal  strength 
before  and  after  it  is  subjected  to  a  tetanizing  current,  the  strength  of  the 
variations  is  found  to  be  increased. 

If  a  second  stimulus  follows  the  first  too  soon,  it  may  be  wholly  ineffec- 
tive ;  at  least  this  has  been  found  to  be  the  case  with  certain  forms  of  proto- 
plasm. It  has  been  shown  that  heart  muscle  has  a  "  refractory  period/7  as  it 
is  called,  responding  very  imperfectly  to  stimuli  applied  to  it  just  before 
and  during  its  systole.2  Apparently  much  the  same  is  true  of  the  nerve. 
Boycott,3  using  contraction  of  muscle  as  a  test,  and  Gotch  and  Burch,4  using 
the  current  of  action  as  a  test,  have  lately  discovered  that  for  a  brief  period 
after  the  nerve  has  been  stimulated  it  is  incapable  of  responding  to  a  second 
stimulus.  The  length  of  the  period  of  lessened  excitability  is  greatly  influ- 
enced by  temperature;  at  4°  C.,  with  maximal  stimuli,  the  "critical  period " 
may  be  0.007-0.008  second ;  at  higher  temperatures  it  is  shorter. 

(b)  Influences  which  favor  the  maintenance  of  the  Normal  Physiological 
Condition  of  Nerve  and  Muscle. — Effect  of  Blood-supply  on  Nerve  and  Muscle. 
— The  vascular  system  is  a  path  of  communication  between  the  several  organs 

1  Waller :  Lectures  on  Physiology,  first  series,  1897,  p.  68. 

2  Gushing :  Journal  of  Physiology,  1897,  vol.  xxi.  p.  214. 

8  Boycott :  Ibid.,  1899,  vol.  xxiv.  p.  144.  *  Gotch  and  Burch:  Ibid.,  p.  410. 


GENERAL    PHYSIOLOGY    OF  MUSCLE  AND    NERVE.       (5, 

and  tissues,  and  the  circulating  blood  is  a  medium  of  exchange.  The  blood 
carries  nutritive  materials  from  the  digestive  organs  and  oxygen  from  the 
lungs  to  all  the  tissues  of  the  body,  and  it  transports  the  waste  materials  which 
the  cells  give  off  to  the  excretory  organs.  In  addition  to  these  functions  it 
has  the  power  to  neutralize  the  acids  which  are  produced  by  the  cells  during 
action,  and  so  maintain  the  alkalinity  essential  to  the  life  of  the  cell ;  it  sup- 
plies all  parts  with  moisture ;  by  virtue  of  the  salts  which  it  contains,  it  secures 
the  imbibition  relations  which  are  necessary  to  the  preservation  of  the  normal 
chemical  constitution  of  the  cell-protoplasm ;  it  distributes  the  heat,  and  so 
equalizes  the  temperature  of  the  body ;  finally,  in  addition  to  these  and  other 
similar  functions,  it  is  itself  the  seat  of  important  chemical  changes,  in  which 
the  living  cells  which  it  contains  play  an  active  part.  It  is  not  strange  that 
such  a  fluid  should  exert  a  marked  influence  upon  the  irritability  of  the  nerves 
and  muscles.  Since  the  metabolism  of  muscles  is  best  understood,  we  will 
first  consider  the  importance  of  the  circulation  to  the  muscle.  Muscles,  even 
in  the  so-called  state  of  rest,  are  the  seat  of  chemical  changes  by  which  energy 
is  liberated,  and  when  they  are  active  these  changes  may  be  very  extensive. 
If  the  cell  is  to  continue  its  work,  it  must  be  at  all  times  in  receipt  of  mate- 
rials to  replenish  the  continually  lessening  store  of  energy-holding  compounds; 
moreover,  as  the  setting  free  of  energy  is  largely  a  process  of  oxidation,  a  free 
supply  of  oxygen  is  likewise  indispensable  to  action.  These  oxidation  pro- 
cesses result  in  the  formation  of  waste  products — such  as  carbon  dioxide,  water, 
lactic  acid — and  these  are  injurious  to  the  muscle  protoplasm,  and  if  allowed 
to  accumulate  would  finally  kill  it.  Of  the  services  which  the  blood  renders 
to  the  muscle  there  are,  therefore,  two  of  paramount  importance,  viz.  the 
bringing  of  nutriment  and  oxygen  and  the  removal  of  waste  matter,  and  sur- 
plus energy,  as  heat. 

A  classical  experiment  illustrating  the  effect  of  depriving  tissues  of  blood 
is  that  of  Stenson,  which  consists  in  the  closure  of  the  abdominal  aorta  of  a 
warm-blooded  animal  by  a  ligature,  or  by  compression.  In  the  case  of  a 
rabbit,  for  example,  the  blood  is  shut  off,  not  only  from  the  limbs  but  from  the 
lower  part  of  the  spinal  cord.  The  effect  is  soon  manifested  in  a  complete 
paralysis  of  the  lower  extremities,  sensation  as  well  as  power  of  voluntary  and 
reflex  movements  being  lost.  The  paralysis  is  due,  in  the  first  instance,  to  the 
loss  of  function  of  the  nerve-cells  in  the  cord  by  which  the  muscles  are  nor- 
mally excited  to  action.  Later,  however,  the  nerves  and  muscles  of  the  limbs 
lose  their  irritability.  Of  the  peripheral  mechanisms  the  motor  nerve-ends 
are  found  to  succumb  before  the  nerves  and  muscles.  This  is  shown  by  the 
fact  that  although  the  muscles  are  still  capable  of  responding  to  direct  irrita- 
tion, they  are  not  affected  by  stimuli  applied  to  the  nerve,  although  the  nerve 
at  the  time,  to  judge  from  electrical  changes  which  occur  when  it  is  excited, 
is  still  irritable.  Since  the  nerve  and  muscle  are  irritable,  the  lack  of  response 
must  be  attributed  to  the  nerve-ends.  The  response  to  indirect  stimulation 
(i.  e.  excitation  of  a  muscle  by  irritating  its  nerve)  is  lost  in  about  twenty 
minutes,  while  the  irritability  of  the  muscle,  as  tested  by  direct  excitation,  is 


6' 

38  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

r 

not  lost  for  four  or  five  hours.  In  this  as  in  so  many  instances  the  loss  of 
irritability  of  the  muscle  is  due  primarily  to  the  disturbance  of  the  respira- 
tion of  the  muscle.  Of  the  substances  supplied  to  the  muscle  by  the  blood, 
oxygen  is  one  the  want  of  which  is  soonest  felt.  The  muscle  contains  within 
itself  a  certain  store  of  oxygen,  but  one  which  is  by  no  means  equal  to  the 
amount  of  oxidizable  substances.  Of  this  oxygen,  that  which  is  in  the  least 
stable  combinations,  and  which  is  available  for  immediate  needs,  is  soon 
exhausted.  A  continual  supply  of  oxygen  is  required  even  for  the  chem- 
ical changes  which  occur  in  the  quiet  muscle.  Of  the  waste  substances  which 
the  blood  removes  from  the  cell,  carbon  dioxide  is  the  one  which  accumu- 
lates most  rapidly  and  is  the  first  to  lessen  the  irritability.  Lactic  acid  and 
waste  products  from  the  breaking  down  of  nitrogenous  materials  of  the  cell 
are  also  injurious. 

The  dependence  of  nerve-fibres  upon  the  blood-supply  is  by  no  means  so 
well  understood.  The  nerve-fibre  is  a  branch  of  a  nerve-cell,  and  it  seems  as 
if  the  nourishment  of  the  fibre  was  largely  dependent  upon  that  of  the  cell- 
body  (see  Fatigue  of  Nerve,  pp.  75  and  95).  Nevertheless,  the  nerve-fibre 
requires  a  constant  supply  of  blood  for  the  maintenance  of  its  irritability. 
The  irritability  of  the  nerve  cannot  long  continue  without  oxygen,  and  a 
nerve  which  has  been  removed  from  the  body  is  found  to  remain  irritable 
longer  in  oxygen  than  in  air,  and  in  air  than  in  an  atmosphere  containing  no 
oxygen.  Waste  products  liberated  by  active  muscles  have  a  deleterious 
effect  on  nerves  ;  whether  such  substances  are  produced  in  the  nerves  them- 
selves will  be  considered  later. 

The  efficacy  of  the  blood  to  preserve  the  irritability  is  to  be  seen  in  such 
experiments  as  those  of  Ludwig  and  Schmidt  j1  they  succeeded  in  maintaining 
the  artificial  circulation  of  defibrinated,  aerated  blood  through  the  muscles 
of  a  dog,  and  kept  them  irritable  for  many  hours  after  death  of  the  animal. 
If  such  an  experiment  is  to  be  successful,  the  blood  must  be  maintained  at  the 
normal  temperature,  be  plentifully  supplied  with  oxygen,  and  be  kept  as  free 
from  carbon  dioxide  as  possible.  Von  Frey 2  made  an  elaborate  experiment  of 
this  nature.  A  dog  was  killed,  the  body  was  cut  in  halves,  and  the  aorta  and 
inferior  vena  cava  were  quickly  connected  with  an  apparatus  for  pumping  the 
blood  at  a  regular  rate  through  the  hind  part  of  the  body.  Before  the  blood 
entered  the  arteries  it  passed  through  coils  in  which  it  was  warmed  to  the  nor- 
mal temperature,  and  an  artificial  lung,  where  it  received  a  supply  of  oxygen 
and  was  relieved  of  its  carbon  dioxide.  Under  these  conditions  the  muscles 
were  kept  alive  for  more  than  seven  hours,  and  so  far  retained  their  normal 
condition  that  throughout  this  period  they  were  able  to  respond  to  stimuli 
sent  to  them  through  their  nerves  and  contract  with  sufficient  vigor  to  raise  a 
considerable  weight.  H.  N.  Martin3  made  a  similar  experiment  on  the  heart 

1  Sitzungsberichte  der  math.-phys.  Clause  der  k.  sticks.  Gesellsehoft  der  Wissenschaften ,  vol.  xx.,  1868. 

2  «  Versuche   iiber  den   Stoffwechsel  des  Muskels,"  Archiv  fur  Anatomic  und  Physiologic^ 
1885;  physiologische  Abtheilung,  S.  533. 

3  Studies  from  the  Biological  Laboratory  of  Johns  Hopkins  University,  1882,  vol.  ii.  p.  188. 


GENERAL    PIIYS1*  >/.m;  \r    OF  MUSCLE   AND    NE, 

of  a  (loir-      Th*'  heart  and  lim^s  were  isolated    from  the  rest  of  tl 
heart  was   led  with  delihriuated    blood    from  a  Mariotte  flask,  and   ; 
were  supplied  with  air  by  an  artificial  respiration  apparatus.     The  hean,  < 
was  kept  moist  and  at  the  normal  temperature,  continued  to  beat  for  four  hours 
and   more.      Porter1  has  succeeded  in  keeping  even  small  pieces  of  the 
triele  of  the   mammalian  heart  alive  by  maintaining  a  good  circulation 
well-oxygenated  blood  through  its  vessels  (see  Section  on  Nutrition  of  the 
Heart). 

Normally  the  blood-supply  to  the  muscle  is  varied  according  to  its  needs. 
When  the  muscle  is  stimulated  to  action  its  blood-vessels  are  at  the  same 
time  dilated,  so  that  it  receives  a  five  supply  of  blood.2  Moreover,  if  mus- 
cular work  is  extensive,  the  heart  beats  faster  and  the  respiratory  movements 
are  quicker,  so  that  a  larger  amount  of  oxygen  is  provided  and  the  carbon 
dioxide  is  removed  more  rapidly.  The  importance  of  the  blood-supply  to  a 
muscle  can  be  best  understood  if  we  consider  it  in  relation  to  the  effects  of 
fatiguing  work  upon  the  muscles  (see  p.  74).  The  relation  of  special  sub- 
stances in  the  blood  to  the  needs  of  the  muscle  can  be  best  considered 
together  with  the  chemistry  of  the  muscle  (see  p.  159). 

Effect  of  Separation  from  the  Central  Nervous  System. — If  a  motor  nerve 
be  cut,  or  if  some  part  of  it  be  so  injured  that  the  fibres  lose  their  power  of 
conduction,  the  portion  of  the  nerve  thus  separated  from  the  central  nervous 
system  sooner  or  later  completely  degenerates  (see  p.  77).  Each  of  the 
motor  nerve-fibres  is  a  branch  of  a  motor  cell  in  the  anterior  horns  of  the 
spinal  cord.  These  nerve-cells  are  supposed  to  govern  the  nutrition  of  their 
processes,  though  how  a  microscopic  cell  can  thus  influence  a  nerve-fibre  a 
meter  or  so  long  is  by  no  means  clear.  Soon  after  the  nerve  is  separated 
from  its  cell  it  exhibits  a  change  of  excitability.  In  general  it  responds 
more  readily  to  the  kathode  than  the  anode,  which  would  imply  an  increased 
irritability ;  at  the  part  near  the  cut,  however,  it  responds  best  to  the  anode.3 
The  increase  is  soon  followed  by  a  gradual  decrease  of  irritability.  In  the  case 
of  mammalian  nerves  loss  of  irritability  may  be  complete  at  the  end  of  three 
or  four  days,  but  the  nerves  of  cold-blooded  animal  may  retain  their  irri- 
tability for  several  weeks.  The  immediate  cause  of  the  loss  of  irritability  is 
the  change  in  the  chemical  and  physiological  structure  of  the  axis-cylinder. 
The  degenerative  changes  result  finally  in  the  complete  destruction  of  the 
nerve-fibres,  and  involve  the  motor  end-organs  as  well,  but  do  not  imme- 
diately invade  the  muscle,  which  may  be  considered  a  proof  that  nerve  and 
muscle  protoplasm  are  not  continuous. 

Though  no  immediate  change  in  the  structure  of  the  muscle  is  observable, 
the  irritability  of  the  muscle  soon  begins  to  alter.  At  the  end  of  a  fortnight  the 
irritability  of  the  muscle  for  all  forms  of  stimuli  is  lessened.  From  this  time 
on,  the  irritability  gradually  undergoes  a  remarkable  change,  the  excitability 

1  Porter:  American  Journal  of  Physiology,  1899,  vol.  ii.  p.  127. 

2  Sczelkow:  Sitzungsber,  d.  k.  Akad.  Wien,  1862,  Bd.  xlv.  Abth.  1. 

3  Blix:  Skandinavisches  Archiv  fur  Physiologic,  1889,  Bd.  i,  S.  184. 


70  AN  AMERICAN    TEXT- BOOK    OF  PHYSIOLOGY. 

for  mechanical  irritants  and  for  direct  battery  currents  (see  p.  54)  beginning  to 
increase,  but  the  power  to  respond  to  electric  currents  of  short  duration, 
as  induction  shocks,  continuing  to  lessen;  indeed,  the  reactions  of  the 
muscle  appear  to  take  on  more  of  the  character  of  those  of  smooth  muscle- 
fibres.  The  condition  of  increasing  irritability  to  direct  battery  currents  and 
mechanical  irritants  reaches  its  maximum  by  the  end  of  the  seventh  week, 
and  from  that  time  on  the  power  to  respond  to  all  forms  of  stimuli  lessens, 
the  excitability  being  wholly  lost  by  the  end  of  the  seventh  or  eighth  month. 
During  the  stage  of  increased  excitability  fibrillary  contractions  are  often 
observed. 

As  in  the  case  of  a  nerve,  so  of  the  muscle  the  loss  of  irritability  is  due  to 
degenerative  changes  which  gradually  lead  to  the  destruction  of  the  muscle 
protoplasm.  The  cause  of  the  change  in  the  muscle  is  still  a  matter  of  doubt, 
some  regarding  it  as  due  to  the  absence  of  some  nutritive,  trophic  influence 
from  the  central  nervous  system,  others  consider  it  to  be  the  result  of  cir- 
culatory disturbances,  consequent  upon  the  lack  of  a  proper  regulation  of  the 
blood-supply,  due  to  the  division  of  the  vaso-motor  nerves,  and  still  others 
attribute  it  to  a  lack  of  exercise,  it  being  no  longer  stimulated  to  action.  As 
regards  the  second  view,  it  may  be  said  that  muscles  whose  vaso-motor 
nerves  are  intact,  the  vessels  being  innervated  through  other  nerves  than 
those  which  supply  the  muscle-tissue  proper,  as  is  the  case  with  some  of  the 
facial  muscles,  undergo  similar  changes  in  irritability  when  their  motor 
nerves  are  cut.  As  regards  the  first  and  last  views,  it  may  be  said  that  if 
the  muscles  be  artificially  excited,  as  by  electric  stimuli,  and  thus  are  exer- 
cised daily,  the  coming  on  of  degeneration  can  be  at  least  greatly  delayed. 
The  question  as  to  whether  the  anabolic  processes  within  the  muscle-cell 
are  dependent  on  the  central  nervous  system,  in  the  sense  of  their  being 
specific  trophic  influences  sent  from  the  nerve-cells  to  the  muscles,  is  still 
under  discussion  and  need  not  be  considered  further  in  this  place.  Without 
doubt  the  reflex  tonus  impulses  which  during  waking  hours  are  all  the  time 
coming  to  the  muscles  are  productive  of  katabolic  changes  and,  indirectly  at 
least,  favor  anabolism. 

(c)  Effect  of  Influences  which  result  from  the  Functional  Activity  of  Nerves 
and  Muscles. — Fatigue  of  Muscles. — The  condition  of  muscular  fatigue  is  cha- 
racterized by  lessened  irritability,  decrease  in  the  rate  and  vigor  with  which 
the  muscle  contracts  and  liberates  energy,  and  a  still  greater  decrease  in  the 
rate  with  which  it  relaxes  and  recovers  its  normal  form.  In  a  sense,  whatever 
induces  such  a  state  can  be  said  to  cause  fatigue,  but  it  is  perhaps  best  to 
restrict  the  term  to  the  form  of  fatigue  which  is  produced  by  excessive 
functional  activity.  The  cause  of  exhaustion  which  results  from  over- 
work is  in  part  the  same  as  the  cause  of  the  loss  of  irritability  and  power 
which  follows  the  cutting  off  of  the  blood-supply.  The  working  cell  liberates 
energy  at  the  expense  of  its  store  of  nutriment  and  oxygen,  and  through  oxi- 
dation processes  forms  waste  products  which  are  poisonous  to  its  protoplasm. 
The  fatigue  which  results  from  functional  activity  has,  therefore,  a  twofold 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    N^liVI-:.       71 


cause,  the  decrease  in  energy-holding  compounds  available  for  work  and  the 
accumulation  of  poisonous  waste  matters. 

It  is  evident  that  the  length  of  time  that  the  cell  can  continue  to  work  will 
depend  very  much  upon  the  rapidity  with  which  the  energy-holding  explosive 
compounds  are  formed  by  the  cell-protoplasm  and  the  waste  products  are 
excreted.  If  a  muscle  is  made  to  contract  vigorously  and  continuously,  as 
when  a  heavy  weight  is  held  up,  fatigue  comes  quickly  ;  on  the  other  hand,  a 
muscle  may  be  contracted  a  great  many  times  if  each  contraction  is  of  short 
duration  and  considerable  intervals  of  rest  intervene  between  the  succeeding 
contractions.  The  best  illustration  of  this  is  the  heart,  which,  though  making 
contractions  in  the  case  of  man  at  the  rate  of  seventy  or  more  times  a  minute, 
is  able  to  beat  without  fatigue  throughout  the  life  of  the  individual.  Each 
of  the  vigorous  contractions,  or  systoles,  is  followed  by  an  interval  of  rest, 
diastole,  during  which  the  cells  have  time  to  recuperate.  The  same  is  true  of 
the  skeletal  muscles.  It  was  found  in  an  experiment  that  if  a  muscle  of  the 
hand,  the  abductor  inditis,  were  contracted  at  regular  intervals,  a  weight  being 
so  arranged  that  it  was  lifted  by  the  finger  each  time  the  muscle  shortened,  a 
light  weight  could  be  raised  at  the  rate  of  once  a  second  for  two  hours  and  a 
half,  i.  e.  more  than  9000  times,  without  any  evidence  of  fatigue.  If,  however, 
the  weight  was  increased,  which  required  a  greater  output  of  energy,  or  if  the 
rate  of  contractions  was  increased,  which  shortened  the  time  of  repose,  the  mus- 
cle fatigued  rapidly.  In  general,  the  greater  the  weight  which  the  muscle  has 
to  lift,  the  shorter  must  be  the  periods  of  contraction  in  proportion  to  the  inter- 
val of  rest  if  the  muscle  is  to  maintain  its  power  to  work.  Maggiora,1  in  his 
interesting  experiments  in  Mosso's  laboratory  at  Turin,  made  a  very  careful  study 
of  this  subject,  and  ascertained  that  for  a  special  group  of  muscles  there  is  for 
each  individual  a  definite  weight  and  rate  of  contraction  essential  to  the  accom- 
plishment of  the  greatest  possible  work  in  a  given  time.  These  experiments 
were  made  on  men,  and  the  height  of  the  succeeding  contractions  was  re- 
corded by  an  apparatus  devised  by  Mosso,  the  ergograph,2  which  made  it 
possible  to  estimate  the  total  amount  of  work  done  by  the  muscles  studied. 
Many  forms  of  apparatus  have  since  been  devised  to  accomplish  this. 
Mosso's  ergograph  consisted  of  two  parts,  an  arm  rest  equipped  with  suitable 
clamps  for  fixing  the  arm  and  hand,  and  a  writing  mechanism  arranged  to 
record  the  movements  of  the  weight  which  was  raised  by  the  flexion  of 
the  second  finger.  Either  increasing  the  weight  or  the  rate  of  contrac- 
tion hastens  the  coming  on  of  fatigue  and  so  lessens  the  power  and  the 
total  amount  of  work.  In  such  an  exercise  as  walking  the  muscles  are 
not  continually  acting,  but  intervals  of  rest  alternate  with  the  periods 
of  work,  and  the  time  for  recuperation  is  sufficiently  long  to  permit  the 
protoplasm  of  the  muscle-cells  to  prepare  the  chemical  compounds  from 
wrhich  the  energy  is  liberated  as  fast  as  they  are  used,  and  get  rid  of  the 

1  Archiv  fur  Anatomic  und  Physiologic,  1890;  physiologische  Abtheilung,  S.  191. 

2  Mosso:  Die  Ermiidung,  Leipzig,  1892,  S.  90;  Lombard:  Journal  of  Physiology,  1892,  vol. 
xiii.  Fig.  1,  Plate  1. 


72  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

waste  products  of  contraction,  so  that  vigorous  muscles  can  be  employed 
many  hours  before  any  marked  fatigue  is  experienced.  Sooner  or  later, 
however,  the  vigor  of  the  muscle  begins  to  decrease.  The  reason  for 
this  is  not  wholly  clear.  It  is  noticeable,  however,  that  not  only  the 
muscles  employed  in  the  work,  but  other  muscles,  such  as  those  of  the 
arms  for  instance,  even  when  purposely  kept  quiet,  have  their  irritability 
reduced.  This  would  suggest  that  the  fatigue  which  finally  asserts  itself  is 
due  to  some  general  rather  than  local  influence.  To  understand  this  we  must 
recall  the  fact  that  all  parts  of  the  body  are  in  communication  by  means  of 
the  circulatory  system.  The  ever-circulating  blood  as  it  is  thrown  out  by  the 
heart  is  divided  into  minute  streams,  which,  after  passing  through  the  many 
organs  of  the  body,  unite  again  on  their  return  to  the  heart.  If  materials  be 
taken  from  the  blood  by  one  part,  they  are  lost  to  all  the  rest,  and  if  materials 
be  added  to  the  blood  by  any  part,  they  are  sooner  or  later  carried  to  all  the  rest. 
During  the  course  of  a  long  march,  the  muscles  of  the  leg  take  up  a  great  deal 
of  nutriment,  and  give  off  many  waste  products,  and  all  the  organs  suffer  in  con- 
sequence. Mosso,1  in  his  experiments  upon  soldiers  taking  long  forced  marches, 
found  that  lack  of  nutriment  is  not  the  only  cause  of  the  general  fatigue 
produced  by  long-continued  muscular  work.  The  soldiers,  though  somewhat 
refreshed  by  the  taking  of  food,  did  not  recover  completely  until  after  a  pro- 
longed interval  of  rest.  He  attributed  this  to  the  fatigue-products  which  he 
supposed  the  muscles  to  have  given  off,  and  concluded  that  they  were  only 
gradually  eliminated  from  the  blood.  To  see  if  there  were  fatigue-products 
in  the  blood  of  a  tired  animal  capable  of  lessening  the  irritability  of  organs 
other  than  those  which  had  been  working,  he  made  the  following  experiment : 
He  drew  a  certain  weight  of  blood  from  the  veins  of  a  dog,  and  then  put  back 
into  the  animal  an  equal  amount  of  blood  from  another  completely  rested  dog. 
The  dog  which  was  the  subject  of  the  experiment  appeared  to  be  all  right  after 
the  operation.  On  another  day  he  repeated  the  experiment,  but  this  time  the 
blood  which  was  put  back  was  taken  from  a  dog  that  was  completely  tired  out 
by  running.  The  effect  of  the  blood  from  the  fatigued  animal  was  very 
marked ;  the  dog  receiving  it  showed  all  the  signs  of  fatigue,  and  crept  off  into 
a  corner  to  sleep.  Mosso  concluded  from  this  experiment,  that  during  mus- 
cular work  fatigue-products  are  generated  in  the  muscles,  pass  thence  into  the 
blood,  and  are  conveyed  to  other  muscles,  where  they  produce  the  lowered 
irritability  and  loss  of  power  characteristic  of  fatigue.  Many  years  before, 
Von  Ranke  extracted  from  the  tired  muscles  of  frogs  substances  which  he 
considered  fatigue  materials.  Lee 2  would  draw  a  sharp  distinction  between 
fatigue  and  exhaustion.  He  considers  the  former  to  be  a  transient  change 
in  the  capacity  for  work  induced  by  the  presence  of  waste  products,  while 
the  latter  is  a  far  more  serious  condition  and  is  due  to  a  lack  of  nutritive 
energy-giving  substance.  He  considers  that  fatigue,  by  lessening  the  irri- 

1  Archiv  fur  Anatomic  und  Physiologic,  1890;   physiologische  Abtheilnng. 

2  Proceedings   of  the   American    Physiological   Society,   Dec.,    1898,   published   in  American 
Journal  of  Physiology,  1899,  vol.  ii.  p.  11. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND    NKHVi:.        73 


lability,  may  exert  a  protective  influence  :m<l  prevent  the  work  from  being 
carried  under  ordinary  conditions  to  the  point  of  exhaustion.  In  favor  of 
this  view  he  states  that  the  muscles  even  of  starving  animals,  although 
incapable  of  long-continued  work,  do  not  make  contractions  of  the  type 
characteristic  of  fatigued  muscles  (see  p.  115);  on  the  other  hand,  muscles 
which  have  been  subjected  to  lactic  acid,  one  of  the  waste  products  resulting 
from  muscular  work,  whether  it  be  free  or  combined,  as  it  probably  is  in  the 
muscle,  with  potassium  or  sodium,  do  make  contractions  of  the  type  shown 
by  fatigued  muscles.  Waller1  has  of  late  laid  much  stress  upon  the  action 
of  CO2  to  stimulate  protoplasm  when  present  in  small  amounts  and  to  anes- 
thetize it  when  in  larger  quantities.  CO2  is  also  a  waste  product  of  muscle, 
but  it  is  doubtful  whether  the  paralyzing  effect  of  large  amounts  can  be 
regarded  as  a  fatigue  effect. 

Maggiora,  in  his  experiments  upon  the  fatigue  of  special  groups  of  mus- 
cles, likewise  found  that  the  taking  of  food  causes  only  a  partial  recovery  of 
the  tired  muscles,  and  that  an  interval  of  rest  is  essential  to  complete  recovery. 
In  these  experiments  the  irritability  of  the  muscles  was  tested  not  only  by 
volitional  impulses,  but  by  the  strength  of  the  electric  current  required  to 
cause  direct  excitation.  A  curve  of  fatigue  of  human  muscles  by  voluntary 
contractions  is  shown  in  Fig.  59,  and  one  resulting  from  electrical  excitation 
of  the  muscle  in  Fig.  58.  In  the  case  of  vigorous  men,  one  and  a  half  hours 
suffice  to  restore  the  muscles  of  the  forearm  which  have  been  completely  tired 
out  by  raising  a  heavy  weight  many  times.  He  also  observed  that  the  time 
required  for  recovery  can  be  greatly  shortened  if  the  circulation  of  the  blood 
and  lymph  in  the  muscles  be  increased  by  massage.  This  suggests  that  the 
power  of  the  cell  to  give  off  its  waste  products  to  the  blood  is  sufficiently 
rapid  to  keep  pace  with  the  ordinary  production,  but  not  with  the  more  rapid 
formation  taking  place  during  fatiguing  work.  This  would  seem  to  be  the 
case  in  spite  of  the  fact  that  circulation  of  the  blood  and  lymph  in  the  mus- 
cles is  increased  during  action.  This  increase  in  the  circulation  through  the 
acting  muscle  is  brought  about  in  part  by  the  fact  that  the  muscle  massages 
itself  by  its  own  contractions.  Jt  is  a  pumping  mechanism,  which  acts  at  the 
time  when  the  increased  taking  of  oxygen  and  nutriment  and  giving  off  of 
waste  products  make  the  rapid  renewal  of  the  restoring  fluids  imperative. 
Every  time  the  muscle  contracts  the  swelling,  tense  fibres  compress  the  lym- 
phatics and  blood-vessels  between  and  about  them,  and  when  it  relaxes  the 
valves  in  the  lymph  vessels  and  veins  prevent  the  return  of  the  fluid  which 
has  been  squeezed  out.  In  addition  to  this,  when  muscles  are  stimulated  to 
action  by  impulses  coming  to  them  from  the  central  nervous  system,  the  mus- 
cles in  the  walls  of  the  blood-vessels  of  the  muscle  are  acted  upon  by  their 
vaso-dilator  nerves,  and,  relaxing,  permit  a  greater  flow  of  blood  through  the 
muscle  ;  when  the  muscles  cease  to  be  excited  the  muscles  in  the  vessel  walls 
gradually  regain  their  tone,  and  the  blood-supply  to  the  muscle  tissue  is 
correspondingly  lessened.  This  arrangement  would  seem  to  suffice  for  the 
1  Lectures  on  Physiology,  first  series,  on  Animal  Electricity,  London,  1897,  p.  47. 


74  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

bringing  of  nutriment  and  oxygen  and  the  removal  of  waste  matters  under 
ordinary  conditions. 

Considerable  difference  of  opinion  exists  as  to  which  of  three  classes  of 
food-stuffs — proteids,  carbohydrates,  and  fats — supply  the  energy  used  by  the 
muscle  in  ordinary  and  excessive  work,  and  how  these  are  employed  by  the 
muscle. 

The  question  has  been  studied  by  examining  the  character  and  quantity 
of  waste  products  liberated  from  the  body  during  and  after  excessive  mus- 
cular work,  as  compared  with  those  given  off  when  the  subject  is  at  rest. 
Another  method  has  been  to  test  the  strength  of  the  muscle  in  ergographic 
experiments,  and  to  find  the  effect  of  different  kinds  of  food  upon  the  time 
required  for  its  recovery.  Experiments  of  Fick  and  Wislicenus,1  Voit  and 
Pettenkofer,2  Voit,3  and  others  caused  the  view  to  become  generally  accepted 
that  the  energy  of  the  muscle  by  violent  muscular  work  comes  largely  from 
the  non-proteid  substances  in  the  muscles.  Later  Pflliger  and  his  pupils 
have  gone  to  the  other  extreme  and  conclude  that  proteid  is  the  chief  source 
of  energy.4 

Very  many  others  have  written  on  both  sides  of  the  subject  and  still  a 
final  conclusion  has  not  been  reached.5 

Probably  the  sugars,  and  possibly  after  these  the  fats  are  employed  by  the 
muscle  as  the  most  available  form  of  energy,  while  the  proteid  forms  a  more 
permanent  part  of  the  muscular  machine,  and  is  only  made  use  of  when  the 
work  is  exhaustive  (see  page  166).  The  taking  of  any  one  of  these  classes 
of  food  hastens  the  recovery  from  fatigue,  and  the  sooner  the  more  readily  it 
is  digested  and  assimilated  (see  Metabolism — effect  of  muscular  work). 

Normally  the  muscles  are  never  completely  fatigued.  It  would  seem 
that  as  the  muscles  tire  and  their  irritability  is  lessened,  the  central  nerve- 
cells  which  send  the  stimulating  impulses  to  them  have  to  work  harder, 
and  that  the  nerve-cells  give  out  sooner  than  the  muscles.  On  the  other 
hand,  certain  experiments  seem  to  show  that  the  nerve-cells  recover  from 
fatigue  more  rapidly  than  the  muscles  do,  so  that  it  is  an  advantage  to 
the  organism  that  they  should  cease  to  excite  the  muscles  before  muscular 
fatigue  is  complete.  With  the  decreasing  irritability  of  the  muscle,  a  feeling 
of  discomfort  in  the  muscle  and  an  increasing  sense  of  effort  are  experienced 
by  the  individual,  both  of  which  tend  to  cause  a  cessation  of  contraction,  and 
prevent  a  harmful  amount  of  work.  That  such  an  arrangement  would  be  of 
service  was  apparent  in  the  experiments  of  Maggiora,  in  which  he  found  that 
if  muscles  are  forced  to  work  after  fatigue  has  developed,  the  time  of  recovery 
is  prolonged  out  of  all  proportion  to  the  extra  work  accomplished. 

At  the  close  of  even  exhaustive  muscular  work  there  is  always  a  large 
amount  of  energy-holding  materials  in  the  blood  and  tissues,  and  the  rapid, 

1  Viertetyahresschrift  der  naturforsche  Gesellschaft  in  Zurich,  1865,  Bd.  x.  S.  317. 

2  Zeitschnft  fur  Biologic,  1866,  Bd.  ii.  3  Ibid.,  1876,  Bd.  vi.  S.  305. 

4  Pfluger's  Archiv,  1899,  Bd.  77,  S.  425. 

5  Schafer's  Text-book  of  Physiology,  1898,  vol.  i.  p.  912. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.       75 

though  partial,  improvement  in  the  condition  on  the  taking  of  food  is  per- 
haps best  explained  as  the  result  of  a  stimulating  effect  on  the  central  nerv- 
ous system.  This  might  be  due  to  the  change  in  the  circulation  which  follow- 
the  taking  of  food,  as  well  as  the  fact  that  a  fresh  supply  of  uncombined  and 
hence  available  energy-holding  substances  is  being  received.  The  effect  of 
the  so-called  stimulants,  alcohol,  tea,  coffee,  etc.,  to  temporarily  increase  the 
ability  to  do  work,  is  probably  chiefly  through  their  action  on  the  central 
nervous  system.  Their  influence  is  a  temporary  one,  and  only  markedly 
increases  the  amount  of  work  when  the  body  has  a  plentiful  supply  of  nutri- 
ment.1 

Fatigue  of  Nerves. — Muscle-,  gland-  and  nerve-cells — in  fact,  almost  every 
form  of  protoplasm — if  excited  to  vigorous  long-continued  action,  deteri- 
orate and  exhibit  a  decline  of  functional  activity.  As  we  have  seen,  in 
the  case  of  muscle  there  are  a  using  up  of  available  energy-holding  com  pounds 
and  a  production  of  poisonous  waste  matters,  and  these  two  effects  induce  the 
condition  known  as  fatigue.  A  priori,  we  should  expect  similar  changes  to 
occur  in  the  active  nerve-fibre  ;  almost  all  the  experimental  evidence  is,  how- 
ever, opposed  to  this  view.  The  form  of  activity  which  is  most  character- 
istic of  muscle  is  contraction ;  that  which  is  most  characteristic  of  nerve  is 
conduction.  In  the  case  of  the  muscle  it  is  exceedingly  difficult  to  distin- 
guish between  the  effects  produced  by  the  processes  associated  with  the  change 
of  form  and  those  which  result  from  the  transmission  of  the  excitatory  change. 
There  is  little  doubt  that  fatigue  is  associated  with  the  former ;  whether 
it  is  associated  with  the  latter  is  not  known.  In  the  case  of  the  nerve,  where 
the  transmission  process  may  be  studied  by  itself,  conduction  does  not  seem 
to  fatigue  (see  p.  95). 

Apparently  the  same  may  be  said  of  the  processes  which  result  in  the 
development  of  what  we  call  the  nerve-impulse.  We  have  already  seen  that 
the  nerve  may  undergo  an  alteration  of  irritability  if  subjected  to  artificial 
irritants.  Such  a  change  at  the  point  of  application  of  the  irritant  is  hardly 
to  be  regarded  as  a  fatigue  effect,  however,  for  in  many  cases,  at  least,  it  is 
due  to  the  direct  effect  of  the  irritant  on  the  physical  or  chemical  structure  of 
the  nerve-protoplasm  rather  than  to  molecular  changes  which  are  peculiar  to 
the  development  of  the  nerve-impulse.  Thus  the  change  of  irritability  which 
results  from  a  series  of  light  blows,  such  as  may  be  given  to  a  nerve  by 
Tigerstedt's  tetanomotor,  cannot  properly  be  said  to  be  the  result  of  fatigue.  It 
has  been  found  that  a  medullary  nerve  may  be  excited  many  times  a  second 
for  hours,  by  an  induced  current,  and  still  be  capable  of  developing  at  the 
stimulated  point  what  we  call  the  nerve-impulse.  The  change  which  is  de- 
veloped at  the  point  of  excitation  and  which  passes  thence  the  length  of  the 
nerve,  would  seem  to  be  the  expression  of  a  form  of  energy  liberated  within 
the  nerve,  and  since  the  liberation  of  energy  implies  the  breaking  down  of 
chemical  combinations,  the  apparent  lack  of  fatigue  of  the  nerve  is  incompre- 
hensible. It  is  the  more  remarkable  since  the  nerve-fibre  is  to  be  considered  a 
1  Schumburg:  Archiv  fur  Analomie  und  Physiologic,  1899,  supplement,  S.  289. 


76  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

branch  of  a  nerve-cell,  and  nerve-cells  appear  to  fatigue  if  frequently  excited 
to  vigorous  action.  Inasmuch  as  we  have  as  yet  no  definite  knowledge  of  the 
nature  of  what  we  call  the  nerve-impulse,  or  of  the  character  of  the  processes 
by  which  it  is  transmitted  along  the  nerve,  we  can  afford  to  leave  this  question 
open,  and  simply  state  that  the  evidence  thus  far  obtained  is  opposed  to  the 
view  that  nerve-fibres  fatigue. 

Effect  of  Use  and  Disuse. — Different  kinds  of  muscle- tissues  possess  very 
different  degrees  of  endurance.  By  endurance  we  mean  the  capacity  to  liber- 
ate energy  during  long  periods  of  time.  This  capacity  is  intimately  associated 
with  irritability,  for  one  of  the  first  marks  of  failure  of  power  is  a  decline  of 
irritability.  In  general,  the  more  irritable  a  muscle  the  less  its  endurance, 
because  with  an  increase  of  irritability  there  is  associated  a  more  rapid  and 
extensive  liberation  of  energy  in  response  to  irritants.  For  example,  the  rap- 
idly responding  and  acting  pale  striated  muscles  of  the  rabbit  have  less  resist- 
ing power  than  the  red  striated  muscles,  while  the  sluggish  unstriated  muscle- 
fibres  can  contract  a  long  time  without  suffering  from  fatigue. 

The  endurance  of  muscles  of  even  the  same  kind  may  differ  very  considera- 
bly in  the  same  individual,  but  the  differences  are  more  striking  in  the  case  of 
different  individuals.  One  of  the  causes  of  this  is  the  extent  to  which  the 
muscles  are  employed.  Use,  exercise,  is  the  most  effective  method  of  increasing 
not  only  the  strength,  but  the  endurance  of  the  muscle.  Though  this  fact  is 
so  well  known  as  to  scarcely  need  repeating,  the  explanation  of  it  is  by  no 
means  so  clear.  Undoubtedly  one  of  the  causes  is  a  more  perfect  circulation 
in  a  muscle  which  is  often  used,  but  this  is  not  all.  It  would  seem  as  if  the 
protoplasm  of  the  muscle-cell  was  educated,  so  to  speak,  to  be  more  expert  in 
assimilating  materials  containing  energy,  in  building  up  the  explosive  compounds 
employed  in  its  work,  and  in  excreting  deleterious  waste  matters. 

The  effect  of  exercise  upon  irritability  has  not  been  thoroughly  worked  out. 
It  would  seem  as  if  there  were  a  normal  degree  of  irritability  for  each  special 
form  of  muscle-tissue,  and  as  if  either  an  increase  or  decrease  of  the  irritability 
above  or  below  this  level  was  a  sign  of  deterioration.  Exercise,  if  not  excess- 
ive, is  favorable  to  the  maintenance  of  this  normal  physiological  condition. 
Without  doubt  many  of  the  differences  which  we  attribute  to  the  muscles  of 
different  men  are  really  due  to  differences  in  the  central  nerve-cells,  the  action 
of  muscles,  rightly  interpreted,  being  rather  an  expression  of  central  nervous 
activity  than  the  result  of  peculiarities  of  the  muscles  themselves.  To  vol- 
untarily exercise  the  muscles  is  to  exercise  the  nerve-cells,  and  the  effects  of 
exercise  upon  these  nervous  mechanisms  is  of  as  much  importance  as  the 
effect  upon  the  muscles.  In  admiring  visible  proportions  we  must  always 
bear  in  mind  that  though  "beef"  is  of  use  to  the  athlete,  the  muscles  are 
merely  the  servants,  and  can  accomplish  nothing  if  the  master  is  sick.  The 
nerve-cells  always  give  out  before  the  muscles,  and  the  man  preparing  for  a 
contest  should  watch  his  nervous  system  more  than  his  muscles.  He  who 
forgets  this  can  easily  over-train,  and  do  himself  a  permanent  injury,  besides 
failing  in  the  race. 


QEXEKAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.       77 


Effect  of  Enforced  Rest.  —  Not  only  is  the  strength  of  the  mu-cl<s 
increased  by  exercise,  but  a  lack  of  exercise  soon  results  in  a  loss  of 
This  is  seen  when  an  individual  is  confined  to  his  bed  for  even  a  comparatively 
short  time,  or  when  a  limb  is  subjected  to  enforced  rest  by  being  placed  in  a 
splint.  The  cause  is  to  be  sought  in  changes  peculiar  to  the  muscle  proto- 
plasm itself,  although  reduced  circulation  may  also  play  a  part.  The  effect  of 
prolonged  rest  on  the  irritability  of  muscles,  is  seen  most  markedly  when  they 
are  separated  from  the  central  nervous  system  by  injuries  of  their  nerves  (see 
p.  70).  The  lowered  irritability  which  results  from  prolonged  rest  is  not 
peculiar  to  muscles,  but  is  shared  by  all  forms  of  protoplasm. 

G.  CONDUCTIVITY. 

Conductivity  is  that  property  of  protoplasm  by  virtue  of  which  a  condition 
of  activity  aroused  in  one  portion  of  the  substance,  by  the  action  of  a  stimulus 
of  any  kind,  may  be  transmitted  to  any  other  portion.  For  example,  if  the 
edge  of  the  bell  of  a  vorticella  (see  Fig.  2,  p.  19)  be  irritated  by  a  hair,  not 
only  do  the  movements  of  the  cilia  cease,  but  the  contractile  substance  of  the 
bell  draws  it  into  a  more  compact  shape,  and  the  fibrilla?  of  the  stalk  shorten 
and  pull  the  bell  away  from  the  offending  irritant.  In  such  a  case  an  exciting 
process  must  have  been  transmitted  throughout  the  cell,  and  through  several  more 
or  less  differentiated  forms  of  protoplasm.  This  property  of  conductivity  is  not 
known  to  be  limited  to  any  one  peculiar  structural  arrangement  of  protoplasm 
distinguishable  with  the  microscope,  but  is  exhibited  by  a  vast  variety  of  forms 
of  cell-protoplasm,  and  by  plants  as  well  as  animals.  The  cytoplasm  of  cells, 
the  part  of  the  protoplasm  surrounding  the  nucleus,  appears  to  be  composed 
of  a  semifluid  granular  material,  traversed  in  all  directions  by  finest  fibrillse 
which  in  some  cases  appear  to  form  an  irregular  meshwork,  the  reticulum,  and 
in  others  to  be  arranged  side  by  side  as  more  or  less  complete  fibrils.  It  is  not 
known  whether  the  power  of  conduction  is  possessed  by  the  whole  of  the  pro- 
toplasmic substance  or  is  confined  to  the  reticular  substance,  but  there  are  cer- 
tain reasons  why  the  former  view  may  be  considered  the  more  probable.  The 
rate  and  the  strength  of  the  conduction  process  varies  greatly  in  different  forms 
of  protoplasm,  and  there  appear  to  be  differences  in  the  facility  with  which 
the  exciting  process  spreads  through  different  parts  of  even  the  same  cell.1  Not 
only  are  such  differences  to  be  noticed  in  many  of  the  ciliated  infusoria,  but 
even  the  substance  of  striated  muscles  seems  to  conduct  in  two  different  ways, 
the  sarcoplasm  appearing  to  conduct  slowly,  and  the  more  highly  differentiated 
fibrillary  portion  of  the  fibre  rapidly.  In  general  the  process  appears  to  be 
more  rapid  and  vigorous  where  a  fibrillated  structure  is  observable.  Smooth 
muscle-tissue,  which  has  a  somewhat  simple  structure,  conducts  comparatively 
slowly;  striated  muscle,  which  is  more  highly  differentiated,  more  rapidly,  and 
the  fibrillated  axis-cylinder  of  the  nerve-fibre,  most  rapidly  of  all. 

Protoplasmic  Continuity  is  Essential  to  Conduction.  —  Effect  of  a 
Break  in  Profoj>/<txin<<'  (.'out  I  nnily.  —  A  break  of  protoplasmic  continuity  in  any 
1  Biedermann  :  Elektrophysiologie,  1895,  S.  137. 


78  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

part  of  a  nerve-  or  muscle-fibre  acts  as  a  barrier  to  conduction.  If  a  nerve  be  cut 
through,  the  irritability  and  conductivity  remain  for  a  considerable  time  in  the 
severed  extremities,  but  communication  between  them  is  lost,  and  remains  absent 
however  well  the  cut  extremities  may  be  adjusted.  The  nerve-impulse  is  not 
transmitted  through  the  nerve-substance  as  electricity  is  transmitted  along  a 
wire :  join  the  cut  ends  of  a  wire,  and  the  contact  suffices  for  the  passage  of 
the  current ;  join  the  cut  ends  of  a  nerve,  and  the  nerve-impulse  cannot  pass. 
Any  severe  injury  to  a  nerve  alters  the  protoplasmic  structure  and  prevents 
the  chemical  and  physical  processes  through  which  conductivity  is  made 
possible.  It  is  probable  that  the  same  may  be  said  of  all  forms  of  liv- 
ing cells,  and  the  absence  of  protoplasmic  continuity  would  seem  to  be  an 
explanation  of  the  fact  that  nerve-  and  muscle-fibres  which  lie  close  together 
may  physiologically  act  as  separate  mechanisms. 

Even  in  the  case  of  apparently  homogeneous  protoplasm  there  is  probably 
a  definite  structural  relation  of  the  finest  particles,  and  upon  this  the  physi- 
ological properties  of  the  substance  depends.  Slight  physical  and  chemical 
alterations  suffice  to  change  the  rate  and  strength  of  the  conduction  process, 
and  the  power  to  conduct  is  altogether  lost  if  the  protoplasm  is  so  altered  that 
it  dies. 

The  relation  of  conductivity  to  structure  of  cell-protoplasm  is  illustrated  in 
the  effects  of  degeneration  and  regeneration  upon  the  physiological  properties  of 
the  nerve-fibre  (see  p.  69).  The  life  of  the  nerve-fibre  is  dependent  on  influ- 
ences exerted  upon  it  by  the  body  of  the  cell  of  which  it  is  a  branch.  When 
any  part  of  the  fibre  is  injured  it  loses  its  power  to  conduct,  and  the  portion 
of  the  fibre  separated  by  this  block  from  the  body  of  the  cell  soon  dies.  The 
irritability  and  conductivity  are  wholly  lost  at  the  end  of  a  period  varying 
from  four  days  to  several  weeks,  the  time  differing  in  different  kinds  of  nerves, 
and  the  fibre  begins  to  undergo  degeneration.  The  axis-cylinder  and  the 
myelin  are  seen  to  break  up  and  are  then  absorbed,  apparently  with  the 
assistance  of  the  nuclei  which  normally  lie  just  inside  the  neurilemma, 
and  which  at  this  time  proliferate  greatly  and  come  to  occupy  most  of 
the  lumen  of  the  tube.  The  process  of  absorption  is  nearly  complete  at 
the  end  of  a  fortnight  after  the  injuy.  Under  suitable  conditions,  however, 
regeneration  may  occur,  and  as  this  takes  place  there  is  a  recovery  of  physi- 
ological activities.  The  order  in  which  conductivity  and  irritability  return  is 
instructive.  Howell  and  Huber 1  made  a  careful  study  of  this  subject.  They 
found  that  the  many  nuclei  which  develop  during  degeneration  are  apparently 
the  source  of  new  protoplasm,  which  is  seen  to  accumulate  in  the  old  sheath  until 
a  continuous  band  of  protoplasm  is  formed.  About  this  thread  of  protoplasm 
a  new  membranous  sheath  develops,  and  thus  is  built  up  what  closely  resembles 
an  embryonic  nerve-fibre.  The  embryonic  fibre  formed  in  the  peripheral  end 
of  the  regenerating  nerve  joins  that  of  the  central  end  in.  the  cicatricial  tissue 
which  has  been  deposited  at  the  point  of  injury.  Thus  a  temporary  nerve- 
fibre  is  formed  and  united  to  the  undegenerated  part  of  the  old  fibre,  and  this 
1  Journal  of  Physiology,  1892,  vol.  xiii.  p.  381. 


GENERAL    PHYSIOLOGY   OF   MUSCLE  AND   NERVE.       79 

new  structure,  though  possessing  neither  myelin  nor  axis-rylindrr,  is  found  to 
be  capable  of  conduction  and  to  have  a  low  form  of  irritability,  being  ex- 
citable to  violent  mechanical  stimuli  but  not  to  induction  currents.  The  pmvn- 
of  conduction  appears  to  return  before  irritability,  and  may  be  observed  first 
at  the  end  of  the  third  week.  Apparently  sensation  is  recovered  before  the 
power  of  making  voluntary  movements;  this  difference  may  well  be  due,  not 
to  any  essential  difference  between  sensory  and  motor  fibres,  but  to  the  fact 
that  extra  time  is  required  for  the  motor  fibres  to  make  connection  with  the 
muscle.  The  embryonic  fibre  gradually  gives  place  to  the  adult  fibre,  new 
myelin  being  formed  all  along  the  fibre,  and  a  new  axis-cylinder  growing  down 
from  the  old  axis-cylinder.  As  the  axis-cylinder  grows  down,  the  irritability 
for  induction  shocks  is  recovered.  Many  months  may  be  necessary  for  the 
complete  recovery  of  function.  Langley  l  reports  that  medullated  fibres  of 
the  sympathetic,  if  cut,  regenerate  and  recover  the  power  to  function  before 
they  regain  a  medullary  sheath.  Such  experiments  show  the  axis-cylinder 
to  be  the  true  conducting  medium,  and  that  the  medullary  sheath  has  a  sub- 
ordinate function. 

The  same  is  true  of  muscle  as  of  nerve  protoplasm, — the  power  of  con- 
duction ceases  with  the  life  of  the  cell-substance ;  thus,  if  the  middle  part  of 
a  muscle-fibre  be  killed,  by  pressure,  heat,  or  some  chemical,  the  dead  proto- 
plasm acts  as  a  block  to  prevent  the  state  of  activity  which  may  be  excited  at 
one  end  from  being  transmitted  to  the  other,  and  the  conduction  power  is  only 
recovered  on  the  regeneration  of  the  injured  tissue. 

Isolated  Conduction  is  the  Rule. — (a)  Conduction  in  Nerve-trunks. — The 
axis-cylinders  of  the  many  fibres  which  run  side  by  side  in  a  nerve-trunk  are 
separated  from  each  other  by  the  neurilemma,  and  in  the  case  of  the  medullary 
nerves  by  the  myelin  substance  as  well,  so  that  there  is  not  even  contiguity, 
much  less  continuity  of  nerve-substance.  Thus  the  many  fibres  of  a  nerve- 
trunk,  some  afferent  and  others  efferent,  though  running  side  by  side,  conduct 
independently  of  one  another.  For  example,  if  the  skin  of  the  foot  be  pricked, 
the  excitation  of  its  sense-organs  is  communicated  to  sensory  nerve-fibres,  and 
is  transmitted  along  them  to  the  spinal  cord,  where  the  stimulus  awakens  cer- 
tain groups  of  cells  to  activity ;  these  cells  in  turn,  by  means  of  their  branches, 
the  motor  nerve-fibres,  transmit  the  condition  of  excitation  down  to  the  mus- 
cle-fibres of  the  legs,  which,  when  stimulated,  contract  and  withdraw  the  foot 
from  the  offending  irritant.  The  sensory  and  motor  nerves  concerned  in  this 
reflex  act  run  for  a  considerable  part  of  their  course  in  the  same  nerve-trunk, 
but  the  sensory  impulses  have  no  direct  effect  on  the  motor  nerve-fibres,  and 
the  roundabout  course  which  has  been  described  is  the  only  way  by  which 
they  can  influence  them'. 

Isolated  conduction  by  separate  fibres  and  their  branches  holds  good  within 
the  central  nervous  system,  as  elsewhere,  otherwise  we  could  scarcely  explain 
the  localization  of  sensations,  or  co-ordinated  movements. 

The  presence  of  a  medullary  sheath  is  not  essential  to  isolated  conduction, 
1  Journal  of  Physiology,  1897,  vol.  xxii.  p.  223. 


80  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

for  it  occurs  in  the  absence  of  this  sheath,  both  in  the  peripheral  nerves  and 
in  the  central  nervous  system.  The  large  class  of  non-raedullated  nerves 
have  the  power  of  isolated  conduction,  and  Donaldson  reports  that  new-born 
rats  can  make  co-ordinated  movements,  although  the  nerves  of  both  the 
peripheral  and  central  nervous  systems  do  not  acquire  a  medullary  sheath 
until  several  days  after  birth.  It  is  not  likely  that  the  neuroglia  cells  are 
essential  to  isolated  conduction  within  the  central  nervous  system,  for  this 
occurs  in  its  absence  in  the  peripheral  nerves.  Although  the  neurilemma,  by 
separating  the  axis-cylinders  of  adjacent  fibres,  may  make  the  insulation  more 
complete,  it  is  probably  not  the  real  cause  of  isolated  conduction.  A  break 
of  the  protoplasmic  continuity  of  the  nerve  protoplasm  stops  conduction,  and 
conduction  fails  wherever  protoplasmic  continuity  is  lacking. 

An  apparent  contradiction  to  the  rule  that  absolute  continuity  of  nervous 
matter  is  essential  to  conduction  by  nerves,  is  to  be  found  in  the  phenomenon 
known  as  "  Hering's  Paradoxical  Contraction."  This  will  be  explained 
later  (see  p.  157,  d). 

(6)  Distribution  of  Excitation  by  Branches  of  Nerves. — Nerve-fibres  some- 
times branch  in  their  passage  along  the  peripheral  nerves,  but  most  of  the 
branches  which  are  seen  to  be  given  off  from  the  nerve-trunks  are  composed 
of  bundles  of  nerve-fibres  which  have  merely  separated  off  from  the  rest. 
After  the  nerves  have  entered  a  peripheral  organ,  or  the  central  nervous  system, 
the  axis-cylinders  may  give  off  branches.  Thus  in  muscles,  and  to  a  still  greater 
degree  in  the  electric  organs  of  certain  fish,  the  nerve-fibre  and  its  axis-cylinder 
may  divide  again  and  again,  or  after  entering  the  spinal  cord  the  fibre  may  be 
seen  to  give  off  a  great  many  lateral  branches — collaterals,  as  they  are  called. 
It  is  not  known  whether  in  such  cases  the  fibrillae  of  the  axis-cylinder  give 
off  branches,  or  whether  they  simply  separate,  a  part  of  them  entering  the 
branch  while  the  rest  of  them  continue  on  in  the  main  fibre.  Though  the 
exciting  process  does  not  pass  from  fibre  to  fibre,  it  probably  involves  in  a 
greater  or  less  degree  all  the  elements  of  the  same  fibre,  and  passes  into  all  its 
branches.  It  is  evident  that  where  it  is  necessary  for  the  irritation  to  be 
localized,  branching  could  not  occur ;  but  where  a  more  general  distribution 
is  permissible,  especially  where  several  4 parts  of  an  organ  ought  to  act  at  the 
same  instant,  conduction  through  a  single  fibre  which  branches  freely  near  its 
termination  would  be  useful. 

(c)  Conduction  in  Muscles. — Each  fibre  of  the  muscles  which  move  the 
bones — the  skeletal  muscles,  as  they  are  sometimes  called — is  physiologically 
independent  of  the  rest.  The  sarcolemma  prevents  not  only  continuity,  but 
contiguity  of  the  muscle-substance  of  the  separate  fibres,  and  there  is  no  cross 
conduction  from  fibre  to  fibre.  That  this  is  so  is  proved  by  an  experiment,  such 
as  was  described  on  page  45,  in  which  unipolar  excitation  of  the  part  of  the 
fibres  of  a  curarized  sartorius  muscle  results  in  a  contraction  strictly  con- 
fined to  the  fibres  which  are  subjected  to  the  irritating  current.  Each  of  the 
separate  muscle-fibres  is  supplied  by  at  least  one  nerve-fibre  or  a  branch 
of  a  fibre,  and,  under  normal  conditions,  only  acts  when  stimulated  by 


GENERAL    PHYSIOLOGY   OF   MUSCLE   AND    NERVE.       81 

the  nerve.  Tn  the  case  of  plant-cells,  and  of  certain  forms  of  mus- 
cle-cells, about  which  there  is  a  more  or  less  definite  wall  or  sheath,  there 
arc  little  bridges  of  protoplasm  binding  the  cells  together.  For  example, 
Engelmann  describes  the  muscle  of  the  intestines  of  the  fly  as  composed  of 
striated  cells,  sheathed  by  sarcolemma,  except  where  bound  together  by  little 
branches  of  sarcoplasma,  which  may  act  as  conducting  wires  between  the  cells. 

There  are  certain  cells,  however,  which  have  been  supposed  to  be  exceptions 
to  the  rule  that  protoplasmic  continuity  is  essential  to  conduction.  The  stri- 
ated muscle-fibres  of  the  heart  are  quite  different  from  those  of  ordinary 
skeletal  muscles,  physiologically  as  well  as  anatomically.  They  are  stumpy, 
quadrangular  cells,  which  are  not  known  to  have  a  sarcolemma,  and  which 
are  united  not  only  by  their  broad  ends,  but  by  lateral  branches.  Engelmann 
and  lately  Porter  l  and  others  have  concluded  that  conduction  takes  place  in 
the  heart  from  cell  to  cell,  without  the  intervention  of  nerves,  and  may 
occur  in  all  directions.  This  question  is  considered  at  length  in  the  section 
on  the  conduction  of  excitation  in  the  heart. 

The  cells  of  the  contractile  substance  of  some  of  the  medusaB  (as  Aurelia), 
have  been  supposed  to  communicate  by  contiguity  rather  than  by  continuity. 
The  same  has  been  thought  to  be  the  case  with  many  forms  of  unstriated 
muscle-tissue ; 2  moreover,  there  are  groups  of  ciliated  cells,  the  members  of 
which  act  in  unison  although  they  have  not  been  found  to  be  connected  either 
directly  or  by  nerves.  These  cells  have  apparently  no  membranous  covering, 
and  though  living  as  independent  units,  are  so  related  that  a  condition  of 
activity  excited  in  one  seems  to  be  transmitted  to  the  rest  by  means  of  contact, 
or  through  the  mediation  of  cement-substance. 

From  what  has  been  said  it  will  be  seen  that  protoplasmic  continuity 
ensures  free  communication  between  different  cells ;  that  protoplasmic  con- 
tiguity, either  directly  or  through  the  mediation  of  the  cement-substance,  may 
possibly  permit  of  conduction  ;  but  that  normally  the  intervention  of  a  dif- 
ferent tissue,  even  as  delicate  as  the  sarcolemma,  suffices  to  cause  complete 
isolation  of  the  cell  from  its  neighbors.  Under  normal  conditions  there  may 
be  a  spread  of  excitation  from  muscle-fibre  to  muscle-fibre,  even  in  the  skel- 
etal muscles.  Kiihne's  experiment  with  the  sartorius  muscle  of  the  frog, 
described  on  page  45,  gives  a  good  proof  that  the  activity  of  a  striated 
muscle-fibre  is  not  normally  transmitted  to  its  neighbors ;  nevertheless, 
Kuhne3  has  found  that  if  the  extremities  of  two  sartorius  muscles  be 
pressed  firmly  together  by  a  suitable  clamp,  care  being  taken  that  the 
pressure  shall  not  be  enough  to  destroy  the  physiological  activity  of  the 
protoplasm,  excitation  of  one  muscle  may  cause  contraction  of  the  other. 
A  satisfactory  explanation  is  lacking. 

Biedermann 4  reported  that  when  a  frog's  muscle  was  partly  dried  a  slight 

1  Porter:  Journal  of  Experimental  Medicine,  1897,  ii.  p.  391  ;  American  Journal  of  Physi- 
ology, 1899,  ii.  p.  127.  •  Engelmann  :  Pfliiger's  Archir,  1871,  Bd.  iv. 

*  Kiihne  :    Untersuchungen  aus  der  physiologischen  Institute  in  Heidelberg,  1880,  Bd.  3,  S.  1. 

*  Biedermann :  Serichte  der  Wiener  Akademie,  1888,  Bd.  97,  Abth.  3,  S.  145. 

VOL.  II  — 6 


82  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

mechanical  excitation  of  one  part  of  it  might  lead  to  a  contraction  of  the 
whole.  Similarly,  a  partly  dried-up  frog  may  be  seen,  if  mechanically 
excited,  to  make  movements  simulating  life.  The  cause  of  these  movements, 
also,  is  not  understood.  Drying  of  the  muscle  in  its  early  stages  greatly 
increases  its  irritability  because  of  the  concentration  of  the  salts,  but  that 
does  not  account  for  the  loss  of  insulation. 

Transmission  of  Excitation  by  Means  of  End-organs. — In  spite  of  the 
rapid  advances  which  have  been  made  in  the  histology  and  physiology  of  the 
nervous  system  during  the  past  few  years,  we  are  still  in  doubt  as  to  the  exact 
way  that  the  axone,  the  exciting  branch  of  the  neurone,  stimulates  the  cell 

~    to   which    it    is    distributed.      In    many 

_  Vl____jL_^— --— — !    cases?  at  'least,  the  axone  terminates  in  an 

^yt/Vc  end-organ  which   is  physiologically  dif- 

r\~~* .  /.   '  ferent  from  the  rest  of  the  cell,  and  this 

end-organ   is  the   exciting  agent.      The 

/  ^ ,  d  relation  of  the  protoplasm  of  the  end- 

^V^-rW^.    /_  organ  to  the  protoplasm  of  the  cell  which 

v*u     j3         j    it  stimulates,  whether  one  of  continuity 

^^^  or  contiguity,   is  not  certain,  but  most 

,  histological    and    physiological    observa- 

FIG.  si.— Nerve-termination  in  voluntary     tions  are  distinctly  in  favor  of  the  latter 

muscle  of  the  rabbit,  stained  in  methylen-blue 

(intra  vitam),  fixed,  sectioned,  and  counter-      V16W. 

stained  in  alum  carmin.    A,  surface  view;  B,  The  physiology  of  the    end-Organs   of 

longitudinal  section  through  nerve-termina-  '  .         - 

tion    and  muscle-fibre;     C,  cross-section :    S,      motor      aXOUCS      distributed      to      striated 

sarcolemma;  n.  I.,  neurilemma.    (From  Text-      muscles  js  best  kllOWU. 

book  of  Histology,  Bohm  and  Davidoff,  revised 

by  G.  C.  Huber.      W.  B.  Sauiiders,  Philadel-  Fig.    31     shows    a     surface     View    and 

phia,  1900).  a  longitudinal    and  cross-section  of  the 

end-organ  of  an  axone  supplying  a  voluntary  muscle  of  a  rabbit.  The  axis- 
cylinder  loses  its  medullary  sheath  shortly  before  reaching  the  fibre,  and  the 
neurilemma  becomes  continuous  with  the  sarcolemma,  so  that  the  axis-cylin- 
der on  penetrating  the  sarcolemma  comes  into  direct  contact  with  the  sarco- 
plasma of  the  muscle.  The  sarcoplasma  is  heaped  together  at  this  place, 
making  a  little  mound,  and  the  axis-cylinder,  after  dividing  into  a  number 
of  fine  terminal  twigs,  ends  in  the  midst  of  this  mass  of  sarcoplasma.  Evi- 
dently the  nerve  and  muscle  protoplasm  come  into  very  close  relation.  On 
the  other  hand,  nerve  and  muscle  protoplasm  retain  each  its  peculiar  reaction 
to  staining-fluids,  and  as  far  as  these  chemical  reactions  can  show  each  main- 
tains its  peculiar  chemical  and  histological  structure.  Moreover,  the  results 
of  physiological  experimentation  have  shown  that,  although  no  definite  histo- 
logical boundary  has  been  found  between  the  axone  and  its  terminal  organ, 
the  exciting  organ  must  be  considered  to  be  a  specially  differentiated  struct- 
ure, differing  widely  from  the  rest  of  the  neurone. 

The  motor  end-organ  uses  up  more  time  in  the  excitation  of  the  muscle 
than  would  be  required  for  transmission  of  the  excitation  through  a  like 
amount  of  nerve-  or  muscle-substance.  It  is  found  by  experiment  that  a 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.       83 

muscle  does  not  contract  so  quickly  if  it  be  excited  through  its  nerve  as  when 
stimulated  directly.  Part  of  the  lost  time  is  spent  in  transmission  of  the 
excitation  through  the  nerve ;  but  after  allowance  has  been  made  for  this  loss 
there  is  a  balance  to  be  accounted  for,  and  this  is  credited  to  the  motor  end- 
organ.  The  time  used  by  the  motor  end-plate  is  found  to  be  0.0032  second.1 

Motor  end-organs  are,  as  we  have  seen,  poisoned  by  curara  (see  p.  26)  and 
a  number  of  other  drugs  which  have  little  influence  on  the  rest  of  the  axone 
or  on  the  muscle. 

If  a  muscle  is  continuously  excited  for  a  considerable  time  by  irritants 
applied  to  its  nerve,  it  will  at  last  cease  to  contract.  Direct  excitation  shows 
that,  though  weakened,  it  is  still  capable  of  contraction,  and  we  know  that 
the  nerve-fibre  does  not  fatigue.  The  cessation  of  contraction  is  due  to 
fatigue  of  the  motor  ends. 

The  motor  end-organ  is  found  to  lose  its  vitality  quicker  than  the  muscle 
or  the  nerve-fibre,  if  it  be  deprived  of  its  normal  blood-supply. 

If  a  motor  nerve  be  cut,  the  part  of  the  axone  separated  from  the  body  of 
the  nerve-cell  and  the  terminal  organ  degenerates,  but  the  degeneration  proc- 
ess stops  at  the  muscle. 

These  facts  show  that  the  motor  end-organ  differs  physiologically  in  many 
respects  from  the  rest  of  the  axone  and  from  the  muscle.  Moreover,  they 
favor  the  idea  that  excitation  is  not  conducted  directly  from  nerve  to  muscle 
protoplasm.  That  this  is  the  case  is  also  made  probable  by  the  fact  that  though 
a  condition  of  excitation  is  transmitted  in  both  directions  through  nerve  and 
muscle  protoplasm  as  long  as  there  is  continuity,  a  condition  of  excitation  in 
muscle  substance  does  not  appear  to  be  transmitted  to  the  motor  nerve.  Ap- 
parently the  protoplasm  of  the  end-organ  and  the  muscle  are  in  contact,  but 
are  not  physiologically  continuous,  and  excitation  of  muscle  protoplasm  by  the 
end-organ  occurs  through  some  special  process.  Various  views  have  been 
advanced  with  reference  to  the  probable  nature  of  such  a  process,  but  as  no 
one  of  them  has  received  general  acceptance  they  need  not  be  dwelt  upon  here. 
One  point  more,  of  interest  in  this  connection,  is  the  fact  that  it  is  the  sarco- 
plasma  rather  than  the  fibrillary  elements  of  the  muscle  that  comes  in  contact 
with  the  nerve  end-organ,  which  would  seem  to  show  that  this  substance  is 
capable  of  being  excited  and  conducting  the  excitation.  If  this  be  true  of 
muscle  substance,  it  is  likely  that  the  semi-fluid  part  of  the  protoplasm  of  the 
nerve,  as  well  as  perhaps  the  fibrillary  part,  may  have  the  power  of  conduction. 

As  a  result  of  a  series  of  remarkable  histological  investigations  on  the 
anatomy  of  the  nervous  system,  the  view  has  come  to  be  generally  accepted, 
that  the  afferent  nerve-fibres  entering  the  spinal  cord  do  not  communicate 
directly  with  the  nerve-cells,  but  terminate  in  brush-like  endings  in  close 
contact  with  some  part  of  the  cells  which  they  excite.  A  similar  arrange- 
ment has  been  found  wherever  nerve-cells  are  excited  to  action  by  nerve- 
fibres.  As  in  the  case  of  the  motor  end-organ,  it  has  remained  a  matter  of 
doubt  whether  the  brush-like  ends  of  the  axones  should  be  considered  to  be 
1  Bernstein:  Archir  fur  Anatomic  und  Physioloyie,  1882,  S.  329. 


84  AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 

in  contact  with  the  bodies  and  dendrites  of  the  cells  to  be  excited,  and  whether 
this  relation  would  be  sufficiently  close  for  a  transmission  of  excitation,  or 
whether  they  should  be  considered  as  specially  differentiated  exciting  mechan- 
isms, which  do  not  simply  transmit  the  condition  of  excitation  by  a  proc- 
ess of  conduction,  but  which  develop  a  special  form  of  physiological  stimulus, 
and  through  this  excite  the  second  neurone  to  activity. 

Of  late,  certain  histologists  claim  to  have  traced  the  fibrilla}  of  the  axone 
of  one  neurone  into  the  cell-body  of  another  neurone,  and  have  even  suggested 
that  the  nerve  impulse  from  the  first  might  be  transmitted  through  the  cell- 
body  of  the  second  and  into  its  branches  without  the  intervention  of  the  pro- 
toplasm of  the  body  of  the  cell. 

It  is  possible  that  in  some  cases  the  axone  of  the  exciting  neurone  may, 
instead  of  ending  close  to  the  neurone  to  be  excited,  penetrate  it  and  end  in 
its  substance,  just  as  the  motor  end-organ  penetrates  into  the  sarcoplasma 
of  the  muscle-fibre.  This  could  happen,  and  yet  the  protoplasms  of  the  two 
cells  might  preserve  their  individuality. 

There  are  many  facts  which  show  that,  physiologically  at  least,  the  two 
neurones  act  as  wholly  independent  mechanisms.  These  will  be  dealt  with 
more  at  length  in  the  section  devoted  to  the  physiology  of  the  central  nervous 
system.  Suffice  it  to  say,  the  end-brush  at  the  extremity  of  the  axone  can 
excite  the  cell  body  of  another  neurone,  but  cannot  be  excited  by  it.  A 
reflex  act  involving  only  two  neurones  requires  more  time  than  could  be 
used  in  simple  conduction  through  the  two  cells.  The  character  of  the 
impulse  sent  out  of  the  spinal  cord  by  the  efferent  cell  may  be  very  different 
from  that  passing  in  along  the  afferent  cell — e.  g.,  the  efferent  impulse  may  be 
stronger  than  the  afferent ;  the  strength  of  efferent  discharge  may  vary  greatly 
within  short  intervals  of  time  even  when  the  strength  of  the  afferent  impulses 
remains  the  same ;  weak  afferent  impulses  may,  by  summation,  lead  to  a 
strong  efferent  discharge,  and  continuous  afferent  stimulation  may  awaken 
rhythmic  efferent  discharges. 

In  short,  physiological  facts  are  all  opposed  to  the  idea  that  there  is  con- 
tinuity of  protoplasm  of  different  nerve-cells,  and  in  favor  of  the  view  that  the 
end-brush,  like  the  motor  end-plate,  acts  as  a  specialized  exciting  mechanism. 

Conduction  in  Both  Directions. — (a)  In  Muscle. — Wherever  proto- 
plasmic continuity  exists,  conductivity  would  seem  to  be  possible ;  moreover, 
the  active  change  excited  by  an  irritant  would  seem  to  be  able  to  pass  in  all 
directions,  though  whether  with  the  same  facility  is  not  known.  Where  the 
spread  of  the  excitatory  process  is  accompanied  by  a  change  in  form,  as  is  the 
case  in  many  of  the  lower  organisms  and  in  muscle-tissue,  it  is  not  difficult  to 
trace  the  process.  The  rate  at  which  the  excitation  spreads  through  the  irrita- 
ble substance  is  very  rapid,  and  special  arrangements  have  to  be  employed  to 
follow  it,  but  the  change  is  not  so  swift  that  its  course  cannot  be  accurately 
determined.  It  has  been  found  that  if  a  muscle-fibre  be  stimulated,  as  nor- 
mally, by  a  nerve-fibre,  the  active  condition  produced  at  the  point  of  stimula- 
tion spreads  along  the  muscle-fibre  in  both  directions  to  its  extremities ;  if  the 


GENERAL   PHYSIOLOGY   OF  MUSCLE    AND   NERVE.       85 

fibre  be  artificially  irritated  at  either  end,  the  exciting  change  runs  the  length 
of  the  fibre,  regardless  of  the  direction,  and  stimulates  every  part  of  it  to  con- 
traction. 

(6)  In  Nerves. — In  the  cases  of  nerves  where  excitation  is  accompanied  by 
no  visible  manifestation  of  activity,  a  definite  answer  to  the  question  is  not  so 
readily  obtained.  As  long  as  a  nerve  is  within  the  normal  body,  the  activity 
of  the  nerve-fibre  can  only  be  estimated  from  the  response  of  the  cell  which  the 
nerve-fibre  excites,  and  there  is  such  an  organ  only  at  one  extremity  of  the  fibre. 

Paul  Bert  made  a  well-known  experiment,  in  which  he  tried  to  reverse  a 
sensory  nerve  in  the  living  animal.  He  succeeded  in  bringing  about  union 
of  the  end  of  the  tail  of  a  rat  with  the  tissues  of  the  back,  and  found,  when 
the  union  was  complete,  after  the  tail  was  cut  off  at  its  base,  it  was  still  capa- 
ble of  giving  sensations  of  pain.  The  experiment  failed  to  throw  light  on 
the  problem,  however,  for  we  now  know  that  the  peripheral  part  of  the  cut 
nerve  dies,  and  the  conduction  power  manifested  in  this  case  was  dependent 
on  new  axis-cylinders  which  had  grown  down  from  the  central  nerve-stump 
(see  p.  79). 

Efforts  have  been  made  to  elucidate  the  problem  by  attempting  to  unite 
the  central  part  of  a  cut  sensory  nerve  with  the  peripheral  part  of  a  divided 
motor  nerve,  and  observing,  after  the  healing  was  complete,  whether  excita- 
tion of  the  sensory  nerve  caused  movements  in  the  part  supplied  by  the 
motor  nerve.  Most  of  these  experiments  have  given  doubtful  results,  but 
lately  Budgett  and  Green  l  have  succeeded  where  others  have  failed,  and 
have  made  cut  sensory  fibres  grow  down  the  degenerated  trunk  of  a  motor 
nerve,  and  connect  with  muscle-fibres,  so  that  the  muscle  contracted  when 
the  peripheral  end  of  the  sensory  fibres  was  stimulated.  The  impulse  went 
up  the  old  sensory  fibres,  and  then  down  the  newly  developed  fibres  in  the 
old  motor  trunk.  Their  method  was  to  cut  the  left  pneumogastric  nerve 
between  the  ganglion  and  the  cranium,  and  to  suture  its  peripheral  cut  end 
to  the  peripheral  cut  end  of  the  hypoglossal.  All  the  fibres  of  the  hypoglossal 
and  the  efferent  fibres  of  the  pneumogastric  must  have  degenerated,  because 
these  fibres  were  separated  from  the  bodies  of  the  cells  of  which  they  were 
branches.  The  sensory  fibres  of  the  pneumogastric,  on  the  other  hand,  be- 
cause still  in  connection  with  the  nerve-cells  of  the  ganglion,  continued  to 
live,  and  the  part  connected  with  the  peripheral  stump  of  the  cut  hypoglossal 
grew  down  this  nerve  and  came  into  relation  with  the  muscles  of  the  tongue. 

Two  or  three  months  after  the  operation  the  left  pneumogastric  was  divided 
just  above  the  thorax,  and  the  combined  vago-hypoglossal  nerve,  together 
with  the  tongue,  was  excised.  When  the  peripheral  end  of  the  pneumogastric 
was  excited  the  muscles  of  the  tongue  were  seen  to  contract.  Mechanical  as 
well  as  electrical  stimuli  were  effective,  and  there  would  seem  to  be  no  escape 
from  the  conclusion  that  the  sensory  fibres  of  the  pneumogastric  had  con- 
ducted the  impulse  centripetally  as  far  as  the  ganglion,  and  then  centrifugally 
down  to  the  muscle  of  the  tongue. 

1  Budgett  and  Green:   American  Journal  of  Physiology,  1899,  iii.  p.  115. 


86  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

There  is,  however,  an  entirely  different  method  of  experimentation  which 
seems  to  prove  that  nerve-,  like  muscle-protoplasm,  can  conduct  in  both  direc- 
tions. This  method  is  based  on  the  fact  that  though  nerve-fibres  rarely  branch 
in  the  peripheral  nerve-trunks  on  their  way  to  an  organ,  they  may  divide  very 
freely  after  reaching  it.  Such  branchings  of  fibres  occur  in  muscle,  and  Kuehne1 
found  that  if  one  of  these  branches  was  stimulated,  the  irritation  passed  up 
the  branch  to  the  nerve-fibre  and  then  down  the  other  branches  to  the  muscle. 
For  example,  he  split  the  end  of  the  sartorius  muscle  of  a 
frog  by  a  longitudinal  cut,  and  then  found  on  exciting  one 
of  the  slips  that  the  other  contracted  (see  Fig.  32).  Since 
cross  conduction  between  striated  muscle-fibres  does  not 
occur,  no  other  explanation  presents  itself.  Perhaps  a  still 
more  striking  example  is  to  be  found  in  an  experiment  of 
Babuchin 2  on  the  nerve  of  the  electric  organ  of  an  electric 
fish,  the  Malopterurus.  The  organ,  consisting  of  many 
thousand  plates,  is  supplied  by  a  single  enormous  nerve- 
fibre  which  after  entering  the  organ  divides  very  freely  so 

FIG.  32.— Kuehne's  /  / 

preparation  of  sarto-      as  to  supply  every  plate.     In  this  case  mechanical  stimu- 
rius  to  show  double      ]  $        f  th         t    T      d    f  f  th        t  branches  of  the 

conduction  in  nerve. 

nerve,  sufficed  to  cause  an  electric  discharge  of  the  whole 
organ.  The  irritation  must  have  passed  backward  along  the  irritated  branch 
until  the  main  trunk  was  reached  and  then  in  the  usual  direction  down  the 
other  branches  to  the  electric  plates. 

Still  another  method  is  that  which  was  employed  by  Du  Bois-Reymond,3 
on  the  fibres  of  the  spinal  nerve-roots.  When  a  nerve  is  excited  to  action  it 
undergoes  a  change  in  electrical  condition,  and  this  change  progresses  along 
the  fibre  at  the  same  rate  and  in  same  direction  as  the  nerve-impulse.  This 
electrical  change,  though  entirely  different  from  the  nerve-impulse  itself,  can 
be  taken  as  an  indication  of  the  direction  of  movement  of  the  process  of 
conduction.  Du  Bois-Reymond  found  that  if  he  stimulated  the  afferent  fibres 
of  the  posterior  spinal  nerve-roots  of  the  sciatic  nerve  of  the  frog,  a  "  nega- 
tive variation  current,"  as  the  current  resulting  from  the  change  in  the  elec- 
trical condition  of  the  nerve  is  called,  passed  down  the  nerve  in  a  direction 
opposite  to  that  which  the  normal  impulse  takes.  Further,  it  was  found 
that  if  the  sciatic  nerve  was  excited,  a  negative  variation  current  could  be 
detected  in  the  anterior  as  well  as  the  posterior  roots.  Normally  the  irritation 
only  passes  up  the  posterior  roots  and  down  the  anterior,  for  normally  the 
sensory  fibres  of  the  posterior  roots  are  excited  only  at  the  peripheral  end  and 
the  motor  fibres  of  the  anterior  roots  only  at  the  central  end.  The  experiment 
showed  both  sensory  and  motor  fibres  to  be  capable  of  conducting  in  both 
directions.  Normally  a  nerve  is  stimulated  only  at  one  end,  and  therefore 
conducts  in  only  one  direction. 

1  Archivfiir  Anatomic  und  Physiologic,  1859,  S.  595. 

3  Ibid.,  1877,  S.  262. 

3  Thierische  Electricitdt,  1849,  Bd.  ii.  S.  587. 


GENERAL    PHYSIOLOGY   OF   MUSCLE   AND    NERVE.        87 

Rate  of  Conduction. — The  activity  of  the  conduction  process  varies 
greatly  in  different  tissues.  The  nerves  of  warm-blooded  animals  conduct  nioiv 
rapidly  than  those  of  cold;  in  a  given  animal  the  nerve-fibres  conduct  more 
rapidly  than  muscle-fibres ;  striated  muscle  conducts  more  rapidly  than  smooth 
muscle;  and  even  within  a  single  cell  different  portions  may  transmit  the  ex- 
citing process  at  different  rates ;  thus  the  myoid  substance  of  the  contractile  fibres 
of  one  of  the  rhizopods  conducts  more  rapidly  than  the  less  highly  differen- 
tiated protoplasm  of  the  cell.  In  general,  it  may  be  said  that,  "  the  power  to 
conduct  increases  with  increase  of  mobility  and  sensitiveness  to  external  irri- 
tants, a  fact  which  reveals  itself  in  the  protozoa,  by  a  comparison  of  the  slowly 
moving  rhizopods  with  the  lively  flagellata  and  eiliata."1  A  study  of  different 
classes  of  muscle-tissue  supports  this  view. 

(a)  Rate  of  Conduction  in  Muscles. — The  conduction  process  is  invisible, 
hence  we  estimate  its  strength  and  rate  by  its  effects.  It  is  most  readily  fol- 
lowed in  such  mechanisms  as  muscle,  where  the  conducting  medium  itself 
undergoes  a  change  of  form  as  the  exciting  influence  passes  along  it. 

Rate  of  Transmission  of  Wave  of  Contraction. — If  a  muscle  be  excited  to 
action  by  an  irritant  applied  to  one  end,  it  does  not  contract  at  once  as  a  whole, 
but  the  change  of  form  starts  at  the  point  which  is  irritated  and  spreads  thence 
the  length  of  the  fibres.  At  the  same  time  that  the  muscle  shortens  it 
thickens,  and  under  certain  conditions  the  swelling  of  the  muscle  can  be  seen 
to  travel  from  the  end  which  is  excited  to  the  further  extremity.  In  the  case 
of  normal,  active,  striated  muscle,  the  rate  at  which  the  change  of  form  spreads 
over  the  muscle  is  far  too  rapid  to  be  followed  by  the  eye,  and  hence  the 
muscle  appears  to  act  as  a  whole.  By  suitable  recording  mechanisms,  evidence 
can  be  obtained  of  the  rate  at  which  the  exciting  influence  and  contraction  pro- 
cess pass  along  the  fibre.  Thus  two  levers  can  be  so  placed  as  to  rest  on  the 
two  extremities  of  a  muscle,  at  the  same  time  that  the  free  ends  of  the  levers 
touch  a  revolving  cylinder,  the  surface  of  which  is  covered  with  paper  black- 
ened with  lampblack.  The  writing-point  of  one  lever  must  be  directly  under 
the  point  of  the  other.  If,  when  the  cylinder  is  revolving,  one  end  of  the  mus- 
cle be  stimulated,  the  record  will 
show  that  the  lever  resting  on  that 
part  is  the  first  to  move,  making 
it  evident  that  that  part  of  the  mus- 
cle begins  to  thicken  first,  and  that 
the  contraction  does  not  begin  at 
the  further  extremity  of  the  mus- 
cle until  somewhat  later.  The  re- 
cord given  in  Figure  33  was  ob-  FIG.  33.— Rate  of  conduction  of  the  contraction  pro- 

tained  in  a  similar  experiment,  but 

One   in    which    the    Contraction    of       fork  waves  record  Tfo  second  (after  Marey). 

the  muscle  was  registered  by  the 

pince  myographique  and  recording  tambour  of  Marey  (see  Fig.  34). 
1  Biedermann :  Eleklrophysioloyie,  1895,  Bd.  i.  S.  124. 


88  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Bernstein l  measured  the  rate  at  which  the  irritating  process  is  transmitted 
along  the  muscle  by  recording  the  latent  period,  the  time  elapsing  between  the 


FIG.  84.— Method  of  recording  the  rate  of  passage  of  the  contraction-  process  along  a  muscle  (after 
Marey).  The  movements  of  the  muscle  are  recorded  by  means  of  air-transmission.  The  pince  myo- 
graphique  consists  of  two  light  bars,  the  upper  of  which  acts  as  a  lever,  moving  freely  on  an  axis  sup- 
ported by  the  lower.  When  the  free  end  of  the  upper  bar  is  raised,  the  other  end  presses  down  on  a 
delicate  rubber  membrane  which  covers  a  little  metal  capsule,-which  is  carried  on  the  corresponding 
extremity  of  the  lower  bar.  The  capsule  is  in  air-communication,  by  a  stiff-walled  rubber  tube,  with 
another  capsule  which  is  similarly  covered  with  rubber  membrane.  A  light  lever  is  connected  with  the 
membrane  of  the  second  tambour,  and  records  its  movements  on  the  surface  of  a  revolving  cylinder. 
The  muscle  is  placed  between  the  free  ends  of  the  bars  of  the  pince  myographique,  and,  when  the  muscle 
thickens  in  contraction,  it  raises  one  end  of  the  lever,  depresses  the  membrane  at  the  other  end,  and 
drives  air  into  the  recording  tambour,  and  thus,  by  automatically  raising  the  writing-point,  records  its 
change  in  form  on  the  cylinder. 

moment  of  irritation  and  the  beginning  of  the  contraction  (see  p.  102).  A 
lever  was  so  connected  with  one  end  of  the  muscle  as  to  record  the  instant  that 
it  began  to  thicken.  The  muscle  was  stimulated  in  one  experiment  at  the  end 
from  which  the  record  of  its  contraction  was  taken,  and  in  another  immediately 
following  experiment  it  was  stimulated  near  the  other  end.  The  distance 
between  the  stimulated  points  being  known,  the  rate  of  transmission  was 
reckoned  from  the  difference  in  the  latent  periods.  In  his  experiments  he 
found  the  rate  of  conduction  in  the  semimernbranosus  of  the  frog  to  be  from 
3.2  to  4.4  meters  per  second.  Hermann  found  the  rate  to  be  2.7  meters  for 
the  curarized  sartorius  of  the  frog.  The  results  obtained  by  Abey  and  some 
others  are  a  little  lower,  but  probably  3  meters  per  second  can  be  accepted  as 
the  average  normal  rate  for  frog's  muscle. 

Length  of  Wave. — By  such  experiments  it  becomes  obvious  that  the  con- 
traction process  passes  over  the  muscle,  in  the  form  of  a  wave.  In  an  experi- 
ment, such  as  Bernstein's,  in  which  the  thickening  of  the  muscle  is  recorded, 
we  can  determine  from  the  length  of  the  curve  written  by  the  contracting 
muscle  how  long  the  contraction  remains  at  a  given  place.  Knowing  this, 
and  the  rate  at  which  the  process  passes  along  the  fibre,  we  can  estimate  the 
length  of  the  contraction  wave,  just  as  we  could  reckon  the  length  of  a  train 

1  Untersuchungen  ilber  die  elektrische  Erregung  von  Muskeln  und  Nerven,  1871,  S.  79. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND   NERVE.       89 

of  cars  if  we  knew  how  fast  it  was  moving  and  how  long  it  required  to  pass 
a  given  station.  Thus,  if  the  contraction  is  found  to  last  at  a  given  point 
on  the  muscle  0.1  second,  and  the  rate  at  which  the  contraction  process  is 
travelling  is  3000  millimeters  per  second,  the  length  of  the  wave  is  300  milli- 
meters. According  to  Bernstein's  determinations,  the  length  of  the  wave  of 
contraction  in  a  frog's  striated  muscle  is  from  198-380  millimeters.  The 
length  of  a  striated  muscle-fibre  is,  at  the  most,  scarcely  more  than  40  milli- 
meters, and  normally  the  muscle-fibre  is  stimulated,  not  as  in  the  above  ex- 
periment at  one  end,  but  near  its  centre,  at  the  point  where  the  nerve  joins 
it ;  the  irritation  process  spreads  along  the  fibre  in  both  directions  from  this 
point,  and  would  pass  over  the  distance  20  millimeters  so  quickly  that  practi- 
cally the  whole  muscle-fibre  would  be  in  the  same  phase  of  contraction  at  the 
same  time. 

Rate  of  Conduction  in  Different  Kinds  of  Muscle. — The  rate  of  conduction 
varies  very  considerably  in  the  muscles  of  different  animals,  and  in  different 
kinds  of  muscle  in  the  same  animal,  just  as  the  contraction  process  itself  dif- 
fers in  its  rate  and  strength. 

Meters  per  second. 

Smooth  muscle-fibres  of  the  ureters  of  the  rabbit  .    .    .    0.02-0.03  (Engelrhann). 

Muscle  of  the  heart- ventricle  of  the  frog 0.1  (Waller). 

Contractile  substance  of  medusae 0.5  (Waller). 

Neck-muscles  of  the  turtle 0.1  -0.5  (Hermann  and  Abey). 

Gracilis  and  semimembranosus  of  the  frog     .....    3.2  -4.4  (Bernstein). 

Cruralis  (red  muscle)  of  the  rabbit 3.4  (Kollet). 

Sterno-mastoid  of  the  dog 3.     -6  (Bernstein  and  Steiner). 

Semimembranosus  (white  muscle)  of  the  rabbit     ...    5.4  -11.4  (Rollet). 

Human  muscle 10.     -13  (Hermann). 

(6)  Rate  of  Conduction  in  Nerves. — Conductivity  is  most  highly  developed 
in  the  case  of  the  nerve-fibre.  The  distances  through  which  it  acts  and  the 
rapidity  of  the  process  excite  our  wonder.  The  process  is  accompanied  by  no 
visible  change  in  the  nerve-fibre  itself,  and  the  strength  and  rate  have  to  be 
estimated  by  the  effect  produced  on  the  organ  which  the  nerve  excites  to  action, 
or  by  the  change  which  takes  place  in  the  electrical  condition  of  the  nerve  as 
the  wave  of  excitation  sweeps  over  it. 

Rate  in  Motor  Nerves. — Helmholtz  was  the  first  to  measure  the  rate  of  con- 
duction in  nerves.1  Originally  he  employed  Pouillet's  method  for  measuring 
short  intervals  of  time.  The  arrangement  is  illustrated  in  Figure  35.  The 
moment  that  a  current  was  thrown  into  the  coils  of  a  galvanometer  (see  p. 
145)  the  current  in  the  primary  coil  of  an  induction  apparatus  was  broken 
and  the  nerve  connected  with  the  secondary  coil  received  a  shock.  An  instant 
after,  the  contraction  of  the  muscle  which  resulted  from  the  stimulation  of  the 
nerve  broke  the  galvanometer  circuit.  The  amount  of  deviation  of  the  magnet 
of  the  galvanometer  varied  with  the  time  that  the  circuit  remained  closed,  and 
therefore  could  be  taken  as  a  measure  of  the  interval  elapsing  between  the 
stimulation  of  the  nerve  and  the  contraction  of  the  muscle.  The  nerve  was 
1  Helmholtz:  Archivfiir  Anatomic  und  Physiologic,  1850,  S.  71-276;  1852,  S.  199. 


90  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

excited  in  two  succeeding  experiments  at  two  points,  at  a  known  distance  apart, 
and  the  difference  in  the  time  records  obtained  was  the  time  required  for  the 
transmission  of  the  nerve-impulse  through  this  distance. 


V  £V\/\y^rv/\> 


FIG.  35.— Method  of  estimating  rate  of  conduction  in  motor  nerve  of  frog,  as  used  ~by  Helmholtz.  The 
horizontal  bar  a-b  is  supported  on  an  axis  in  such  a  mariner  that  when  the  contact  is  made  at  a  it  is 
broken  at  6,  therefore  at  the  same  instant  a  current  is  made  in  the  galvanometer  circuit  g  and  opened  in. 
the  primary  circuit  of  the  induction  apparatus  p.  When  the  muscle  contracts,  the  galvanometer  circuit 
is  broken  at  c.  The  nerve  was  stimulated  in  two  successive  experiments  at  d  and  e. 

Later,  Helmholtz  devised  a  method  by  which  a  muscle  would  record  its 
contractions  on  a  rapidly  moving  surface,  and  employed  this  to  measure  the 
rate  of  conduction  in  motor  nerves.  He  stimulated  the  nerve  as  near  as 
possible  to  the  muscle  and  let  the  contraction  be  recorded ;  then  he  stimulated 
the  nerve  as  far  as  possible  from  the  muscle,  and  again  had  the  contraction 
recorded.  The  difference  in  time  between  the  moment  of  excitation  and  the 
beginning  of  the  contraction  in  the  two  experiments  was  due  to  the  difference 
in  the  distance  that  the  nerve-impulse  had  to  pass  in  the  two  cases,  and,  this 
distance  being  known,  the  rate  of  conduction  could  be  readily  calculated. 
By  this  means  he  found  the  rate  of  transmission  in  the  motor  nerves  of  the 
frog  to  be  27  meters  per  second.  In  similar  experiments  upon  men  he 
recorded  the  contractions  of  the  muscles  of  the  ball  of  the  thumb,  and  noted 
the  difference  in  the  time  of  the  beginning  of  the  contractions  when  the 
median  nerve  was  excited  through  the  skin  at  two  different  places.  He 
found  the  average  normal  rate  for  man  to  be  about  34  meters  per  second,  a 
rate  which  is  considerably  quicker  than  that  of  our  fastest  express  trains, 
but  a  million  times  less  than  the  rate  at  which  an  electric  current  is  trans- 
mitted along  a  wire.  These  determinations  are  still  accepted  as  approxi- 
mately correct  for  human  nerves,  although  they  are  found  to  vary  very  con- 
siderably under  different  conditions,  a  high  temperature  and  strong  irritation 
quickening  the  rate  to  90  or  more  meters  per  second,  while  cooling  may 
gradually  slow  the  rate  and  finally  stop  conduction.  Moreover,  considerable 
differences  exist  in  nerves  controlling  different  functions,  even  in  the  same 
animal.  Thus  Chauveau  gives  the  rate  for  the  fibres  of  the  vagus  nerve, 
which  supply  the  rapidly  contracting  striated  muscles  of  the  larynx,  as  66.7 
meters  per  second ;  and  the  rate  for  vagus  fibres,  controlling  the  slower 
smooth  muscles  of  the  oesophagus,  as  8.2  meters  per  second.  The  rate  of 
transmission  in  the  non-medullated  nerves  of  invertebrates  appears  to  be  still 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.        91 

slower ;  the  nerve  for  the  claw-muscles  of  the  lobster  conducts  at  a  rate  of 
from  6  to  12  meters  per  second,  according  as  the  temperature  is  low  or  high 
(Fredericq  and  Vandervelde).  The  rate  in  non-medullated  nerves  of  the 
Cephalopodia  is  3.5-5.5  meters  per  second  (Boruttau). 

Contrary  to  the  view  frequently  expressed  (Pfliiger1  and  others),  all  parts 
of  the  nerve  have  the  same  rate  of  conduction.2 

Rate  in  Sensory  Nerves. — We  have  no  definite  knowledge  of  the  rate  of 
conduction  in  sensory  nerves.  The  attempt  has  been  made  to  measure  it  by 
stimulating  the  sensory  fibres  of  a  nerve-trunk  at  two  different  points  and 
noting  the  difference  in  the  time  of  the  beginning  of  the  resulting  reflex  acts ; 
or,  in  experiments  on  men,  the  difference  in  the  length  of  the  reaction  time 
has  been  taken  as  an  indication.  By  reaction  time  is  meant  the  interval 
which  elapses  between  the  moment  that  the  irritant  is  applied  and  the  signal 
which  is  made  by  the  subject  as  soon  as  he  feels  the  sensation.  Oehl  found 
the  mean  normal  rate  of  conduction  in  the  sensory  nerves  of  men  to  be  36.6 
meters  per  second.3  Dolley  and  Cattell,4  by  employing  the  reaction-time 
method,  found  the  rate  for  the  sensory  fibres  of  the  median  nerve  of  one  of 
them  to  be  21.1  meters  per  second,  and  for  the  other  49.5  meters  per  second, 
while  the  posterior  tibial  nerve  gave  rates,  for  one  of  them  31.2  meters,  and 
for  the  other  64.9  meters.  They  attribute  these  wide  variations  in  part  to 
differences  in  the  effectiveness  of  the  irritant  at  different  parts  of  the  skin, 
but  chiefly  to  differences  in  the  activity  of  the  central  nervous  processes 
involved  in  the  act. 

Schelske5  observed  similar  differences  in  different  men — for  one  25.3 
meters,  for  another  32.6  meters,  and  for  still  another  31.05  meters  per 
second. 

In  spite  of  the  great  difficulty  of  getting  definite  measurements  in  experi- 
ments on  men,  we  may  conclude  from  the  work  of  these  and  other  observers 
that  the  rate  of  conduction  in  sensory  fibres  is  about  the  same  as  in  motor 
fibres ;  in  the  case  of  man  about  35  meters  per  second. 

Another  method  applicable  to  isolated  nerves  is  based  on  the  fact  that  the 
passage  of  the  nerve-impulse  along  a  nerve  is  accompanied  by  a  change  in 
its  electrical  condition.  The  rate  of  conduction  can  be  ascertained  by  finding 
the  rate  at  which  this  electrical  change  is  transmitted. 

Influences  which  Alter  the  Rate  and  Strength  of  the  Conduction-proc- 
ess.— (a)  Effect  of  Death-processes. — Normally,  the  rate  of  conduction  in  mus- 
cle-fibres is  so  rapid  that  the  whole  muscle  appears  to  contract  at  the  same  time ; 
but  there  are  certain  conditions  under  which  the  transmission  of  the  exciting 
influence  is  very  much  slowed,  or  even  altogether  prevented,  so  that  the  stimu- 
lation of  a  given  part  of  the  muscle  results  in  a  local  swelling,  or  welt,  limited 

1  Pfliiger:    Untersuchungen  uber  die  Physiologic  des  Electrotonus,  Berlin,  1859,  S.  465. 

2R.  du  Bois-Reymond  :  CeniraJblatt  fiir  Physiologic,  1899,  Bd.  xiii.  S.  513. 

3  Oehl :  Archives  italiennes  de  Biologic,  1895,  xxi.  3,  p.  401. 

4Dolle3r  and  Cattell :  Psychological  Review,  New  York  and  London,  1894,  i.  p.  159. 

5 Schelske:  Archiv fur  Anatomic und  Physiologic,  1864,  S.  151. 


92  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

to  the  excited  area.  When  a  muscle  is  dying,  the  rate  of  conduction  as  well 
as  the  rapidity  of  contraction  is  lessened.  The  muscles  of  warm-blooded  ani- 
mals exhibit  more  striking  differences  than  those  of  cold-blooded,  but  both  are 
similarly  affected.  If  a  dying  muscle  be  mechanically  stimulated,  as  by  a  direct 
blow,  a  localized  swelling  develops  at  the  place ;  and  if  the  muscle  be  stroked 
with  a  dull  instrument,  a  wave  of  contraction  may  be  seen  to  follow  the  instru- 
ment, the  contraction  being  quite  strictly  limited  to  the  excited  area,  so  that 
one  can  write  on  the  muscle.  The  strict  localization  of  the  contraction  to  the. 
irritated  parts  makes  it  evident  that  the  nerves  take  no  part  in  it,  hence  Schiff 
called  it  an  idio-muscular  contraction,  in  distinction  from  the  normal  neuro- 
muscular  contraction.  In  the  dying  nerve  as  in  the  dying  muscle  the  rate  of 
transmission  is  found  to  be  slowed. 

(6)  Effect  of  Mechanical  Conditions. — The  effect  of  pressure  to  lessen  the 
conduction-power  of  nerves  is  one  which  everyone  has  had  occasion  to  demon- 
strate on  himself.  For  example,  if  pressure  be  brought  to  bear  on  the  ulnar 
nerve  where  it  crosses  the  elbow,  the  region  supplied  by  the  nerve  becomes  numb, 
"goes  to  sleep,"  so  to  speak.  It  is  noticeable  that  only  a  slightly  greater 
effort  is  required  to  move  the  muscles,  at  a  time  when  no  sensations  are  received 
from  the  hand.  For  some  unexplained  reason  the  sensory  nerve-fibres  appear 
to  be  less  resistant  than  the  motor.  Gradually  applied  pressure  may  paralyze 
the  nerve  without  exciting  it,  but  on  the  removal  of  the  pressure  the  recovery 
of  function  of  the  sensory  fibres  is  accompanied  by  excitation  processes,  which 
are  felt  as  pricking  sensations  referred  to  the  region  supplied  by  the  nerve.  The 
exact  reason  of  the  loss  of  functional  power  caused  by  pressure  which  is  insuf- 
ficient to  produce  permanent  injury  is  not  altogether  clear.  Stretching  a  nerve 
may  act  to  lessen,  and  if  severe  destroy,  conductivity.  It  is  in  one  sense  another 
way  of  applying  pressure,  since  the  calibre  of  the  sheath  is  lessened  and  through 
the  fluids  pressure  is  brought  to  bear  on  the  axis-cylinder.  Of  course,  if  the 
stretching  were  excessive,  the  nerve-fibres  would  be  ruptured  and  degenerate. 

Whether  stretching  can  alter  the  rate  of  conduction  in  nerves  is  not  known. 
Apparently  it  does  not  do  so  in  muscles,  although  because  of  the  greater 
length  of  the  muscle  it  appears  to  do  so.1 

(c)  Effect  of  Temperature  on  Conduction. — Helmholtz  and  Baxt  found  that 
by  cooling  motor  nerves  they  could  lower  the  rate  of  conduction,  and  by  heat- 
ing them  they  could  increase  it  even  more  markedly.  By  altering  the  tem- 
perature of  the  motor  nerves  of  man,  they  observed  rates  varying  from  30  to 
90  meters  per  second.  The  rate  of  the  motor  nerves  of  other  animals  is  like- 
wise greatly  altered  by  heat  and  cold.  This  is  true  of  the  invertebrates  as  well 
as  the  vertebrates ;  the  rate  in  the  nerves  of  the  claw-muscles  of  the  lobster, 
for  example,  changes  from  6  to  12  meters  per  second  as  the  temperature  is 
varied  from  10°  to  20°  C.  Sensory  nerve-fibres  are  similarly  influenced  by 
temperature.  Oehl 2  found  by  cooling  and  heating  the  nerves  of  men,  varia- 
tions of  from  30  to  25  meters  per  second  on  cooling,  and  from  30  to  50  meters 

^chenck  :  Pfliiger's  Archiv,  1896,  Bd.  64,  S.  179. 

3 Oehl:  Archives  italiennes  de  Biologie,  1895,  xxiv.  p.  231. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND  NERVE.       93 

a  second  on  heating.  Both  the  sympathetic  and  the  vagus  nerve-fibres  in  the 
frog  have  their  influence  on  the  heart-beat  decreased  by  cold  and  increased  by 
heat.1  The  favorable  influence  of  heat  on  the  conduction  power  seems  common 
to  all  nerves,  but  only  within  certain  limits.  The  motor  fibres  of  the  sciatic  of 
the  frog  lose  their  power  to  conduct  at  41  °  to  44°  C.,  but  may  recover  the  power 
if  quickly  cooled  ;  if  the  temperature  has  reached  50°  C.  conductivity  is  per- 
manently lost. 

Nerves  of  like  function  in  different  animals  may  lose  the  power  of  conduc- 
tion at  different  temperatures.  Thus  the  motor  fibres  of  the  sciatic  nerve  of 
the  dog  cease  to  conduct  at  6°  C.,  those  of  the  cat  at  5°  to  3°  C.,  of  the  frog  at 
about  0°  C.  The  inhibitory  fibres  of  the  vagus  nerve  of  the  dog  show  dimin- 
ished activity  at  3°  C.,  and  become  wholly  inactive  at  0°  C. ;  the  inhibitory 
fibres  of  the  vagus  of  the  rabbit  become  inactive  at  15°  C. 

Different  kinds  of  fibres  in  the  same  nerve-trunk  may  be  differently  affected 
by  temperature,  and  this  difference  may  be  sufficiently  marked  in  some  cases  to 
be  used  as  a  means  of  distinguishing  them.2  For  example,  the  temperature 
limits  at  which  the  vaso-constrictor  fibres  of  the  sciatic  of  the  cat  can  conduct 
are  2°-3°  C.  to  47°  C.,  while  the  limits  for  the  dilator  fibres  are  both  lower 
,  and  higher  than  for  the  constrictors.  If  cold  be  applied  to  the  sciatic  nerve, 
the  fibres  supplying  the  extensor  muscles  seem  to  fail  before  those  which  in- 
nervate the  flexors. 

Further,  it  has  been  observed  that  if  cold  be  applied  locally  to  a  nerve,  the 
part  affected  loses  its  power  to  conduct,  and  acts  as  a  block  to  the  passage  of 
the  nerve-impulse  generated  in  another  part  of  the  nerve.  Application  of 
extreme  cold  to  the  ulnar  nerve  of  man  at  the  elbow  results  in  a  complete 
loss  of  feeling  in  the  parts  which  the  nerve  supplies.3  On  the  other  hand, 
the  strength  of  an  impulse  is  increased  by  passage  through  a  region  which  has 
been  warmed.  These  facts  remind  us  of  the  effect  of  heat  and  cold  on  the 
activity  of  other  forms  of  protoplasm  and  would  find  a  comparatively  easy 
explanation  were  we  content  to  look  upon  conduction  as  the  result  of  chemical 
change  in  the  axis-cylinder.  The  fact  that  conduction  does  not  cause  fatigue 
is  opposed  to  such  an  explanation,  and  so  we  take  refuge  in  the  idea  that  heat 
is  favorable  and  cold  unfavorable  to  molecular  activity  in  general. 

(d)  Effect  of  Chemicals  and  Drugs. — The  conductivity,  like  the  irritability, 
of  nerve  and  muscle  is  greatly  influenced  by  anything  which  alters  the  chemical 
constitution  of  active  substance.  In  general,  influences  which  increase  or 
decrease  the  one  have  a  similar  effect  upon  the  other.  There  are  important 
exceptions  to  the  rule,  however.  The  direct  application  of  alcohol,  ether, 
etc.,  to  the  nerve  may  destroy  the  conductivity  without  greatly  lessening  the 
irritability,  while  carbon  dioxide4  or  cocain  will  destroy  the  irritability  very 
much  sooner  than  the  conductivity.  Such  observations  suggest  that  con- 

1  Stewart :  Journal  of  Physiology,  1891,  vol.  xii.  No.  3,  p.  22. 

'Howell,  Budgett,  and  Leonard  :  Journal  of  Physiology,  vol.  xvi.  Nos.  3  and  4,  1894. 
3  Weir  Mitchell:  Injuries  of  Nerves  and  their  Consequences,  Philadelphia,  1872,  p.  59. 
*Griinhagen  :  Pjliiger's  Archiv,  1872,  vi.  S.  180. 


94  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

ductivity  is  dependent  on  other  properties  of  the  nerve  than  irritability,  and 
there  are  some  other  facts  pointing  in  the  same  direction  ;  for  example, 
regenerating  nerves  acquire  the  power  to  conduct  before  they  recover  their 
irritability.  The  usual  explanation  of  those  who  regard  conduction  as  due 
to  the  excitation  of  each  succeeding  part  of  the  nerve  by  the  one  just  pre- 
ceding it  is,  that  external  excitation  is  a  coarse  affair  as  compared  with  the 
normal  internal  excitation  process,  and  the  effect  of  the  former  may  be  lost 
when  the  latter  is  still  effective. 

(e)  Effect  of  a  Constant  Battery  Current. — A  constant  electric  current,  if 
allowed  to  flow  through  a  nerve  or  muscle,  not  only  alters  the  irritability,  but 
also  the  conductivity.  The  change  in  the  conductivity  affects  both  the 
strength  and  rate  of  the  conduction  process.  Von  Bezold 1  found  that  weak  and 
medium  currents  have  little  effect  on  the  conductivity,  but  that  strong  currents 
completely  destroy  the  power  of  the  nerve  to  transmit  the  nerve-impulse.  As 
the  strength  of  the  current  is  increased,  the  first  effect  is  observed  at  the  anode, 
and  shows  itself  in  a  slowing  of  the  passage  of  the  exciting  impulse.  This 
action  is  the  greater  the  more  of  the  nerve  exposed  to  the  current,  the  stronger 
the  current,  and  the  longer  it  is  closed.  The  loss  of  conduction  power  is  asso- 
ciated with  changes  at  the  place  where  the  current  enters  and  where  it  leaves 
the  nerve  rather  than  with  alterations  within  the  intrapolar  region.  Engelmann, 
in  his  experiments  on  the  smooth  muscle-fibres  of  the  ureter,  saw  a  decline  of 
power  of  conduction  at  the  anode  by  weak  currents,  which  when  the  strength 
of  the  current  was  increased  appeared  also  at  the  kathode ;  the  conductivity 
was  wholly  lost  at  both  poles  when  the  current  was  very  strong.  In  the  case 
of  a  striated  muscle,  such  as  tne  sartorius  of  the  frog,  the  kathode  has  been 
found  to  become  impassable  after  strong  currents  have  flowed  through  a  muscle 
for  a  considerable  time.  The  same  is  true  of  nerves. 

It  is  not  surprising  that  a  current  which  can  greatly  decrease  the  irritability 
at  the  anode,  and  even  inhibit  a  contraction  which  may  be  present  when  it  is 
applied,  should  be  found  to  decrease  the  conductivity  as  well,  but  that  the  con- 
ductivity should  be  decreased  at  the  kathode,  where  the  irritability  is  greatly 
increased,  was  not  to  be  expected.  Rutherford  2  found  that  with  weak  currents 
the  rate  of  the  conduction  power  at  the  kathode  was  increased  rather  than 
diminished,  and  that  it  was  only  when  strong  currents  acted  a  considerable 
time  that  the  conduction  power  lessened  at  the  kathode.  Biedermann  explains 
this  on  the  ground  that  the  increased  excitability  at  the  kathode  leads  in  the 
muscle  to  a  condition  of  latent  contraction  and  therefore  to  fatigue,  and  that 
it  is  this  which  lessens  the  conductivity.  The  lessened  power  to  conduct  con- 
tinues at  the  kathode  after  the  removal  of  the  current.  There  is  little  doubt 
that  fatigue  interferes  with  the  conduction  power  of  muscle,  but  this  explana- 
tion would  hardly  apply  to  nerves  which  are  not  known  to  fatigue  at  the  point 
of  stimulation,  i.  e.  if  we  limit  the  term  fatigue  to  changes  resulting  from 
physiological  activity.  Undoubtedly  chemical  and  physical  alterations  may 

1  Untersuchungen  iiber  die  ekktrische  Erregung  der  Nerven  und  Muskeln,  Leipzig,  1861. 

2  Journal  of  Anatomy  and  Physiology,  1867,  vol.  2,  p.  87, 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.       95 

occur  in  nerves  as  a  result  of  the  passage  of  an  electric  current  through  them, 
and  it  would  seem  as  if  the  loss  of  conductivity  which  they  show  when  sub- 
jected to  strong  currents  is  to  be  accounted  for  by  such  electrolytic  changes. 

The  changes  produced  in  the  conductivity  of  nerves  by  strong  currents 
explain  the  failure  of  the  closing  of  the  ascending  current  and  opening  of  the 
descending  current  to  irritate  the  muscle  (see  Pfluger's  law,  p.  50).  In  the 
former  case  the  anode  region  of  decreased  conductivity  intervenes  between  the 
kathode,  where  the  closing  stimulus  is  developed,  and  the  muscle.  In  the 
latter  case  the  irritation  developed  at  the  anode,  on  the  opening  of  the  current, 
is  unable  to  pass  the  region  of  decreased  conductivity  which  is  formed  at  the 
kathode,  and  which  persists  after  the  current  is  opened. 

Practical  Application  of  Alterations  produced  by  Battery  Currents. — The 
alterations  produced  by  strong  battery  currents  in  the  irritability  and  conduc- 
tivity of  nerves  and  muscles  may  be  made  use  of  by  the  physician.  If  the 
effect  of  only  one  pole  is  desired,  it  may  be  applied  as  a  small  electrode  im- 
mediately over  the  region  to  be  influenced,  while  the  other  pole  may  be  a  large 
electrode  placed  over  some  distant  part  of  the  body  where  there  are  no  import- 
ant organs.  The  size  of  the  electrodes  used  determines  the  density  of  the 
current  leaving  or  entering  the  body  and  consequently  the  intensity  of  its 
action.  The  application  of  the  anode  to  a  region  of  increased  excitability,  by 
decreasing  the  irritability,  may  for  the  time  lessen  irritation;  on  the  other 
hand  the  kathode  may  heighten  the  irritability  of  a  region  of  decreased 
excitability.  The  sending  of  a  strong  polarizing  current  through  a  motor 
nerve,  by  lessening  the  conductivity,  may  prevent  abnormal  motor  impulses 
from  reaching  muscles,  and  so  stop  harmful r"  cramps ;"  or  the  sending  of 
such  a  current  through  a  sensory  nerve  may,  during  the  flow  of  the  cur- 
rent, keep  painful  impulses  from  reaching  the  central  nervous  system.  In 
applying  a  strong  battery  current  to  lessen  irritability  or  conductivity  it 
must  be  remembered  that  the  after-effect  of  such  a  current  is  increased 
irritability. 

(/)  Effect  of  Conduction  and  Fatigue  of  Nerves. — Many  experiments  have 
been  made  in  the  hope  of  detecting  some  form  of  chemical  change  as  a  result 
of  conduction.  The  nerve  has  been  stimulated  for  many  hours  in  succession 
with  an  electric  current,  and  then  been  examined  with  the  utmost  care  to 
find  whether  there  had  been  an  accumulation  of  some  waste  product,  as 
carbon  dioxide,  or  some  other  acid  body.  The  gray  matter  of  the  spinal 
cord,  which  is  largely  composed  of  nerve-cells,  is  found  to  become  acid  as  a 
result  of  activity,1  but  this  cannot  be  found  to  be  the  case  with  the  white 
matter  of  the  cord,  which  is  chiefly  made  up  of  nerve-fibres,  nor  has  an  acid 
reaction  been  obtained  with  certainty  in  nerve-trunks.2 

1  Funke:  Archivfur  Anatomic  und  Physioloyie,  1859,  S.  835.    Ranker  Centralblatt filr  medicin- 
ische  Wissenschaft,  1868  and  1869. 

2  Heidenhain :  Studien  aus  dem  physiologischen  Inslitut  zu  Breslau,  ix.  S.  248 ;  Cenlralblatt  fur 
Medicin,  1868,  S.  833.     Tigerstedt:  "  Studien  iiber  mechanische  Nervenreizung,"  Acta  Societatis 
Scientiarum  Fennicce,  1880,  torn.  xi. 


96  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Not  only  has  an  attempt  to  discover  this  or  other  waste  products  which 
might  be  supposed  to  result  from  chemical  changes  within  the  nerve-fibre 
failed,  but  observers  have  been  unable  to  obtain  evidence  of  the  liberation 
of  heat,  which  one  would  expect  to  find  were  the  nerve-fibre  the  seat  of  chem- 
ical changes  during  the  process  of  conduction.1  Stewart  writes  :  "  Speaking 
quite  roughly,  I  think  we  may  say  that  in  the  nerves  of  rabbits  and  dogs  there 
is  not  even  a  rise  of  temperature  of  the  general  nerve-sheath  of  ^oW  of  a 
degree  during  excitation.7' 

Many  experiments  have  been  made  to  ascertain  whether  a  nerve  would 
fatigue  if  made  to  conduct  for  a  long  time.  Most  of  these  have  been  made 
upon  motor  nerves,  the  amount  of  contraction  of  the  muscle,  in  response  to  a 
definite  stimulus  applied  to  the  nerve,  being  taken  as  an  index  of  the  activity 
of  the  nerve.  Since  the  muscle  would  fatigue  if  stimulated  continuously  for 
a  long  time,  various  means  have  been  employed  to  block  the  nerve-impulse 
and  prevent  it  from  reaching  the  muscle,  except  at  the  beginning  and  end  of 
the  experiment.  This  block  has  been  established  by  passing  a  continuous 
current  through  the  nerve  near  the  muscle,  thus  inducing  an  electrotonic 
change  and  non-conducting  area ; 2  or  the  nerve-ends  were  poisoned  with 
curare  (see  p.  26),  and  the  nerve  excited  until  the  effect  of  the  drug  wore  off, 
and  the  nerve-impulse  was  able  to  reach  the  muscle ; 3  or  the  part  of  the  nerve 
near  the  muscle  was  temporarily  deprived  of  its  conducting  power  by  an 
anesthetic,  such  as  ether.  Another  method  of  experimentation  consisted  in 
using  the  negative  variation  current  of  a  nerve  (see  p.  150)  as  an  indication 
of  its  activity,  the  presence  of  the  current  being  observed  with  the  galvanom- 
eter.4 Other  experimenters  have  examined  the  vagus  nerve,  to  see  if  after 
long-continued  stimulation  it  was  still  capable  of  inhibiting  the  hearf,  the 
effect  of  the  stimulation  being  prevented  from  acting  on  the  heart  muscle 
during  the  experiment  by  atropin,5  or  by  cold,  applied  locally  to  the  nerve.6 
Still  another  method  was  to  study  the  effect  of  long-continued  stimulation  on 
the  secretory  fibres  of  the  chorda  tympani,  the  exciting  impulse  being  kept 
from  the  gland-cells  by  atropin.7  Most  of  these  experiments  have  yielded  nega- 
tive results,  and  it  is  doubtful  whether  nerves  are  fatigued  by  the  process  of 
conduction. 

These  results,  of  course,  do  not  show  that  the  nerve-fibres  can  live  and 
function  independently  of  chemical  changes.  As  has  been  said,  nerves  lose  their 
irritability  in  time  if  deprived  of  the  normal  blood-supply,  and  undoubtedly 
they  are,  like  all  protoplasmic  structures,  continually  the  seat  of  metabolic 

1  Helmholtz :  Archiv  fur  Anatomie  und  Physiologic,  1848,  S.  158.  Heidenhain :  op.  cit. 
Rolleston:  Journal  of  Physiology,  1890,  vol.  xi.  p.  208.  Stewart:  ibid.,  1891,  vol.  xii.  p.  424. 

*  Bernstein  :  Pftilger's  Archiv,  1877,  xv.  S.  289.     Wedenski :  Centralblatt  fur  die  medicinischen 
Wissenschaften,  1884. 

*  Bowditch :  Journal  of  Physiology,  1885,  vi.  p.  133. 

4  Wedenski :  loc.  cit.     Maschek  :  Siteungsberichte  der  Wiener  Academic,  1887,  Bd.  xcv.  Abthl.  3. 

5  Szana:  Archiv  fur  Anatomie  und  Physiologic,  1891,  S.  315. 

6  Ho  well,  Budgett,  and  Leonard:  Journal  of  Physiology,  1894,  xvi.  p.  312. 

7  Lambert:  Comptes-rendus  de  la  Societe  de  Biologic,  1894,  p.  511. 


GENERAL  PHYSIOLOGY   OF  MUSCLE  AND   NERVE.       97 

processes.  The  normal  function  of  the  nerve,  however,  the  conduction  of  the 
nerve-impulse,  seems  to  take  place  without  any  marked  chemical  change. 

Nature  of  the  Conduction  Process. — There  have  been  a  great  many 
views  as  to  the  nature  of  the  conduction  process,  one  after  the  other  being 
advanced  and  combated  as  physiological  facts  bearing  on  the  question  have 
been  accumulated.  It  has  been  suggested  that  the  whole  nerve  moved  like  a 
bell-rope ;  that  the  nerve  was  a  tube,  and  that  a  biting  acid  flowed  along  it ; 
that  the  nerve  contained  an  elastic  fluid  which  was  thrown  into  oscillations ; 
that  it  conducted  an  electric  current,  like  a  wire ;  that  it  was  composed  of 
definitely  arranged  electro-motor  molecules  which  exerted  an  electro-dynamic 
effect  on  each  other ;  that  it  was  made  up  of  chemical  particles,  which  like 
the  particles  of  powder  in  a  fuse,  underwent  an  explosive  change,  each  in 
turn  exciting  its  neighbor ;  that  the  irritant  caused  a  chemical  change,  which 
produced  an  alteration  of  the  electrical  condition  of  such  a  nature  as  to  excite 
neighboring  parts  to  chemical  change  and  thereby  to  electrical  change,  and 
so  alternating  chemical  and  electrical  changes  progressed  along  the  fibre  in  the 
fo^m  of  a  wave ;  finally,  that  the  molecules  of  the  nerve-substance  underwent 
a  form  of  physical  vibration  analogous  to  that  assumed  for  light. 

A  discussion  of  these  different  theories,  none  of  which  can  be  regarded 
as  entirely  satisfactory,  cannot  be  entered  upon  here. 

Although  the  exact  nature  of  the  conduction  process  is  not  determined, 
there  seems  little  doubt  that  it  is  of  the  same  type  in  all  forms  of  protoplasm. 
In  all  cases  it  is  a  property  of  the  living  substance  of  the  cell  and  is  lost 
when  the  cell  dies :  the  state  of  activity  spreads  like  a  wave  in  all  directions 
through  the  living  substance,  and  is  markedly  altered  by  physical  and  chem- 
ical influences  which  change  the  irritability  of  the  living  substance,  and  in 
much  the  same  way  as  this  is  altered ;  continuity  of  protoplasm  is  absolutely 
essential  to  conduction,  hence  the  spread  of  the  excitation  change  is  limited 
to  the  one  cell,  unless  the  cell  is  connected  by  protoplasmic  bridges  with 
other  cells,  or  possesses  a  specially  differentiated  exciting  end-organ. 

In  its  details  the  conduction  process  exhibits  many  peculiarities  in  differ- 
ent cells  and  even  in  the  different  parts  of  the  same  cell.  The  receiving 
organs  at  the  extremities  of  the  dendrites  of  different  classes  of  neurones 
differ  widely  in  respect  to  structure,  and  in  their  capacity  to  react  to  different 
kinds  of  stimuli  and  to  transmit  the  state  of  excitation  to  the  dendrite.  The 
exciting  organs  at  the  extremities  of  the  axones  of  different  classes  of  neurones 
are  of  different  types,  and  behave  differently,  the  discharge  of  the  exciting 
process  upon  a  muscle,  gland,  or  nerve-cell  being  adjusted  to  the  capacity  for 
reaction  possessed  by  the  organ  in  question.  In  each  neurone  the  strands  of 
protoplasm  which  connect  these  distant  receiving  and  exciting  mechanisms 
with  the  cell  body,  and  the  body  of  the  cell  itself,  work  each  according  to  its 
own  nature.  For  example,  the  time  spent  by  the  phase  of  activity  in  the 
body  of  a  ganglion-cell  of  the  posterior  spinal  root-ganglion,  is  far  longer 
than  that  used  in  a  corresponding  length  of  protoplasm  in  the  dendrite  of  the 
cell.  Although  the  conduction  process  differs  in  its  details  even  in  different 

VOL.  II.— 7 


98  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

parts  of  the  same  neurone,  the  condition  of  activity  which  spreads  through  the 
neurone  and  which  we  call  the  nerve  impulse,  has  the  same  general  character- 
istics in  all  forms  of  nerves  whether  medullated  or  non-medullated,  motor, 
sensory,  or  secretory.  The  character  of  a  movement  or  secretion  depends  on 
the  character  of  the  organ  excited,  and  not  on  the  nature  of  the  change  trans- 
mitted along  the  efferent  nerve,  and  the  specific  character  of  a  special  sensa- 
tion depends  on  the  form  of  psychic  activity  developed  in  the  central  nervous 
system,  and  not  on  the  nature  of  the  process  of  transmission  in  the  afferent 
neurone.  This  view  that  the  nerve  impulse  is  to  be  regarded  merely  as  an 
excitatory  process,  and  that  it  has  the  same  general  characteristics  in  all 
kinds  of  nerves,  is  strengthened  by  two  sets  of  experiments  which  have  been 
reported  lately. 

One  of  these  sets  of  experiments  was  reported  by  Langley.1  He  found 
that  preganglionic  sympathetic  fibres — i.  e.  fibres  between  the  ganglion  and 
the  cord — if  cut  centrally  from  the  ganglion,  after  a  time  regenerate  and  make 
new  connections  with  the  nerve-cells  of  the  ganglion.  In  some  cases  they 
unite  with  cells  of  their  own  class,  and  sometimes  with  other  cells;  for 
example,  pupillo-dilator  fibres  were  found  to  have  established  connection 
with  pilo-motor  neurones — i.  e.  with  ganglion-cells  which  send  their  axones  to 
the  erector  muscles  of  the  hairs.  Further,  by  section  of  post-ganglionic 
fibres — i.  e.  fibres  between  the  ganglion  and  the  periphery — it  was  found,  after 
regeneration  had  occurred,  that  pilo-motor  fibres  can  form  nerve-endings  in 
the  iris  and  become  pupillo-dilator  fibres.  Evidently  ganglion-cells  and 
muscle-fibres  can  be  excited  by  nerve  impulses  developed  in  other  nerves 
than  those  normally  connected  with  them. 

A  still  more  remarkable  result  was  obtained  by  Budgett  and  Green. 
A  description  of  this  experiment  is  given  on  page  85.  They  succeeded  in 
causing  sensory  fibres  of  the  pneumogastric  to  grow  down  a  degenerated 
motor  trunk,  the  hypoglossal,  and  connect  with  the  muscles  of  the  tongue. 
In  this  case  excitation  of  the  peripheral  part  of  the  afferent  nerve  caused 
muscular  contractions.  If  we  should  think  of  the  nerve  which  was  excited, 
we  would  be  inclined  to  say  that  a  sensory  impulse  was  generated ;  if  we 
should  think  of  the  effect  on  the  muscle,  we  would  call  it  a  motor  impulse, 
and  the  latter  would  be  the  proper  term.  Evidently  the  condition  of 
activity  which  can  be  aroused  in  a  sensory  nerve  is  capable  under  suitable 
conditions  of  exciting  muscles,  and  sensory  nerves  cannot  be  considered  to 
be  the  seat  of  specific  forms  of  energy  different  from  those  generated  in 
motor  nerves. 

D.  CONTRACTILITY. 

Contractility  is  the  property  of  protoplasm  by  virtue  of  which  the  cell  is 

able  to  change  its  form  when  subjected  to  certain  external  influences  called 

irritants,  or  when  excited  by  certain  changes  occurring  within    itself.     The 

change  of  form  does  not  involve  a  change  of  size.     The  contraction  is  the 

1  Langley  :  Journal  of  Physiology,  1897,  xxii.  p.  215. 


GENERAL    PHYSIOLOGY  OF  MUSCLE  AND   NERVE.       99 

result  of  a  change  in  the  position  of  the  more  fluid  parts  of  the  cell-protoplasm, 
and  the  effect  is  to  cause  the  cell  to  approach  a  spherical  shape.  In  the  case  of 
an  amoeba,  for  instance,  excitation  causes  a  drawing  in  of  the  pseudopods,  and 
as  the  material  in  them  flows  back  into  the  cell  the  body  of  the  cell  expands 
and  acquires  a  globular  form.  In  the  simpler  forms  of  contractile  protoplasm 
the  movement  does  not  appear  to  be  limited  to  any  special  direction,  but  in  the 
case  of  the  highly  differentiated  forms,  such  as  muscle,  both  contraction  and 
relaxation  occur  on  definite  lines. 

When  a  muscle  is  excited  to  action,  energy  is  liberated  through  chemical 
change  of  certain  constituents  of  the  muscle-substance,  and  this  energy  in  some 
unknown  way  causes  a  rearrangement  of  the  finest  particles  of  the  muscle-sub- 
stance, and  the  consequent  change  of  form  peculiar  to  the  contracted  state. 
When  the  'irritation  ceases  and  relaxation  takes  place,  there  is  a  sudden  return 
of  the  muscle-substance  to  the  position  of  rest,  either  because  of  elastic  recoil  or 
of  some  other  force  at  work  within  the  muscle  itself.  That  the  recovery  of 
the  elongated  form  peculiar  to  the  resting  muscle  is  not  dependent  on  external 
influences  is  evidenced  by  the  fact  that  a  muscle  floating  on  mercury,  and 
subjected  to  no  extending  force,  will  on  the  cessation  of  irritation  assume  its 
resting  form.  The  relaxation  no  less  than  the  contraction  must  be  regarded 
as  an  active  process,  but  on  account  of  their  flexibility  muscle-fibres  are  incap- 
able of  exerting  an  expansion  force,  therefore  cannot  by  relaxing  do  external 
work. 

Both  the  histological  structure  and  physiological  action  of  the  striated  mus- 
cles which  move  the  bones  show  them  to  be  the  most  highly  differentiated,  the 
most  perfect  form  of  contractile  tissue.  It  is  by  means  of  these  structures  that 
the  higher  animals  perform  all  those  voluntary  movements  by  which  they  change 
their  position  with  reference  to  external  objects,  acquire  nourishment,  protect 
themselves,  and  influence  their  surroundings.  An  exact  knowledge  of  the 
method  of  action  of  these  mechanisms  and  the  influences  which  affect  them  is 
therefore  of  the  greatest  importance  to  us. 

1.  Simple  Muscle -Contractions  Studied  by  the  Graphic  Method. — 
When  a  muscle  makes  a  single  contraction,  in  response  to  an  electric  shock  or 
other  irritant,  the  change  of  form  is  too  rapid  to  be  followed  by  the  eye.  To 
acquire  an  adequate  idea  of  the  character  of  the  movement  it  is  necessary  that 
we  should  obtain  a  continuous  record  of  the  alterations  in  shape  which  it  un- 
dergoes. This  can  be  done  by  connecting  the  muscle  with  a  mechanism  which 
enables  it  automatically  to  record  its  movements. 

If  one  moves  a  pencil  vertically  up  and^down  on  a  piece  of  paper,  a  straight 
line  is  written ;  if  while  the  vertical  movements  are  continued  the  paper  be 
drawn  along  at  a  regular  rate  in  a  direction  at  right  angles  to  the  move- 
ment of  the  pencil,  a  curve  will  be  traced.  If  the  paper  be  moved  at  a  regular 
rate,  the  shape  of  the  curve  will  depend  on  the  rate  at  which  the  pencil  is 
moved,  and,  if  the  speed  of  the  paper  be  known,  the  rate  of  movement  of  the 
pencil  can  be  readily  determined.  This  principle  is  employed  in  recording  the 
movements  of  muscles.  The  muscle  is  connected  with  a  mechanism  which 


100 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


rises  and  falls  as  the  muscle  contracts  and  relaxes,  and  records  the  movements 
of  the  muscle  on  a  surface  which  passes  by  the  writing-point  at  a  regular 
speed  (see  Fig.  38) ;  such  a  record  is  called  a  myogram. 

The  Myograph. — The  writing  mechanism,  together  with  the  apparatus 
which  moves  the  surface  on  which  the  record  of  the  movement  of  a  contracting 
muscle  is  taken  is  called  a  myograph.  The  writing  mechanism  has  usually  the 
form  of  a  light,  stiff  lever,  which  moves  very  easily  on  a  delicate  axis ;  the 
lever  is  so  connected  with  the  muscle  as  to  magnify  its  movements.  The  point 
of  *he  lever  rests  very  lightly  against  a  glass  plate,  or  surface  covered  with 
glazed  paper,  which  is  coated  with  a  thin  layer  of  soot.  The  point  of  the  lever 
scratches  off  the  soot,  and  the  movements  are  recorded  as 
a  very  fine  white  line.  At  the  close  of  the  experiment 
the  record  is  made  permanent  by  passing  it  through  a 
thin  alcoholic  solution  of  shellac.  The  recording  surface 
in  some  cases  is  in  the  form  of  a  plate,  in  others  of  a  cyl- 
inder, and  is  moved  at  a  regular  rate  by  a  spring,  pendu- 
lum, falling  weight,  clockwork,  electric  or  other  motor.1 
The  record  which  is  traced  with  the  myograph  lever 
by  the  muscle  has  the  form  of  a  curve.  From  the  height 
of  the  curve  we  can  readily  estimate  the  amount  that 
the  muscle  changes  its  length,  but  in  order  to  accu- 
rately determine  the  duration  of  the  contraction  process 
and  the  time  relations  of  different  parts  of  the  curve, 
it  is  necessary  to  know  the  exact  rate  at  which  the 
recording  surface  is  moving.  The  shape  of  the  curve 
drawn  by  the  muscle  will  depend  very  largely  on  the 
rate  of  the  movement  of  the  surface  on  which  the  record 
is  taken.  This  is  illustrated  by  the  four  records  repro- 
duced in  Figure  36.  These  were  all  taken  from  the 
same  muscle  within  a  few  minutes  of  each  other  and 
under  exactly  the  same  conditions,  except  that  in  the 
successive  experiments  the  speed  of  the  drum  on  which 
the  record  was  traced  was  increased. 

A  glance  at  these  records  shows  that  a  knowledge 
of  the  rate  of  movement  of  the  surface  on  which  the  record  is  taken  is  indis- 
pensable to  an  understanding  of  the  time  relations  of  the  different  parts  of  the 
curve  written  by  the  muscle.  The  rate  of  movement  of  the  recording  surface 
can  be  registered  by  an  instrument  called  a  chronograph. 

The  chronograph  (g,  Fig.  37),  consists  of  one  or  two  coils  of  wire  wound 
round  cores  of  soft  iron,  and  a  little  lever  bearing  a  strip  of  iron,  which  is 
attracted  to  the  soft-iron  cores  whenever  they  are  magnetized  by  an  elec- 
tric current  flowing  through  the  coils  of  wire  about  them.  When  the  current 
ceases  to  flow  and  the  iron  ceases  to  be  magnetized,  a  spring  draws  the  lever 

1  See  O.  Langendorff:  Physiologische  Graphik,  Franz  Deuticke,  Leipzig,  1891  ;  J.  S. 
Brodie :  The  Essentials  of  Experimental  Physiology,  London,  1898. 


FIG.  36.— Records  of  four 
contractions  of  a  gas- 
trocnemius  muscle  of  a 
frog:  a,  recording  sur- 
face at  rest;  6,  surface 
moving  slowly ;  c,  sur- 
face moving  more  rapidly ; 
d,  surface  moving  even 
faster. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND    NP1RVE.      101 

away  from  the  iron.     Many  of  the  instruments  employed  for  this  purpose  are 
very  delicate,  and  are  capable  of  responding  to  very  rapid  interruptions  of  the 


FIG.  37.— Method  of  interrupting  an  electric  circuit  by  a  tuning-fork,  and  of  recording  the  intci .  up- 
tions  by  means  of  an  electro-magnet:  a,  battery;  6,  tuning-fork,  with  platinum  wire  at  the  extremity 
of  one  of  its  arms,  which  with  each  vibration  of  the  fork  makes  and  breaks  contact  with  the  mercury 
in  the  cup  below ;  c,  mercury  cup ;  e,  electro-magnet  which  keeps  the  fork  vibrating ;  g,  chronograph. 
The  current  from  the  battery  a,  passes  to  the  fork  6,  then,  by  way  of  the  platinum  wire,  to  the  mercury 
in  cup  c,  then  to  the  binding-post  d,  where  it  divides,  a  part  going  through  the  coils  of  wire  of  the 
chronograph  g,  and  thence  to  the  binding-post  /,  the  rest  through  the  coil  of  wire  of  electro-magnet 
e,  and  then  to  the  post/,  from  which  the  united  threads  of  current  flow  back  to  the  battery.  The 
electro-magnet  e  keeps  the  fork  in  vibration,  because  when  the  platinum  wire  enters  the  mercury 
at  c,  the  circuit  is  completed  and  the  electro-magnet  magnetizes  its  soft-iron  core,  which  attracts  the 
arms  of  the  fork,  and  thus  draws  the  wire  out  of  the  mercury  and  so  breaks  the  circuit.  When  the 
current  is  broken  the  fork,  being  released,  springs  back,  dips  the  wire  into  the  mercury,  and  by 
closing  the  circuit  causes  the  process  to  be  repeated. 

current.  The  electric  current  is  made  and  broken  at  regular  intervals  by  a  clock, 
tuning-fork  (6,  Fig.  37),  or  other  interrupting  mechanism,  and  the  lever  of  the 
chronograph,  which  has  a  writing-point  at  its  free  end,  moves  correspondingly 


FIG.  38.— Myogram  from  gastrocnemius  muscle  of  frog ;  beneath,  the  time  is  recorded  in  0.005  second : 
a,  moment  of  excitation ;  b,  beginning  of  contraction  ;  c,  height  of  contraction  ;  d,  end  of  contraction. 

and  traces  an  interrupted  line  on  the  recording  surface  of  the  myograph  (see 
Fig.  38).  The  space  between  the  succeeding  jogs  marked  by  the  chronograph 
lever  is  a  measure  of  the  amount  of  the  surface  which  passed  the  point  of  the 
chronograph  in  one  second,  ^0  second,  or  T^g-  second,  as  the  case  may  be. 

Myogram  of  Simple  Muscle-contraction. — The  rate  of  the  movement  of  the 
muscle  during  every  part  of  its  contraction  can  be  readily  determined  by  com- 
paring the  record  it  has  drawn  with  that  of  the  chronograph. 

Figure  38  is  the  reproduction  of  a  single  contraction  of  a  gastrocnemius 
muscle  of  a  frog.  The  rise  of  the  curve  shows  that  the  contraction  began 
comparatively  slowly,  soon  became  very  rapid,  but  toward  its  close  was  again 
gradual  •  the  relaxation  began  almost  immediately,  and  took  a  similar  course, 


102  AN   AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

though  occupying  a  somewhat  longer  time.  The  electric  current  which 
actuated  the  chronograph  was  made  and  broken  by  a  tuning-fork  which 
made  200  complete  vibrations  per  second,  therefore  the  spaces  between  the 
succeeding  peaks  of  the  chronograph  curve  each  represents  0.005  second.  A 
comparison  of  the  movements  of  the  muscle  with  the  tuning-fork  curve 
reveals  that  about  yf^  second  elapsed  between  the  point  6,  at  which  the  muscle 
curve  began  to  rise,  and  c,  the  point  at  which  the  full  height  of  the  contraction 
was  reached,  and  that  about  yj^  second  was  occupied  by  the  return  of  the 
muscle  curve  from  c  to  point  d,  at  the  level  from  which  it  started.  The  muscle 
employed  in  this  experiment  was  slightly  fatigued,  and  the  movements  were 
in  consequence  a  little  slower  than  normal. 

Latent  Period. — The  time  that  elapses  between  the  moment  that  a  stim- 
ulus reaches  a  muscle  and  the  instant  the  muscle  begins  to  change  its  form  is 
called  the  latent  period.  In  the  experiment  recorded  in  Fig.  38  the  muscle 
received  the  shock  at  the  point  a  on  the  curve,  but  the  lever  did  not  begin  to 
rise  until  the  point  b  was  reached.  The  latent  period  as  recorded  in  this  ex- 
periment was  about  0.006  second.  The  latent  period  and  the  time  relations  of  the 
muscle-curve  were  first  measured  by  Helmholtz,  who  introduced  the  use  of  the 
myograph.1  Helmholtz  concluded  from  his  experiments  that  the  latent  period 
for  a  frog's  "muscle  is  about  yj-^  second,  that  the  rise  of  the  curve  occupies 
about  yJ-Q-,  and  the  fall  about  yjj-g-  second,  the  total  time  occupying  about  -fa 
second.  These  rates  can  be  considered  approximately  correct,  excepting  for 
the  latent  period,  which  has  been  found  by  more  accurate  methods  to  be  con- 
siderably shorter.  Tigerstedt  connected  a  curarized  frog's  muscle  with  a  myo- 
graph lever,  which  was  so  arranged  as  to  break  an  electric  contact  at  the 
instant  that  the  muscle  made  the  slightest  movement ;  the  break  in  the  electric 
circuit  was  recorded  on  a  rapidly  revolving  drum,  by  an  electro-magnet  similar 
to  the  chronograph.  By  this  means  he  found  the  latent  period  of  a  frog's 
muscle  may  be  as  short  as  0.004  second.  Tigerstedt2  did  not  regard  this  as 
the  true  latent  period,  however ;  he  expressed  the  belief  that  the  muscle  proto- 
plasm must  have  begun  to  respond  to  the  excitation  much  sooner  than  this. 
The  contraction  of  the  whole  muscle  is  the  result  of  a  shortening  of  each  of  the 
myriad  of  light  and  dark  disks  of  which  each  of  the  muscle-fibres  is  composed 
(see  Fig.  39).  The  distance  to  be  traversed  by  the  finest  particles  of  muscle- 
substance  is  microscopic,  hence  the  rapidity  of  the  change  of  form  of  the  whole 
muscle.  Even  such  a  change  would  require  time,  however,  and  it  is  probable 
that  the  muscle  protoplasm  becomes  active  before  any  outward  manifestation 
occurs.  That  this  view  is  correct  has  been  proved  by  electrical  observations. 

When  muscle  protoplasm  passes  from  a  state  of  rest  to  one  of  action  it 
undergoes  an  alteration  in  electrical  condition.  This  change  can  be  detected  by 
the  galvanometer  (Fig.  62,  p.  144)  or  by  the  capillary  electrometer  (Fig.  63, 
p.  146).  Burdon  Sanderson 3  has  found  that  by  the  aid  of  the  latter  instru- 

1  Archivfiir  Anatomie  und  Physiologic,  1850,  S.  308. 

2  Ibid.,  1885,  Suppl.  Bd.,  S.  111. 

3  Journal  of  Physiology,  1898,  vol.  xxiii.  p.  350. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND    NERVE.     103 

ment  an  alteration  of  the  electrical  condition  of  the  muscle  of  a  frog  can  be 
detected  less  than  0.001  second  after  the  stimulus  has  been  applied  to  it. 
Since  some  slight  interval  of  time  must  have  been  lost  even  by  this  delicate 
method,  it  would  seem  that  muscle  protoplasm  begins  to  be  active  at  the 
instant  it  is  stimulated. 

According  to  this  view,  muscle-substance  has  no  latent  period ;  neverthe- 
less we  can  still  speak  of  the  latent  period  of  the  muscle  as  a  whole.  It  will 
be  necessary,  however,  to  distinguish  between  the  electrical  latent  period  and 
the  mechanical  latent  period  :  by  the  former  we  mean  the  time  which  elapses 
between  the  moment  of  excitation  and  the  first  evidence  obtainable  of  a  change 
in  the  electrical  condition  of  the  muscle ;  by  the  latter,  the  time  between  exci- 
tation and  the  earliest  evidence  of  movement  which  can  be  observed.  In  the 
case  of  the  striated  muscles  of  a  frog  the  electrical  latent  period  is  less 
than  0.001  second,  and  the  mechanical  about  0.004  second.  Mendelssohn l 
estimated  the  mechanical  latent  period  of  the  muscles  of  man  to  be  about 
0.008  second.  There  can  be  little  doubt,  however,  that  this  figure  is  too 
large. 

Bernstein2  found  that  if  a  normal  frog's  muscle  be  excited  indirectly, 
by  the  stimulation  of  its  nerve,  the  mechanical  latent  period  is  somewhat 
longer  than  when  it  is  directly  excited.  Of  course  a  certain  length  of  time  is 
required  to  transmit  the  excitation  through  the  length  of  nerve  intervening 
between  the  point  stimulated  and  the  muscle  fibres.  If  this  time  be  deducted, 
there  still  remains  a  balance  of  about  0.003  second,  which  can  only  be  ac- 
counted for  on  the  assumption  that  the  motor  nerve  end-plates  require  time  to 
excite  the  muscle-fibres.  The  motor  end-plates  are  therefore  said  to  have  a 
latent  period  of  0.002-0.003  second. 

The  latent  period,  and  the  time  required  for  the  rise  and  fall  of  the  myo- 
graph  curve,  are  found  to  be  very  different  not  only  for  the  muscles  of  differ- 
ent animals,  but  even  for  the  different  muscles  of  the  same  animal.  Moreover, 
the  time  relations  of  the  contraction  process  in  each  muscle  are  altered  by  a 
great  variety  of  conditions. 

Before  considering  the  effect  of  various  influences  upon  the  character  of  the 
muscle  contraction,  let  us  give  a  glance  at  the  finer  structure  of  the  muscle, 
and  the  change  of  form  which  the  microscopic  segments  of  the  muscle-fibre 
undergo  during  contraction. 

2.  Optical  Properties  of  Striated  Muscle  during1  Rest  and  Action. — 
An  ordinary  striated  muscle  is  composed  of  a  great  number  of  very  long 
muscle-cells,  fibres  as  they  are  called,  arranged  side  by  side  in  bundles,  the 
whole  being  bound  together  by  a  fine  connective-tissue  network.  Each  muscle- 
fibre  consists  of  a  very  delicate  elastic  sheath,  the  sarcolemma,  which  is  com- 
pletely filled  with  the  muscle-substance.  Under  the  microscope  the  fibres  are 
seen  to  be  striped  by  alternating  light  and  dark  transverse  bands,  and  on  focus- 
ing, the  difference  in  texture  which  this  suggests  is  found  to  extend  through 

1  Archives  de  Physiologic,  1880,  2d  series,  t.  vii.  p.  197. 

2  Untemuchungen  uber  den  Erregungsvorgang  im  Nerven  und  Muakelsystem,  1871. 


104 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


the  fibres,  i.  e.  the  light  and  dark  bands  correspond  to  little  disks  of  substances 
of  different  degrees  of  translucency.     More  careful  study  with  a  high  power, 

shows  under  certain  circumstances  other 
cross  markings  (see  Fig.  39,  A),  the  light 
band  is  found  to  be  divided  in  halves  by 
a  fine  dark  line,  Z,  and  parallel  to  it  is 
another  faint  dark  line,  n,  while  the  dark 
band,  §,  is  found  to  have  a  barely  per- 
ceptible light  line  in  its  centre. 

The  fine  dark  lines,  Z,  which  run 
through  the  middle  of  the  light  bands, 
were  for  a  time  supposed  to  be  caused  by 
delicate  membranes  (Krause's  membrane), 
which  were  thought  to  stretch  through 
the  fibre  and  to  divide  it  into  a  series  of 
little  compartments,  each  of  which  had 
exactly  the  same  construction.  Kuehne 
FIG.  39.-schema  of  histoiogicai  structure  of  cnanced  to  see  a  minute  nematode  worm 

muscle-fibre  :  A,  resting  fibre  as  seen  by  ordinary 

light  ;  B,  resting  fibre  seen  by  polarized  light  ;  C,    moving  along  inside   a    muscle-fibre,  and 

'"  ^served  that  it  encountered  no  obstruc- 


tion,  such  as  a  series  of  membranes,  how- 

ever delicate,  would  have  caused.  As  it  moved,,  the  particles  of  muscle-sub- 
stance closed  in  behind  it,  the  original  structure  being  completely  recovered. 
This  observation  did  away  with  the  view  that  the  fibre  is  divided  into  com- 
partments, but  the  arrangement  shown  in  Figure  39,  A,  repeats  itself  through- 
out the  length  of  the  fibre  and  indicates  that  it  is  made  up  of  a  vast  succession 
of  like  parts. 

Muscle-substance  consists  of  two  materials,  which  differ  in  their  optical 
peculiarities  and  their  reaction  to  stains.  If  a  muscle-fibre  be  examined  by 
polarized  light,  it  is  found  that  there  is  a  substance  in  the  dark  bands  which 
refracts  the  light  doubly,  is  anisotropic,  while  the  bulkpf  the  substance  in  the 
light  bands  is  singly  refractive,  isotropic  (£,  Fig.  39).  The  anisotropic  sub- 
stance is  found  to  stain  with  ha3inatoxylin,  while  the  isotropic  is  not  thus 
stained  ;  on  the  other  hand,  the  isotropic  substance  is  often  colored  by  chloride 
of  gold,  which  is  not  the  case  with  the  anisotropic.  By  means  of  these  reac- 
tions it  has  been  possible  to  ascertain  something  as  to  the  arrangement  of  these 
substances  within  the  muscle-fibre,  though  the  ultimate  structure  has  not  been 
definitely  decided.  It  appears  that  the  isotropic  material  is  the  sarcoplasma, 
which  is  distributed  throughout  the  fibre  and  holds  imbedded  within  it  the 
particles  of  the  anisotropic  substance,  these  particles  having  a  definite  arrange- 
ment. Striated  muscle-fibres  present  not  only  cross  markings,  but  under 
favorable  conditions  longitudinal  striations,  these  being  most  evident  in  the 
dark  bands.  These  longitudinal  striatious  are  looked  upon  with  great  interest 
as  indicating  that  the  particles  of  anisotropic  material  are  arranged  in  long 
chains  as  incomplete  fibrillaB.  According  to  this  view  the  muscle-fibre  is  com- 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND   NERVE.     105 

posed  of  semifluid  isotropic  substance,  in  which  are  the  particles  of  anisotropic 
material,  arranged  to  form  vast  numbers  of  parallel  fibrillse  of  like  structure, 
and  so  placed  as  to  give  the  effect  of  transverse  disks  (Z,  n,  Q,  Fig.  39). 

When  a  striated  muscle  contracts,  each  of  its  fibres  becomes  shorter  and 
thicker,  and  the  same  is  true  of  the  dark  and  light  disks  of  which  the  fibres 
are  composed.  If  we  examine  a  muscle-fibre  which  has  been  fixed  by  ramie 
acid  at  a  time  when  part  of  it  was  contracting,  we  see  that  in  the  contracted 
part  the  light  and  dark  bands  have  both  become  shorter  and  wider,  but  that 
the  volume  of  the  dark  bands  (§,  Fig.  39,  C)  has  increased  at  the  expense  of 
the  light  bands. 

Further,  the  dark  bands  are  seen  to  be  lighter  and  the  light  bands  darker 
in  the  contracted  part,  while  examination  with  polarized  light  shows  that 
though  the  anisotropic  substance  does  not  seem  to  have  changed  its  position, 
(Fig.  39,  Z>),  the  original  dark  bands  have  less  and  the  lighter  bands  greater 
refractive  power.  These  appearances  would  seem  to  be  explained  by  Engel- 
mann's  view  that  contraction  is  the  result  of  imbibition  of  the  more  fluid 
part  of  the  sarcoplasm  by  the  anisotropic  substance.  He  has  advanced  the 
theory  that  the  cause  of  the  imbibition  is  the  liberation  of  heat  by  chemical 
changes  which  occur  at  the  instant  the  muscle  is  excited.  In  support  of 
this  theory  Engelmaim1  showed  that  dead  substance  containing  anisotropic 
material,  such  as  a  catgut  string,  can  change  its  form,  by  imbibition  of 
fluid  under  the  influence  of  heat,  and  give  a  contraction  curve  in  many 
respects  similar  to  that  to  be  obtained  from  muscle.  This  theory  of 
the  method  of  action  of  the  muscle-substance,  though  attractive,  can  be 
accepted  only  as  a  working  hypothesis,  and  is  not  to  be  regarded  as  proved. 
Various  other  theories  have  been  advanced  to  explain  the  connection  between 
the  chemical  changes  which  undoubtedly  occur  during  contraction  and  the 
alteration  of  form,  but  none  have  been  generally  accepted.  Enough  has  been 
said  to  show  that  the  contraction  of  the  muscle  as  a  whole  is  the  result  of 
a  change  in  the  minute  elements  of  the  fi  brills,  and  that  the  various  condi- 
tions which  influence  the  activity  of  the  process  of  contraction  must  act  chiefly 
through  alterations  produced  in  these  little  mechanisms. 

3.  Elasticity  of  Muscle. — The  elasticity  and  extensibility  of  muscle  are 
of  great  importance,  for  by  every  form  of  muscular  work  the  muscle  is  sub- 
jected to  a  stretching  force.  Elasticity  of  muscle  is  the  property  by  virtue  of 
which  it  tends  to  preserve  its  normal  form,  and  to  resist  any  external  force 
which  would  act  to  alter  that  form.  The  shape  of  muscles  may  be  altered  by 
pressure,  but  the  change  is  one  of  form  and  not  of  bulk ;  since  muscles  are 
largely  made  up  of  fluid,  their  compressibility  is  inconsiderable.  The  elasticity 
of  muscles  is  slight  but  quite  perfect,  by  which  is  meant  that  a  muscle  yields 
readily  to  a  stretching  force,  but  on  the  removal  of  the  force  quickly  recovers 
its  normal  form.  Most  of  the  experiments  upon  muscle  elasticity  have  been 
made  after  the  muscle  had  been  removed  from  the  body,  hence  under  abnormal 

1  Ueber  den  Ursprung  der  Muskelkraft,  Leipzig,  1893. 


106 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


FIG.  40. — a,  Curve  of  extensibility 
and  elasticity  of  a  rubber  band ;  b,  curve 
of  extensibility  and  elasticity  of  a  sar- 
torius  muscle  of  a  frog.  The  weights 
employed  were  10  grams  each.  The 
same  length  of  time  was  allowed  to 
pass  between  the  adding  and  subtract- 
ing of  the  weights. 


conditions.  Under  these  circumstances  it  is  found  that  if  a  number  of  equal 
weights  be  added  to  a  suspended  muscle,  one  after  the  other,  the  extension  pro- 
duced is  not,  like  that  of  an  inorganic  body 
such  as  steel  spring,  proportional  to  the  weight, 
but  each  weight  stretches  the  muscle  less  than 
the  preceding.  If  the  weights  be  removed 
in  succession,  an  elastic  recovery  is  observed, 
which,  although  considerable,  is  incomplete. 
If  the  change  in  the  length  be  recorded  by 
a  lever  attached  to  the  muscle,  the  surface 
being  moved  along  just  the  same  amount  after 
each  weight  is  added  or  removed,  a  curve  is 
obtained  such  as  is  shown  in  Fig.  40,  b. 
Above  this  is  a  record  taken  in  a  similar  way 
from  a  piece  of  rubber  (a).  The  rubber  resem- 
bles a  steel  spring  in  that  equal  weights  stretch 
it  to  like  amounts,  but  the  elastic  recovery, 
though  more  complete  than  that  of  the  muscle, 
is  imperfect. 

In  such  an  experiment  it  is  found  that  the 
full  effect  of  adding  the  weights,  or  removing 
them  from  the  muscle,  does  not  occur  immedi- 
ately, but  when  a  weight  is  added  there  is  a 
gradual  yielding  to  the  stretching  force,  and,  on  the  removal  of  a  weight,  a 
gradual  recovery  of  form  under  the  influence  of  the  elasticity.  This  slow 
after-action  makes  it  difficult  to  say  just  what  is  to  be  considered  the  proper 
curve  of  elasticity  of  muscle,  especially  as  the  physiological  condition  of  the 
muscle  is  always  changing.  The  elasticity  of  muscles  is  dependent  on  normal 
physiological  conditions,  and  is  altered  by  death,  or  by  anything  which  causes 
a  change  in  the  normal  constitution  of  the  muscles,  as  the  cutting  off  of  the 
blood-supply.  The  dead  muscle  is  less  extensible  and  less  elastic  than  the 
normal  living  muscle.  Heating,  within  limits,  increases,  and  cooling  decreases 
the  elasticity,  possibly  by  altering  the  mobility  of  the  semifluid  materials  of 
the  muscle,  and  hence  changing  the  internal  friction.1  Contraction  is  accom- 
panied by  increased  extensibility,  i.  e.  lessened  elasticity — and  the  changes 
caused  by  fatigue  lessen  the  elasticity.  It  is  interesting  to  note  in  this  con- 
nection that  the  elasticity  is  decreased  by  weak  acid  solutions  and  increased 
by  weak  alkaline  solutions  (Brunton  and  Cash).2 

The  elasticity  of  a  muscle  within  the  body  is  generally  considered  to  be 
more  perfect  than  that  of  the  isolated  muscle,  but  even  here  one  can  observe 
the  after-stretching  described  by  Weber  and  the  contraction  remainder 
described  by  Hermann.  Mosso 3  suggests  the  following  experiment  on  man  : 

1  Blix  :  Skandinavisches  Archivfur  Physiologic,  1893,  iv.  S.  392. 

2  Philosophical  Transactions,  1884,  p.  197. 

3  Mosso :  Archives  italiennes  de  Biologic,  1895,  xxv.  p.  27. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND   NERVE.     107 

Place  the  subject  in  a  sitting  position,  make  the  suspended  leg  immovable 
by  suitable  clamps,  strap  a  board  to  the  bottom  of  the  foot,  and  connect  the 
toe  end  of  the  board  with  a  weight  by  means  of  a  cord  passing  over  a  pulley. 
As  the  weight  is  increased  or  decreased  the  foot  is  more  or  less  flexed,  and 
the  gastrocnemius  muscle  is  stretched  more  or  less.  A  pointer  fastened  to 
the  foot-board  moves  over  a  scale  and  indicates  the  amount  the  muscle 
changes  its  length  when  subjected  to  various  weights.  Mosso  reports  that 
though  the  curve  of  elasticity  has  about  the  same  character  as  that  of  isolated 
frog  muscle,  the  curve  of  extensibility  is  different,  each  of  the  added  weights 
causing  greater  amount  of  stretching.  This  is  probably  due  to  the  fact  that 
a  muscle  within  the  body  is  always  being  influenced  by  the  central  nervous 
system.  Its  length  at  any  given  moment  is  due  not  only  to  its  elasticity  as 
compared  with  that  of  its  antagonist,  but  also  to  the  strength  of  the  nervous 
impulses,  reflex  and  voluntary  (often  unintentional),  coming  to  it.  The  sub- 
ject would  have  to  be  under  an  anaesthetic  or  in  very  deep  sleep  for  such  an 
experiment  to  give  a  true  picture  of  its  elasticity.  Mosso  describes,  in  fact, 
movements  of  the  foot  accompanying  the  respirations,  due  to  variations  in 
the  tonus  impulses  coming  to  the  muscles  in  inspiration  and  expiration.  In 
spite  of  the  innate  difficulties  of  such  an  experiment,  we  can  ascertain  that 
in  general  the  conclusions  arrived  at  by  studying  the  isolated  muscles  of  a 
frog  apply  to  the  muscles  when  in  the  living  body. 

The  elasticity  of  a  muscle  within  the  normal  body  suffices  to  preserve  the  ten- 
sion of  the  muscle  under  all  ordinary  conditions.  The  muscles  are  attached  to 
the  bones  under  elastic  tension,  as  is  shown  by  the  separation  of  the  ends  in  case 
a  muscle  be  cut.  This  elastic  tension  is  very  favorable  to  the  action  of  the 
muscle,  as  it  takes  up  the  slack  and  ensures  that  at  the  instant  the  muscle 
begins  to  shorten  the  effect  of  the  change  shall  be  quickly  imparted  to  the 
bones  which  it  is  its  function  to  move.  The  extensibility  of  the  muscle  is 
a  great  protection,  lessening  the  danger  of  rupture  of  the  muscle-fibres  and 
ligaments,  and  the  injury  of  joints  when  the  muscles  contract  suddenly  and 
vigorously,  or  when  they  are  subjected  to  sudden  strains  by  external  forces. 
The  importance  of  extensibility  and  elasticity  to  muscles  which  act  as  antag- 
onists is  evident.  When  a  muscle  suddenly  contracts  against  a  resisting  force 
such  as  the  inertia  of  a  heavy  weight,  the  energy  of  contraction,  which  puts  the 
muscle  on  the  stretch,  is  temporarily  stored  in  it  as  elastic  force,  and  as  the 
weight  yields  to  the  strain,  is  given  out  again ;  thus  the  effect  of  the  contrac- 
tion force  is  tempered,  the  application  of  the  suddenly  developed  energy  being 
prolonged  and  softened.  Elasticity  is  very  important  to  the  function  of  the 
non-striated  muscles  of  the  blood-vessels,  bladder,  intestine,  etc.  This  is 
especially  true  of  the  sphincter  muscles,  for  it  is  an  important  factor  in 
securing  the  continued  tension  characteristic  of  their  action. 

4.  Influences  which  Affect  the  Activity  and  Character  of  the  Con- 
traction.— (a)  The  Character  of  the  Muscle. — Attention  has  been  called  to 
the  fact  that  irritability  and  conductivity  may  be  different  not  only  in  different 
kinds  of  muscle-tissue,  and  in  muscles  of  different  animals,  but  even  in  similar 


108  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

kinds  of  muscle-tissue  in  the  different  muscles  of  the  same  animal ;  the  same 
may  be  said  of  contractility.  Although  irritability,  conductivity,  and  contrac- 
tility are  to  be  regarded  as  different  properties  of  muscle  protoplasm,  they  are 
usually  found  to  be  developed  to  a  corresponding  degree  in  each  muscle. 
Those  forms  of  muscle  which  require  for  their  excitation  irritants  of  slow  and 
prolonged  action,  are  found  to  conduct  slowly  and  to  make  slow  and  long- 
drawn-out  contractions,  and  muscles  which  are  excited  by  irritants  acting 
rapidly  and  briefly  are  noted  for  the  quickness  with  which  they  contract 
and  relax. 

Differences  in  the  activity  of  the  contraction  process  are  made  evident 
by  the  duration  of  single  contractions  of  different  forms  of  muscle-tissue. 
The  duration  of  the  contraction  of  the  striated  muscles  of  different  animals 
differs  greatly,  e.  g.  of  the  frog  ^  second,  of  the  turtle  1  second,  of  certain 
insects  only  -^  second.  Even  muscles  of  apparently  the  same  kind  in  the 


Pectoralis  major 
Omohyoid. 


FIG.  41. — Records  of  maximal  isotonic  contractions  of  four  different  muscles  from  a  turtle,  each 
weighted  with  30  grams :  Pectoralis  major ;  omohyoid ;  gracilis ;  palmaris.  The  dots  mark  i  second,  and 
the  longer  marks  seconds  (after  Cash).2  J 

same  animal  exhibit  different  degrees  of  activity.  Cash l  reports  the  following 
differences  in  the  duration  of  the  contractions  of  different  striated  muscles  of 
a  frog  in  fractions  of  a  second:  Hyoglossus,  0.205;  rectus  abdominis,  0.170; 
gastrocnemius,  0.120;  semimembranosus,  0.108  ;  triceps  femoris,  0.104.  Sim- 
ilar differences  are  found  to  exist  between  different  muscles  in  other  animals 
— in  the  turtle,  for  instance,  as  is  shown  by  the  myograms  in  Fig.  41. 

It  is  interesting  to  connect  the  rate  of  the  contraction  process  in  different 
muscles  with  their  function.  The  omohyoid  muscle  of  the  turtle  is  capable  of 
comparatively  rapid  contractions,  and  the  action  of  this  muscle  is  to  draw  back 
the  head  beneath  the  projecting  shell ;  the  pectoralis,  on  the  other  hand, 
although  strong,  contracts  slowly;  it  is  a  muscle  of  locomotion  and  has  to 
move  the  heavy  body  of  the  animal.  Unstriated  muscles,  which  are  remark- 
able for  the  slowness  and  the  duration  of  their  contractions,  are  found  chiefly 
in  the  walls  of  the  intestines,  blood-vessels,  etc.,  which  require  to  remain  in  a 
state  of  continued  contraction  for  considerable  periods  and  do  not  need  to  alter 
rapidly.  It  is  the  business  of  the  heart-muscle  to  drive  fluids  often  against 
considerable  resistance,  and  a  strong,'  not  too  rapid,  slightly  prolonged  contrac- 
tion, such  as  is  peculiar  to  it,  would  be  best  adapted  to  its  function.  The  bulk 
of  the  muscles  of  the  bodies  of  warm-blooded  animals  are  capable  of  rapid 
contraction  and  relaxation,  but  the  rate  normal  to  the  muscle  is  found  to  vary 
with  the  form  of  work  to  be  done.  The  muscles  which  control  the  vocal 
organs,  for  instance,  have  a  very  rapid  rate  of  relaxation  as  well  as  of  con- 

1  Archivfilr  Anatomic  und  Physiologie,  1880,  Suppl.  Bd.,  S.  147.  a  Op.  cit.,  S.  157. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     109 

traction.  The  muscles  which  move  the  bones  appear  to  have  different  rates 
of  contraction  and  relaxation  according  to  the  weight  of  the  parts  to  be  moved ; 
those  which  control  the  lighter  parts,  as  the  hand,  being  capable  of  rapid  con- 
tractions, while  those  which  have  to  overcome  the  inertia  of  heavier  parts,  to 
which  rapidity  of  action  would  be  a  positive  disadvantage,  react  more  slowly. 
In  general,  where  rapid,  brief,  and  vigorous  contractions  are  required,  pale 
striated  muscles  are  found;  where  more  prolonged  contractions  are  needed, 
red  striated  muscles  occur.  The  accompanying  myograms  (Fig.  42)  illustrate 


100 


50, 


100 


FIG.  42.  —A,  maximal  contractions  of  the  gastrocnemius  medialis  of  the  rabbit  (pale  muscle),  weighted 
with  50, 100, 300,  and  500  grams  ;  B,  maximal  contractions  of  the  soleus  of  the  rabbit  (red  muscle),  weighted 
with  50, 100,  and  200  grams  (after  Cash). 

the  difference  in  the  rate  of  contractions  of  pale  and  red  striated  muscles  of 
the  rabbit.  Ranvier  says  the  latent  period  of  red  muscle  of  rabbit  is  four 
times  as  long  as  that  of  the  pale ;  and  Tigerstedt  states  the  latent  period  of 
red  muscles  of  the  frog  to  be  0.02  second  and  of  the  pale  muscles  0.005 
second. 

Pale  and  red  striated  fibres  are  found  united  in  the  same  muscle  in  certain 
instances,  and  in  these  cases  it  is  supposed  that  the  former,  which  are  capable 
of  very  rapid  and  powerful  but  short-lived  contractions,  start  the  movement, 
while  the  slower  red  muscles  continue  it.  Bottazzi l  would  explain  many  of 
the  peculiarities  of  muscle  contraction  on  the  theory  that  both  the  isotropic 
and  anisotropic  substances  are  contractile,  and  that  they  react  differently 
under  varying  conditions.  The  isotropic  substance,  the  sarcoplasma,  is 
responsible  for  the  slow,  prolonged  movements  of  the  muscle  and  the  aniso- 
tropic substance  for  the  rapid,  brief  movements.  In  ordinary  contractions 
they  both  act,  though  to  different  degrees. 

(6)  Effect  of  Tension  Caused  by  Weights  and  Myograph-lever  on  the  Extent 

and  Course  of  the  Contraction. — As  we  have  seen,  the  rate  of  the  contraction 

of  an  ordinary  striated  muscle  is  much  too  rapid  to  be  followed  by  the  eye, 

and  to  study  the  course  of  the  change  in  form  it  is  necessary  to  employ  some 

kind  of  recording  mechanism.     Every  mechanical  device  for  recording  the 

rnents  of  the  muscle  has  inertia,  and,  if  given  motion,  acquires  moinen- 

1  Bottazzi :  Journal  of  Physiology,  1897,  xxi.  p.  1. 


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AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


turn.     Both  of  these  factors  would  tend  to  alter  the  shape  of  the  record,  and 
the  more  the  greater  the  weight  of  the  recording  apparatus. 

A  weight,  or  tension,  can  be  applied  to  a  muscle  in  various  ways,  and  the 
form  of  the  contraction  will  be  correspondingly  changed.  If  a  muscle  is  made 
to  work  with  a  considerable  weight  hanging  on  it,  we  speak  of  it  as  loaded; 
if  the  weight  be  connected  with  the  muscle,  but  so  supported  that  it  does 
not  pull  on  it  until  the  muscle  begins  to  shorten,  the  muscle  is  said  to  be  after- 
loaded;  if  the  weight  is  the  same  throughout  the  contraction,  as  when  the 
muscle  has  only  to  lift  a  light  weight,  applied  close  to  the  axis  of  the  lever,  the 
contraction  is  said  to  be  isotonic  ;  if  on  the  other  hand  the  contracting  muscle 
is  made  to  work  against  a  strong  spring,  so  that  it  can  shorten  very  little,  i.  e. 
has  almost  the  same  length  throughout  the  contraction,  the  contraction  is  said 

to  be  isometric.1  The  shape  of  the 
myogram  recorded  as  a  result  of 
the  same  stimulus  would  evidently 
be  very  different  in  these  four 
cases.  The  effect  of  a  weight  to 
alter  the  myogram  is  illustrated  in 
the  record  given  in  Figure  43. 
Increasing  the  weight  prolonged 
the  latent  period,  and  lessened  the 
height  and  duration  of  the  con- 
tractions. 

The  alterations  liable  to  occur 
in  the  form  of  the  myogram  by 
the  isotonic  method,  as  a  result 
of  the  mechanical  conditions  under 
which  the  work  is  done,  are — 

(1)  Prolongation  of  the  latent 
period.  There  can  be  no  move- 
ment of  the  lever  until  the  inertia 
of  the  weight  has  been  overcome, 
and  the  first  effect  of  the  contrac- 
tion is  to  stretch  the  muscle,  a 
part  of  the  energy  of  contraction  being  changed  to  elastic  force,  which  on  the 
recoil  assists  in  raising  the  weight.  Thus  the  myogram  may  fail  to  reveal 
the  instant  that  the  contraction  process  starts.  Indeed,  inasmuch  as  tension 
increases  the  activity  of  muscle  protoplasm,  it  is  probable  that  the  presence 
of  the  weight  really  hastens  the  liberation  of  energy  at  the  same  time  that  it 
delays  the  recording  of  the  contraction. 

(2)  Alteration  in  the  shape  of  the  ascending  limb  of  the  myograph  curve.     The 

weight  will  either  lessen  the  rate  at  which  the  curve  rises  and  decrease  the 

height,  or,  if  the  weight  be  not  great,  it  may  acquire  a  velocity  from  the  energy 

suddenly  imparted  to  it  by  the  muscle,  which  will  carry  the  record  higher 

1  Fick  :  Mechanische  Arbeit  und  W armeentwickelung  bei  der  Muskelthatigheit,  Leipzig,  1882. 


FIG.  43.—  Effect  of  the  weight  upon  the  form  of  the 
myogram.  The  gastrocnemius  muscle  of  a  frog  excited 
by  maximal  breaking  induction  shocks  five  times,  the 
weight  being  increased  after  each  contraction,  and  in  the 
intervals  supported  at  the  normal  resting  length  of  the 
muscle ;  i.  e.  the  muscle  was  after-loaded :  1,  muscle 
weighted  only  with  very  light  lever;  2,  weight  five 
grams ;  3,  ten  grams ;  4,  twenty -five  grams ;  5,  fifty  grams. 
The  perpendicular  line  marks  the  moment  of  excitation. 
The  time  is  recorded  at  the  bottom  of  the  curve  by  a 
chronograph,  actuated  by  a  tuning-fork  vibrating  50  times 
per  second. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     Ill 

than  the  absolute  contraction  of  the  muscle.  The  part  of  the  myogram  cor- 
responding to  the  height  of  the  contraction  of  the  muscle  can  be  distinguished 
from  that  due  to  the  throw  of  the  lever  by  a  method  suggested  by  Kaiser.1 
If  the  rising  lever  strikes  a  check,  it  remains  in  contact  with  the  check  as 
lonir  as  the  muscle  continues  to  contract,  but  falls  immediately  if  not  held 
there  by  the  contraction  process.  By  varying  the  height  of  the  check,  the 
point  corresponding  to  the  true  contraction  height  can  be  ascertained. 

(3)  The  fall  of  the  curve  may  be  altered.  The  weight,  suddenly  freed  by  the 
rapidly  relaxing  muscle,  may  acquire  a  velocity  in  falling  which  will  stretch 
the  muscle-tissue,  carry  the  record  lower  than  the  actual  relaxation  of  the 
muscle  would  warrant,  and  lead  to  the  development  of  artificial  elastic  after- 
oscillations.  It  must  not  be  supposed,  however,  that  the  relaxation  of  the 
muscle  is  merely  a  passive  aifair,  and  that  it  returns  to  its  original  shape 
because,  when  it  ceases  to  develop  energy,  it  is  stretched  by  the  weight.  The 
relaxation,  like  the  contraction  process,  is  an  active  event,  and  it  is  antago- 
nistic to  the  contraction  process.2 

These  sources  of  error  can  be  in  part  overcome  by  the  employment  of  an 
exceedingly  light,  stiff  writing-lever,  and  by  bringing  the  necessary  tension  on 
the  muscle  by  placing  the  extending  weight  very  near  the  axis  of  the  lever,  so 
that  it  shall  move  but  little  and  hence  acquire  little  velocity. 

(c)  Effect  of  Rate  of  Excitation  on  Height  and  Form  of  Muscular  Contrac- 
tion.— If  a  muscle  be  excited  a  number  of  times  by  exactly  the  same  irritant 
and  under  the  same  external  conditions,  the  amount  and  course  of  each  of 
the  contractions  should  be  exactly  the  same,  provided  the  condition  of  the 
muscle  itself  remains  the  same.  The  condition  of  the  muscle  is,  however, 
altered  every  time  it  is  excited  to  contraction,  and  each  contraction  leaves 
behind  it  an  after-effect.  This  altered  condition  is  not  permanent ;  as  we  have 
seen,  increased  katabolism  is  accompanied  by  increased  anabolism,  and,  if  the 
excitations  do  not  follow  each  other  too  rapidly,  the  katabolic  changes  occur- 
ring in  contraction  are  compensated  for  by  anabolic  changes  during  the  suc- 
ceeding interval  of  rest.  Normally,  a  muscle,  under  the  restorative  influence 
of  the  blood,  rapidly  recovers  from  the  alterations  produced  by  the  contraction 
process,  and,  therefore,  if  not  excited  too  frequently,  will  give,  other  things 
being  equal,  the  same  response  each  time  it  is  called  into  action.  The  best 
illustration  of  this  is  the  heart,  which  continues  to  beat  at  a  regular  rate 
throughout  the  life  of  the  individual.  Tiegel  found  that  one  of  the  skeletal 
muscles  of  a  frog,  while  in  the  normal  body,  can  make  more  than  a  thousand 
contractions  in  response  to  artificial  stimuli  without  showing  fatigue ;  finally 
the  effect  of  the  work  shows  itself  in  a  lessening  of  the  power  to  contract. 
Every  muscle  contains  a  surplus  of  energy-holding  compounds  and  also  sub- 
stances capable  of  neutralizing  waste  products,  and  even  a  muscle  which  has 
been  separated  from  the  rest  of  the  body  retains  for  a  considerable  time  the 
abiln\  to  recover  from  the  effects  of  excitation.  It  is  evident  that  when  a 

1  Kaiser:  Zeitschrift fur  Biologic,  1896,  xxxiii.  S.  157,  360. 
'ick,  v.  Kries,  and  others. 


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AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


muscle  is  excited  repeatedly,  a  certain  interval  of  rest  must  be  permitted 
between  the  succeeding  excitations  if  its  normal  condition  is  to  be  maintained, 
and  that  the  more  extensive  the  chemical  changes  produced  by  the  excita- 
tions the  longer  must  be  the  periods  allowed  for  recovery.  This  being  the 
case,  the  rate  of  excitation  and  consequent  length  of  the  interval  of  rest  will 
have  a  great  effect  upon  the  condition  of  the  muscle  and  its  capacity  for  work. 

(1)  Effect  of  Frequent  Excitations  on  the  Height  of  Separate  Muscular 
Contractions.— -Other  things  being  equal,  the  height  to  which  a  muscle  can  con- 
tract when  excited  by  a  given  irritant  can  be  taken  as  an  index  of  its  capacity 
to  do  work,  and  if  a  muscle  be  excited  many  times  in  succession,  the  effect  of 
action  upon  the  strength  of  the  contraction  process,  the  endurance,  and  the 
coming  on  of  fatigue  can  be  estimated  from  the  height  of  the  succeeding  con- 
tractions. One  might  expect  that  every  contraction  would  tend  to  fatigue  and 
to  lessen  the  power  of  the  muscle,  but  almost  the  first  effect  of  action  is  to 
increase  the  irritability  and  mobility  of  muscle  protoplasm. 

Introductory  and  Staircase  Contractions. — The  peculiar  effect  of  action  to 
increase  muscular  activity  was  first  observed  by  Bowditeh,1  when  studying 
the  effect  of  excitations  upon  the  heart.  He  found  that  repeated  excitations 
of  equal  strength  applied  to  the  ventricle  of  a  frog's  heart  caused  a  series  of 
contractions  each  of  which  was  greater  than  the  preceding.  If  the  contrac- 
tions were  recorded  on  a  regularly  moving  surface,  the  summits  of  the  succes- 
sive contractions  were  seen  to  rise  one  above  the  other  like  a  flight  of  steps. 
This  peculiar  phenomenon  received  the  name  of  the  "  staircase  contractions  " 
(see  Fig.  44). 


FIG.  44.— Staircase  contractions  of  a  frog's  ventricle  in  response  to  a  series  of  like  stimuli,  written  on 
a  regularly  revolving  drum  by  the  float  of  a  water  manometer  connected  with  the  chamber  of  the 
ventricle  (after  Bowditeh).  The  record  is  to  be  read  from  right  to  left. 

This  effect  of  repeated  excitations  was  later  observed  by  Tiegel,2  on  the 
skeletal  muscles  of  frogs;  by  Eossbach,3  on  the  muscles  of  warm-blooded 
animals,  and  by  Komanes 4  on  the  contractile  tissues  of  Medusae. 

The  following  series  of  contractions  (Fig.  45),  which  closely  resembles  the 
above,  was  obtained  from  the  gastrocnemius  muscle  of  a  frog,  excited  at  a 
regular  rate  by  a  series  of  equal  breaking  induction  shocks. 

The  contractions  in  Figure  45  did  not  begin  to  increase  in  height  imme- 
diately ;  on  the  contrary,  each  of  the  first  four  contractions  was  slightly  lower 
than  the  one  which  preceded  it.  A  decline  in  the  height  of  the  first  three  or 
four  contractions  is  the  rule  when  a  normal  resting  muscle  is  called  into  action 


1  Berichte  der  koniglichen  sdchsischen  Gesellschaft  der  Wissenschaft,  1871. 

3  Pftuger's  Archiv,  1882,  1884,  Bd.  xiii.,  xv. 

4  Romanes :  Jelly-fish  and  Star-fish,  International  Science  Series,  p.  54. 


Ibid.,  1875. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.     113 

(see  Figs.  46  and  49),  and  these  contractions  at  the  beginning  of  a  series  have 
received  the  name  of  the  "  introductory  contractions."  The  introductory  con- 
tractions appear  to  indicate  that  the  first  effect  of  action  is  to  lessen  irritability, 
or  that  anabolic^  changes  are  too  slow  to  compensate  for  katabolic  changes,  and 
each  of  the  first  few  contractions  leaves  behind  it  a  fatigue  effect.  It  is  not 
long,  however,  before  the  influence  of  activity  to  heighten  anabolism  and 
increase  irritability  shows  itself  in  the  growth  of  the  height  of  the  succeeding 
contractions,  and  the  "  staircase  contractions"  are  observed.  This  growth  of  the 
height  of  contractions  must  necessarily  reach  a  limit,  and  the  amount  of 
increase  is  found  to  gradually  lessen  until  the  succeeding  contractions  have  the 
same  height.  Sometimes  the  full  height  of  the  staircase  is  not  reached  before 
more  than  a  hundred  contractions  have  been  made.  These  maximal  contractions 
may  be  repeated  many  times ;  sooner  or  later,  however,  an  antagonistic  effect  of 
the  work  manifests  itself  and  the  height  of  the  contractions  begins  to  lessen. 

Effect   of  Fatigue.— A.   decline   in   the   height  of  the   contractions   is  an 
evidence  of  fatigue,  and  indicates  that  anabolism  is  failing  to  keep  pace  with 


FIG.  45.— Staircase  contractions  of  gastrocnemius  muscle  of  a  frog,  excited  once  every  two  seconds  by 
strong  breaking  induction  shocks. 

katabolism,  or  that  the  waste  products  which  result  from  the  work  are  col- 
lecting faster  than  they  can  be  removed  or  neutralized  and  are  exerting  a 
paralyzing  influence  on  the  muscle  protoplasm  (see  p.  70).  From  this  time 
on,  the  height  of  the  succeeding  contractions  continually  lessens,  and  often 
with  great  regularity,  so  that  a  line  drawn  so  as  to  connect  the  summits 
of  the  declining  contractions,  the  "curve  of  fatigue,"  as  it  is  called,  may 
be  a  straig  t  line.  In  the  experiment,  parts  of  the  record  of  which  are 
.reproduced  ir>  Figure  46,  an  isolated  gastrocnemius  muscle  of  a  frog  was 
excited  with  maximal  breaking  induction  shocks  at  the  rate  of  25  times 
a  minute  for  about  one  and  one-half  hours ;  the  contractions  were  isotonic,  and 
the  total  weight  of  lever  and  load  did  not  exceed  20  grams ;  the  records  of 
the  succeeding  contractions  were  recorded  on  a  slowly  moving  cylinder.  The 
experiment  counted  of  two  parts — in  the  first  66  contractions,  in  the  second 
over  1700  contractions  were  made;  an  interval  of  rest  of  five  minutes  was 
permit t«-d  U-r  •  t>n  the  two  series. 

In  the  first  part  c»f  the  experiment  there  was  a  decline  in  the  height  of  the 
contractions  for  the  first  five  contractions,  the  "introductory  contractions," 
then  during  the  next  sixty-one  contractions  a  gradual  rise  in  the  height  of  the 


114 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


contractions,  the  "  staircase  contractions."  These  phenomena  repeat  themselves 
in  the  second  part  of  the  experiment,  that  following  the  interval  of  rest.  The 
contractions  at  the  beginning  of  the  second  series  were  not  so  high  as  those  at 
the  end  of  the  first  series,  though  somewhat  higher  than  those  seen  at  the 
beginning  of  the  first  series;  the  rest  of  five  minutes  was  not  sufficient  to 
entirely  do  away  with  the  stimulating  influence  of  the  preceding  work.  The 
contractions  of  the  second  series  took  the  following  course :  The  first  four 
introductory  contractions  gradually  declined,  then  came  the  staircase  contrac- 
tions, which  continued  to  rise  until  the  100th  contraction,  when  a  gradual 
lessening  of  the  height  of  the  contractions  began.  This  decline  continued 


600 


700 


800 


900 


1000 


1100         1200 


1300 


1400 


1500 


1600 


1700 


FIG.  46.— Effect  of  fatigue  on  the  height  of  muscular  contractions.  The  figure  is  a  reproduction  of 
parts  of  a  record  of  over  1700  contractions  made  by  an  isolated  gastrocnemius  muscle  of  a  frog.  The  con- 
tractions were  isotonic,  the  weight  being  about  20  grams.  The  stimuli  were  maximal  breaking  induction 
shocks,  and  were  applied  directly  to  the  muscle,  at  the  rate  of  25  per  minute.  Between  the  first  group  of 
66  contractions  and  the  following  groups  a  rest  of  five  minutes  was  given ;  after  this  r  ,st  the  work  was 
continued  without  interruption  for  about  one  and  a  half  hours.  The  second  group  of  contractions,  that 
immediately  following  the  period  of  rest,  contains  the  first  twenty  contractions  of  tlte  new  series ;  the 
next  group  the  100th  to  the  110th ;  the  next  the  200th  to  the  210th,  and  so  on. 

throughout  the  long  series  of  more  than  1 700  contractions  g:ven  in  the  record, 
and,  had  the  experiment  been  continued,  would  have  undoubtedly  gone  on 
until  the  power  was  completely  lost.  The  curve  of  fatigue  was  not  a  straight 
line,  but  fell  somewhat  more  rapidly  during  the  early  part  of  the  work  than 
toward  the  end. 

That  the  peculiar  changes  in  the  height  of  the  contractions  which  occur  in 
the  early  part  of  an  experiment  such  as  that  which  we  have  described  are  not 
abnormal,  and  the  result  of  the  artificial  conditions  under  which  the  work  is 
done,  is  shown  not  only  by  the  fact  that  they  are  observed  when  a  muscle 


GENERAL    PHYSIOLOGY   or    J/r,SY7,/v    AM)    NEMVJE.      115 

which  has  its  normal  blood-supply  is  rhythmically  excited  to  a  large  number 
of  contractions,  but  by  the  personal  experience  of  every  one  accustomed  to 
violent  muscular  exercise.  Everyone  is  conscious  that  he  cannot  put  out  the 
greatest  muscular  effort  until  he  has  "warmed  up  to  the  work."  The-  runner 
precedes  the  race  by  a  short  run ;  the  oarsman  takes  a  short  pull  before  going 
to  the  line ;  in  all  the  sports  one  sees  the  contestants  making  movements  to 
••limber  up"  before  they  enter  upon  the  work  of  the  game.  These  prelim- 
inary movements  are  performed  not  only  to  put  the  muscles  in  better  condition 
for  action,  but  to  ensure  more  accurate  co-ordination — that  is  to  say,  the  facts 
ascertained  for  the  muscle  can  be  carried  over  to  the  central  nervous  system. 
The  finely  adjusted  activities  of  the  nerve-cells  which  control  the  muscles  reach 
their  perfection  only  after  repeated  action. 

In  such  experiments  as  that  recorded  in  Figure  46  the  record  shows  to 


FIG.  47.— Effect  of  excitation  upon  the  form  of  separate  contractions.  In  this  experiment  the  records 
of  the  muscular  contractions  were  taken  upon  a  rapidly  revolving  drum.  The  muscle  was  the  gas- 
trocnemius  of  the  frog ;  the  contractions  were  isotonic ;  the  weight  was  very  light,  about  10  grams ;  the 
stimuli  were  maximal  breaking  induction  shocks ;  and  the  rate  of  stimulation  was  twenty-three  per 
minute.  1  marks  the  first  contraction;  2,  the  100th;  3,  the  200th ;  4,  the  300th.  The  muscle  was  excited 
automatically  by  an  arrangement  carried  by  the  drum,  and  the  excitation  was  always  given  when  a 
definite  part  of  the  surface  of  the  drum  was  opposite  the  point  of  the  lever  which  recorded  the  con- 
tractions. 

a  remarkable  degree  the  fact  that  at  any  given  time  the  muscle  has  a  definite 
capacity  for  work.  A  suitable  explanation  of  this  is  lacking.  The  corre- 
spondence in  the  height  of  the  contractions  of  the  same  group,  and  the  differ- 
ence in  the  height  of  different  groups  of  contractions,  must  be  attributed  to  the 
existence  within  the  muscle-cell  of  some  automatic  mechanism  which  regulates 
the  liberation  of  energy  and  which  has  its  activity  greatly  influenced  by  the 
alterations  which  result  from  action.  Whether  this  supposed  automatic  regu- 
^latory  mechanism  controls  both  the  preparation  of  the  final  material  from 
which  the  energy  displayed  by  the  muscle  is  liberated,  and  the  amount  of  the 
explosive  change  which  results  from  the  application  of  the  irritant,  cannot  be 
definitely  said. 

(2)  Effect  of  Frequent  Excitations  upon  the  Form  of  Separate  Contractions. 
-The  effect  of  activity  is  not  only  observable  in  the  change  in  the  height 


116  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

of  the  muscular  contractions,  but  in  the  length  of  the  latent  period,  in  the  rate 
at  which  the  muscle  shortens,  and  in  the  rate  at  which  the  muscle  relaxes. 
The  effect  of  a  large  number  of  separate  contractions,  made  in  quick  succes- 
sion, upon  the  rate  at  which  the  muscle  changes  its  form  during  contraction, 
is  illustrated  in  the  myograms  reproduced  in  Figure  47. 

In  Figure  47  only  the  1st,  100th,  200th,  and  300th  contractions  were  re- 
corded. The  perpendicular  line  marks  the  point  at  which  the  stimulus  was 
given.  In  this  experiment  the  latent  period  for  each  of  the  succeeding  con- 
tractions is  seen  to  be  longer ;  the  height  is  lessened  ;  the  rise  of  the  curve  of 
contraction  is  slowed  and  the  curve  of  relaxation  is  even  more  prolonged.  These 
and  certain  other  changes  are  to  be  observed  in  the  records  of  Figure  48,  which 
were  taken  in  an  experiment  made  under  the  same  conditions  as  the  last,  except 
that  the  rate  of  excitation  was  80  per  minute,  instead  of  23,  as  in  the  preced- 
ing experiment,  and  the  record  of  every  50th  contraction  was  recorded. 


FIG.  48.— Effect  of  frequent  excitation  on  the  form  of  separate  contractions.  The  method  employed 
to  obtain  this  record  is  the  same  as  in  the  preceding  experiment,  except  that  the  drum  is  revolving  more 
rapidly,  and  every  50th  contraction  is  recorded:  1  marks  the  first  contraction;  2,  the  50th:  3,  the  100th; 
4,  the  150th ;  5,  the  200th ;  6,  the  250th ;  7,  the  300th. 

A  comparison  of  the  first  with  the  50th  contraction  gives  a  number  of 
points  of  interest.  The  stimulating  effect  of  action  upon  the  contraction  pro- 
cess is  shown  by  the  fact  that  the  latent  period  of  the  50th  (2  of  Fig.  48)  is 
shorter  than  that  of  the  first,  the  rise  of  the  curve  is  somewhat  steeper,  and 
the  height  is  considerably  greater.  It  is  noticeable,  however,  that  the  crest 
is  prolonged,  and  consequently  the  total  length  of  the  contraction  is  increased. 
Such  a  prolongation  of  the  contraction  is  known  as  "  Contracture."  In  con- 
sidering the  greater  activity  of  the  contraction  process  of  this  50th  contraction 
as  compared  with  the  first,  we  must  recall  that  it  represents  one  of  a  series 
of  staircase  contractions,  such  as  we  noticed  in  Figure  46.  If  we  examine 
the  1 00th  contraction  (3  of  Fig.  48),  we  see  the  evidences  of  the  beginning  of 
fatigue ;  although  the  latent  period  is  nearly  as  quick  as  in  the  first,  the  rise, 
of  the  curve  is  less  rapid,  the  height  is  less,  and  rate  of  relaxation  is  very 
much  slowed.  These  changes  are  to  be  seen  in  a  more  marked  degree  in  the 
150th  contraction  (4  of  Fig.  48),  and  the  prolongation  of  the  crest  of  the 
contraction  and  the  decreased  rate  of  relaxation  are  particularly  noticeable. 
The  same  sort  of  differences  is  to  be  observed  in  the  later  contractions.  By 


GENERAL    PHYSIOLOGY   OF   MUSCLE    AND    NERVE.     117 

still  more  rapid  rates  of  excitation  these  alterations  in  the  contraction  curve 
are  not  only  exaggerated,  but  develop  more  quickly,  and  play  a  very 
important  part  in  producing  the  peculiar  form  of  continued  contraction 
known  as  tetanus. 

Lee1  states  that  the  slowing  of  the  contraction  process,  which  is  compara- 
tively slight  in  the  muscles  of  the  frog,  is  very  marked  in  the  muscles  of  the 
turtle,  but  practically  absent  from  the  white  muscles  of  the  cat.  Moreover, 
the  prolongation  of  the  relaxation  which  is  very  noticeable  in  the  case  of  the 
muscles  of  frogs  and  turtles,  is  very  slight  in  those  of  the  cat.  Contracture 
effects  have,  however,  been  seen  on  both  the  red  and  pale  muscles  of  the 
rabbit  and  on  the  muscles  of  man.  Although  the  muscles  of  different  animals 
show  certain  peculiarities,  the  facts  illustrated  in  the  above  experiments  can 
be  considered  as  in  general  true  of  most  striated  muscles. 

(3)  Effect  of  Frequent  Excitations  to  Produce  Tetanus. — As  we  have  seen,  the 
normal  muscle  the  first  time  that  it  is  excited  relaxes  almost  as  quickly  as  it 
contracts,  but  if  it  be  excited  rhythmically  a  number  of  times  a  minute,  gradu- 
ally loses  its  power  of  rapid  relaxation.  The  tendency  to  remain  contracted 
begins  to  show  itself  in  a  prolongation  of  the  crest  of  the  contraction  curve, 
even  before  fatigue  comes  on,  and  increases  for  a  considerable  time  in  spite  of 
the  effect  of  fatigue  in  lessening  the  height  of  the  contractions.  If  a  skeletal  mus- 
cle of  a  frog  be  excited  many  times,  at  a  rate  of  about  once  every  two  seconds, 
the  gradual  increase  in  the  duration  of  the  contractions  will  have  the  effect  of 
preventing  the  muscle  from  returning  to  its  normal  length  in  the  intervals  be- 
tween the  succeeding  stimuli,  for  contraction  will  be  excited  before  relaxation 
is  complete.  As  is  shown  in  the  record  of  the  experiment  reproduced  in  Figure 
49,  there  will  come  a  time  in  the  work  when  the  base-line  connecting  the  lower 
extremities  of  the  succeeding  myograms  will  be  seen  to  rise  in  the  form  of  a 
curve,  the  change  being  at  first  gradual,  then  more  and  more  rapid,  and  then 
again  gradual  (see  6,  Fig.  49).  The  effect  of  the  change  in  the  power  to  relax 
is  to  make  it  appear  as  if  the  muscle  were  the  seat  of  two  contraction  processes, 
the  one  acting  continuously,  the  other  intermittently  in  response  to  the  suc- 
cessive excitations.  Such  a  condition  as  that  exhibited  in  section  c,  Figure  49, 
is  spoken  of  as  an  incomplete  tetanus,  complete  tetanus  being  a  condition  of 
continuous  contraction  caused  by  rhythmical  excitations,  in  which  none  of  the 
separate  contraction  movements  are  visible.  In  complete  tetanus  the  muscle 
writes  an  unbroken  curve. 

The  slowing  of  the  relaxation  of  the  muscle  and  consequent  state  of  con- 
tinued shortening  which  is  to  be  seen  in  the  latter  part  of  the  above  experiment 
is  the  result  of  the  developing  contracture.  The  amount  of  contracture 
increases,  within  limits,  with  the  increase  in  the  strength  and  rate  of  exci- 
tation. The  intensity  and  rate  of  stimulation  required  for  the  production 
of  this  condition  depend  very  largely  upon  the  character  of  the  muscle  and 
its  condition  at  the  time.  In  the  experiment  recorded  in  Figure  50  the 
development  of  the  condition  of  contracture  was  more  marked  than  in  the 
lLee  :  American  Journal  of  Physiology,  1899,  ii.  3,  p.  11. 


118  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

above  experiment,  and  the  resulting  condition  of  continued  contraction  caused 
first  incomplete  and  finally  complete  tetanus. 

Although  frequent  excitations  appear  to  be  essential  to  the  development 
of  contracture,  it  is  not  to  be  considered  a  fatigue  effect,  since  the  contracted 
state  which  it  produces  may  be  increasing  at  the  time  that  fatigue  is  lessen- 
ing the  height  of  the  ordinary  contraction  movements,  and  since  the  form  of 
contraction  peculiar  to  contracture  is  itself  seen  to  lessen  as  fatigue  becomes 
excessive.  Both  of  these  facts  are  illustrated  in  Figure  50,  but  are  more 
strikingly  shown  in  Figure  51,  in  which  a  more  rapid  rate  of  excitation  was 
used.  The  effect  of  fatigue  to  prolong  muscular  contractions  and  the  relation 
of  contracture  to  fatigue  effects  will  be  considered  later  (see  p.  130). 

The  record  in  Figure  51  shows  many  points  of  interest :  a  to  6,  a  rapidly 


PIG.  49.— Effect  of  frequent  stimuli  to  gradually  produce  incomplete  tetanus.  Series  of  isotonic  con- 
tractions of  a  gastrocnemius  muscle  of  a  frog,  excited  once  every  two  seconds  by  strong  breaking  induc- 
tion shocks.  Only  a  part  of  the  record  is  shown,  70  contractions  have  been  omitted  between  the  end  of  the 
section  marked  a  and  the  beginning  of  section  b,  and  200  contractions  between  the  end  of  section  band  the 
beginning  of  c.  The  increase  in  the  extent  of  the  relaxations  seen  at  the  close  of  the  record  was  due 
to  the  slowing  of  the  rate  of  excitations  at  that  time. 

developing  staircase,  which  is  accompanied  by  a  rising  of  the  base  line,  which 
indicates  that  contracture  began  to  make  itself  felt  from  the  moment  the  work 
began ;  6  to  c,  a  rapid  and  then  a  gradual  fall  in  the  height  of  contractions 
due  to  fatigue  effects ;  c  to  d,  a  rise  in  the  top  of  the  curve  in  spite  of  the 
lessening  height  of  the  contractions,  due  to  the  increasing  contracture;  d  to  e, 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND    NKIiVI-:.      11!> 


a  gradual  fall  of  the  curve  of  incomplete  tetanus,  due  to  the  effect  o 

on  the  contracture  ;  e,  complete  tetanus,  but  continued  gradual  decline  in  the 

height  of  the  curve  under  the  influence  of  fatigue. 


FIG.  50.— Effect  of  frequent  excitations  to  gradually  produce  tetanus.  Experiment  on  a  gastrocnemius 
muscle  of  a  frog,  similar  to  the  last.  The  weight  was  only  10  grams.  The  rate  of  excitation  was  100  per 
minute.  This  muscle  had  been  worked  a  short  time  before  this  series  of  contractions  was  taken,  and,  as 
a  result,  the  introductory  and  staircase  contractions  were  absent  and  contracture  began  much  sooner 
than  in  the  experiment  recorded  in  Figure  48.  The  record  in  section  6  is  a  continuation  of  that  in 
section  a. 

The  following  experiment,  Figure  52,  differs  from  those  which  have  preceded 
it,  in  that  the  muscle,  instead  of  being  directly  excited,  was  stimulated  indirectly 
by  irritation  of  its  nerve.  Each  shock  applied  to  the  nerve  was  represented 
by  a  separate  contraction  process  in  the  muscle.  The  experiment  illustrates 
well  the  combined  effect  of  the  staircase  and  the  contracture  to  raise  the  height 


FIG.  51.— Development  and  fatigue  of  contracture.  Experiment  on  a  gastrocnemius  muscle  of  a  frog. 
The  weight  was  10  grams.  As  in  the  preceding  experiments  strong  maximal  breaking  induction  shocks 
were  used  to  excite.  The  rate  of  excitation  was  5  per  second.  The  record  appears  as  a  silhouette  for  the 
reason  that  the  drum  was  moving  very  slowly. 

of  the  contractions.     On  account  of  the  more  rapid  rate  of  excitation,  the 
contracture  came  on  more  quickly  than  in  the  preceding  experiments ;  it  did 


120  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

not  become  sufficient  during  the  few  seconds  that  this  experiment  lasted  to 
prevent  the  separate  relaxations  from  being  seen,  and  an  incomplete  tetanus 
was  the  result. 

In  the  experiment  the  record  of  which  is  given  in  Figure  53,  the  muscle  was 
directly  stimulated,  and  the  rate  of  excitation  was  rapid,  33  per  second.  Not 
even  this  rate  sufficed  to  cause  complete  tetanus,  and  the  crest  of  the  curve 


FIG.  52.— Development  of  incomplete  tetanus  and  contracture,  by  indirect  stimulation.  A  gas- 
trocnemius  muscle  of  a  frog  was  indirectly  stimulated  by  breaking  induction  shocks,  of  medium 
strength,  applied  to  the  sciatic  nerve.  The  rate  was  about  8  per  second,  as  shown  by  comparison  of  the 
seconds  traced  at  the  bottom  of  the  figure  with  the  oscillations  caused  by  the  separate  contractions.  The 
weight  was  somewhat  heavier  than  in  the  preceding  experiment.  The  drum  was  revolving  much  faster 
than  in  the  other  experiments,  hence  the  difference  in  the  apparent  duration  of  the  contractions. 

shows  fine  waves,  which  represent  the  separate  contractions  the  combined  effect 
of  which  resulted  in  the  almost  unbroken  curve  seen  in  the  record.  Had  the 
rate  been  a  little  more  rapid,  no  waves  could  have  been  detected  and  the  tetanus 
would  have  been  complete  from  the  start.  The  effects  of  the  staircase  and  con- 
tracture are  merged  into  one  another,  and  a  very  rapid  high  rise  of  the  curve 
of  contraction  is  the  result.  It  is  noticeable  that  the  summit  of  the  curve  is 
rising  throughout  the  experiment,  owing  to  the  increasing  contracture. 

It  is  evident  that  the  condition  of  contracture  which  is  developed  in  a 
rapidly  stimulated  muscle  will  tend  to  maintain  a  condition  of  continuous  con- 


FIG.  53.— Effect  of  rapid  excitations  to  produce  tetanus.  Experiment  with  a  gastrocnemius  muscle 
of  a  frog,  excited  directly,  with  breaking  induction  shocks  of  medium  strength,  at  the  rate  of  33  per 
second.  The  weight  was  about  15  grams.  The  drum  was  moving  much  more  slowly  than  in  the  pre- 
ceding experiment.  The  time  record  gives  fiftieths  of  a  second. 

traction,  there  being  no  opportunity  for  the  muscle  to  relax  in  the  intervals 
between  the  succeeding  excitations. 

4.  Explanation  of  the  Great  Height  of  Tetanic  Contractions. — We  have 
now  to  seek  an  explanation  of  the  fact  that  a  muscle  when  tetanized  will  con- 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.      \'1\ 

tract  much  higher  than  it  will  as  a  result  of  a  single  excitation.  As  we  have 
seen,  repeated  excitations  cause,  in  the  case  of  a  fresh  muscle,  a  gradual  increase 
in  irritability  and  consequently  a  gradual  rise  in  the  height  of  the  succeeding 
contractions,  but  the  staircase  sooner  or  later  reaches  its  upper  limit,  and  will 
not  alone  account  for  the  great  shortening  which  occurs  in  tetanus. 

Effect  of  Two  Rapidly  Following  Excitations. — Helniholtz  was  the  first  to 
investigate  the  effect  of  rate  of  excitation  on  the  height  of  combined  contrac- 
tions. For  the  sake  of  simplicity,  he  excited  a  muscle  with  only  two  breaking 
induction  shocks,  of  the  same  strength,  and  observed  the  effect  of  varying  the 
interval  between  these  two  excitations.  He  concluded  that  if  the  second  stim- 
ulus is  given  during  the  latent  period  of  the  first  contraction,  the  effect  is  the- 
same  as  if  the  muscle  has  received  but  one  shock ;  if  the  second  shock  be  applied— 
at  some  time  during  the  contraction  excited  by  the  first,  the  second  contraction 
behaves  as  if  the  amount  of  contraction  present  when  it  begins  were  the  resting 
state  of  the  muscle,  i.  e.  the  condition  of  activity  caused  by  the  first  shock  has 
no  influence  on  the  amount  of  activity  caused  by  the  second,  but  the  height 
of  the  second  contraction  is  simply  added  to  the  amount  of  the  first  contraction 
present.  Were  this  rule  correct,  as  a  result  of  this  summation,  if  the  second 
contraction  occurred  when  the  first  was  at  its  height,  the  sum  of  the  two  con- 
tractions would  be  double  the  height  of  either  contraction  taken  by  itself. 

Helmholtz'  conclusion,  that  the  condition  of  activity  awakened  by  the  first 
excitation  has  no  effect  upon  that  caused  by  the  second  excitation,  has  not  been 
substantiated  by  later  observers.  Von  Kries  *  has  found  that  the  presence  of* 
the  first  contraction  hastens  the  development  of  the  contraction  process  result-' 
ing  from  the  second  excitation ;  and  Von  Frey  2  has  ascertained  that  Helrn- 
holtz's  rule  of  summation  applies  only  to  weighted  muscles.  In  the  case  of 
unweighted  muscles  the  summation  effect  is  greatest  when  the  second  contrac- 
tion starts  during  the  period  of  developing  energy  caused  by  the  first  excita- 
tion, i.  e.  during  the  rise  of  the  first  contraction.  If  the  second  contraction 


FIG.  54.— A  schema  of  the  effect  of  double  excitations  upon  the  gracilis  muscle  of  a  frog,  by  differ- 
ent intervals  of  excitation.  To  obtain  this  figure,  the  results  of  different  experiments  were  super- 
imposed (after  Von  Frey). 

starts  during  the  period  of  relaxation  of  the  first,  the  second  may  be  not 
even  as  high  as  when  occurring  alone  (see  Fig.  54). 

1  Archiv  fur  Anatomie  und  Physiologic,  1888.  2  IKd.,  S.  213. 


122 


AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


The  fact  that  the  second  contraction  is  higher  if  it  starts  during  the  ascent 
of  the  first,  may  be  explained  as  due  to  a  summation  of  the  condition  of  ex- 


FIG.  55.— Effect  of  support  on  height  of  contractions  (after  Von  Frey) :  a,  gastrocnemius  muscle  of  a 
frog,  separate  contractions,  tetanus,  separate  contractions,  and  group  of  supported  contractions  ;  weight 
10.5  grams ;  b,  the  same,  by  weight  of  0.5  grams. 

citation  awakened  by  the  two  irritants,  and  hence  the  liberation  of  a  greater 
amount  of  energy.  Nevertheless,  the  increased  irritability,  indicated  by  stair- 
case contractions,  and  the  summation  of  excitation  effects  which  occur  by  rapidly 
repeated  excitations,  shown  by  the  above  experiment,  do  not  suffice  to  wholly 
explain  the  great  shortening  of  the  muscle  seen  in  tetanus.  Helmholtz'  idea, 
that  there  is  a  support  afforded  by  the  first  contraction  to  the  second,  must 
also  play  an  important  part,  and  we  must  turn  to  this  for  the  completion  of  the 
explanation  of  the  great  height  acquired  by  the  tetanus  curve. 

Effect  of  Support  on  the  Height  of  Contractions. — Von  Kries l  and  Von 
Frey 2  found  that,  in  general,  the  shorter  the  distance  the  muscle  has.  to  raise 
a  weight,  the  higher  it  can  contract,  and  that  if  a  muscle  be  excited  at  a  regu- 
lar rate,  and  the  support  for  the  weight  be  raised  between  each  of  the  succeed- 
ing contractions,  at  a  certain  height  of  the  support  the  contractions  may  be 
as  high  as  during  tetanus  (see  Fig.  55)  This  effect  can  be  got  with  a  fresh 
muscle  when  the  interval  between  the  excitations  is  such  that  there  can  be  no 
summation  in  Helmholtz'  sense. 

The  importance  of  this  discovery  to  our  understanding  of  tetanus  is  very 
great,  for  it  has  been  found  that  if  an  unsupported  muscle  be  rapidly  excited, 
effects  are  observed  which  closely  resemble  those  obtained  by  the  aid  of  a  sup- 
port; this  we  have  seen  in  the  experiments  recorded  in  Figures  50,  51,  p.  119. 
After  a  certain  amount  of  excitation,  a  change  occurs  in  the  condition  of  a 
muscle,  owing  to  which  it  acts  as  if  it  had  received  an  upward  push,  and  as 
if  a  new  force  had  been  developed  within  it,  which  aids  the  ordinary  con- 
traction process  in  raising  the  weight.  The  new  aid  to  high  contraction  is 
the  support  afforded  by  the  developing  condition  of  contracture.  That  con- 
1  Archivf'dr  Anatomie  und  Physiologic,  1886.  2  Ibid.,  1887. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND   NKin'i:.      \'2:\ 

tracture  offers  an  internal  support  to  the  muscle,  and  raises  the  total  height 
of  the  contraction  curve  just  as  von  Frey  found  an  external  support  to  do, 
can  he  seen  in  Figure  57. 

5.  Effect  of  Gradually  Increasing  the  Rate  of  Excitation. — One  of  the 
most  instructive  methods  of  exciting  tetanus  is  to  send  into  the  muscle  a  series 
of  breaking  induction  shocks  of  medium  intensity,  at  a  gradually  increasing 
rate.  The  record  of  such  an  experiment  has  been  reproduced  in  Figure  5(\ 


FIG.  56.— Effect  of  a  gradually  increasing  rate  of  excitation.  Excitation  of  a  gastrocnemius  muscle 
of  a  frog  with  breaking  induction  shocks  of  medium  strength.  The  time  was  recorded  directly,  by  n 
tuning-fork  making  100  vibrations  per  second.  The  rate  of  excitation  was  gradually  increased,  and 
then  gradually  decreased.  The  ascending  curve,  a-b,  shows  the  effect  of  increasing,  and  the  descending 
curve,  c-d,  of  decreasing  the  rate  of  stimulation.  Excitation  was  given  by  means  of  a  special  mechanism 
for  interrupting  the  primary  circuit  of  an  induction  apparatus  and  at  the  same  time  short-circuiting  the 
making  shocks.  This  interrupter  was  run  by  an  electric  motor  which  was  allowed  to  speed  up  slowly, 
and  was  slowed  down  gradually. 

At  the  beginning  of  the  experiment,  a,  one  complete  contraction  with  a 
wave  of  elastic  after- vibration  was  recorded ;  this  was  followed  by  two  con- 
tractions of  less  height,  "  introductory  contractions ;"  then  came  three  contrac- 
tions each  of  which  was  higher  than  the  preceding,  "staircase  contractions;" 
these  were  followed  by  three  contractions,  which,  in  spite  of  the  developing 
contracture,  were  of  less  height,  "fatigue  effect."  The  rate  of  excitation  at 
this  place  was  about  17  per  second.  From  this  point  on,  the  developing  con- 
tracture supported  the  muscle  more  and  more  and  the  contraction  waves  became 
less  and  less,  until  finally,  when  the  rate  had  become  36  a  second,  the  effect 
of  the  separate  stimuli  could  scarcely  be  detected,  although  the  curve  continued 
to  rise.  This  is  as  far  as  the  record  shows,  but  the  rate  was  increased  -till 
further,  and  the  contraction  curve  continued  to  rise,  although  less  and  less, 


124  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

until  finally  an  almost  straight,  unbroken  line  was  drawn.  After  a  little  time 
this  was  seen  to  begin  to  fall,  the  contracture  yielding  to  the  effect  of  fatigue. 

As  the  drum  had  nearly  revolved  to  the  place  at  which  the  experiment  had 
been  begun,  the  rate  of  excitation  was  then  slowly  decreased.  With  the  lessen- 
ing rate,  the  curve  fell  more  and  more  rapidly,  and  oscillations  began  to  show 
themselves.  The  character  of  the  record  during  the  rest  of  the  experiment  is 
shown  in  the  curve  c-d,  Figure  56.  At  c  the  rate  was  about  17,  and  at  d  it 
was  so  slow  that  separate  contractions  were  recorded,  nevertheless  the  curve  as 
a  whole  kept  up.  Indeed,  even  after  the  excitation  had  altogether  ceased,  the 
muscle  maintained  a  partially  contracted  state  for  a  considerable  time,  on 
account  of  the  contracture  effect,  which  only  gradually  passed  off. 

6.  Summary  of  the  Effects  of  Rapid  Excitation  which  produce  Tetanus. — 
Muscle- tetanus  is  the  result  of  the  combined  action  of  a  great  many  different 
factors,  but  the  essential  condition  is  that  the  muscle  shall  be  excited  at  short 
intervals,  so  that  the  effect  of  each  excitation  shall  have  an  influence  on  the 
one  to  follow  it.  This  influence  is  exerted  in  several  different  ways  :  1.  In- 
crease of  irritability  resulting  from  action,  and  leading  to  the  production 
of  staircase  contractions ;  21_Summation  of  excitation  effects,  as  when  each 
of  the  succeeding  stimuli  begins  to  act  before  the  contraction  process  excited 
by  its  predecessor  has  ceased ;  3.  Support  given  by  the  contracting  muscle  to 
itself,  especially  the  support  offered  by  contracture. 

The  experiment,  the  record  of  which  is  reproduced  in  Figure  57,  was  made 
on  the  gastrocnemius  muscle  of  a  frog  during  the  latter  part  of  the  winter, 
and  when  the  muscle  had  begun  to  show  the  effects  of  spring  irritability.  A 
light  weight  was  used.  The  muscle  was  first  tested  with  four  separate  break- 
ing induction  shocks  given  at  intervals  of  two  seconds  ;  it  was  then  subjected 
for  nine  seconds  to  a  tetanizing  current;  and  in  order  that  the  condition  of  the 
muscle  during  this  period  might  be  ascertained,  the  tetanizing  current  was 
shut  off  from  the  muscle  by  a  short-circuiting  mechanism  for  a  brief  period 
every  two  seconds.  Finally,  at  the  close  of  the  tetanus,  the  condition  of  the 
muscle  was  again  tested  by  single-breaking  shocks  of  the  same  intensity  as 
those  used  before  the  tetanus.  The  curve  reveals  many  points  of  interest. 

a.  The  first  four  single  contractions  show  the  "  introductory  "  effect  and 
the  beginning  of  a  "  staircase  "   effect  such  as  is  usually  observed  by  serial 
excitations. 

b.  Each  of  the  short  tetani  starts  with  a  sharp  rise  of  the  curve,  making 
what  has  been  called  the  "  introductory  peak."     These  introductory  peaks, 
which  are  caused  by  the  throw  of  the  recording  lever,  give  an  evidence  of  the 
intensity  of  the  summation  effects  at  these  times.     It  is  interesting  to  observe 
that  the  first  is  high,  the  second  low,  and  the  third,  fourth,  and  fifth  show  a 
staircase-like  growth,  which  is  indication  of  the  fact  that  excitation  increases 
the  activity  of  the  contraction  processes. 

c.  The  amount  that  the  curve  falls  in  the  short  interval  separating  the 
succeeding  periods  of  tetanus  reveals  the  extent  of  the  contracture  present  at 
these  times. 


GENERAL    PHYSIOLOGY   OF   MUSCLE   AND    NERVE.     125 

d.  The  height  of  the  base-line  after  the  tetanus  shows  the  persistence  of  the 
contracture  condition. 

e.  The  height  of  the  separate  contractions  following  the  period  of  tetanic 
excitation  was  22  mm.,  while  the  height  of  the  first  of  the  single  contractions 
preceding  the  tetani  was  14  mm.,  which  well  illustrates  how  excitation  may 
increase  irritability. 

/.  The  total  height  to  which  the  curve  was  carried  by  the  separate  shocks 
after  the  period  of  tetanic  excitation  exhibits  the  effect  of  the  support  offered 


FIG.  57.— Effect  of  tetanizing  excitations  to  increase  the  irritability  of  a  muscle  and  at  the  same  time 
to  produce  a  condition  of  contracture.  The  gastrocnemius  muscle  of  a  winter  frog,  connected  with  a 
very  light  lever  and  a  small  weight,  was  arranged  to  write  isotonic  contractions  on  a  slowly  moving 
kymograph  drum.  The  time  was  recorded  in  seconds  at  the  bottom  of  the  record,  and  above  this  the 
movement  of  the  interrupter  of  the  induction  apparatus  was  written  by  an  electric  signal.  The  muscle 
was  excited  four  times  by  breaking  induction  shocks  at  intervals  of  two  seconds ;  then  it  was  subjected 
to  a  tetanizing  current,  this  being  short-circuited  for  brief  periods  at  intervals  of  two  seconds ;  finally 
it  was  again  excited  at  two  second  intervals  with  breaking  induction  shocks  of  the  same  strength  as 
those  used  at  the  beginning  of  the  experiment. 

by  the  contracture  to  increase  the  total  height  of  contraction,  and  corroborates 
von  Frey's  statement  that  supported  single  contractions  may  carry  the  curve 
as  high  as  tetanus. 

g.  The  rapid  growth  in  the  height  of  the  crests  of  succeeding  short  tetani, 
taken  in  connection  with  the  lessening  amount  of  relaxation  during  the  inter- 
val when  the  tetanizing  current  was  shut  off,  and  the  curve  of  contraction 
seen  at  the  close  of  the  tetani,  all  go  to  show  how  contracture  may  aid  sum- 
mation and  staircase  effects  to  give  the  great  height  to  the  tetanus  curve. 
Finally,  it  may  be  stated  that  the  elasticity  of  the  muscle  gradually  increases 
as  a  result  of  tetanic  excitations,  and  this  may  aid  in  the  support  of  the  weight 
during  long-continued  tetanic  contractions. 

7.  Number  of  Excitations  required  to  Tetanize. — The  number  of  stimuli  per 
second  required  to  tetanize  a  muscle  depends  largely  on  the  nature  of  the 


126  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

muscle,  for  this  decides  the  character  of  the  separate  contractions,  and,  through 
them,  the  effect  of  their  combined  action. 

The  duration  of  the  separate  contractions,  and  the  tendency  of  the  muscle 
to  enter  into  contracture,  are  the  predominant  factors  in  determining  the  result. 
Complete  tetanus  can  only  be  obtained  in  the  case  of  a  fresh  muscle,  when  the 
interval  between  succeeding  stimuli  is  shorter  than  is  required  for  the  muscle 
to  reach  its  maximal  contraction  by  a  single  stimulus.  Thus  the  prolonged 
contractions  of  smooth  muscles  permit  of  the  development  of  a  form  of  tetanus 
by  successive  closures  of  the  galvanic  current  at  intervals  of  several  seconds. 
The  non-striated  muscle  of  the  bladder  of  the  cat  can  be  tetanized  by  induc- 
tion shocks  given  at  a  rate  of  a  little  less  than  one  in  two  seconds.1  The 
contraction  of  some  of  the  muscles  of  the  turtle  may  last  nearly  a  second,  and 
two  or  three  excitations  a  second  suffice  to  tetanize.  The  muscles  of  mar- 
mots during  the  winter  sleep  can  be  tetanized  by  5  excitations  per  second 
(Patrizi).  Tetanus  of  the  red  (slowly  contracting)  striated  muscles  of  the 
rabbit  can  be  obtained  by  10  excitations  per  second,  while  20-30  per  second 
are  required  to  tetanize  the  pale  (active)  striated  muscles  (Kronecker  and 
Sterling)  ;  100  stimuli  per  second  are  needed  to  tetanize  the  muscles  of  some 
birds  (Richet),  and  over  300  per  second  would  be  required  to  tetanize  the 
muscles  of  some  insects  (Marey).  Any  influence  that  will  prolong  the  contrac- 
tion process  will  lessen  the  rate  of  excitation  required  to  tetanize. 

8.  Effect  of  Exceedingly  Rapid  Excitations. — The  question  arises,  Is  there  an 
upper  limit  to  the  rate  of  excitation  to  which  muscles  will  respond  by  tetanus? 
There  is  no  doubt  that  this  is  the  case,  but  there  is  a  difference  of  opinion  as 
to  what  the  limit  is,  and  how  it  shall  be  explained. 

Striated  muscles  and  nerves  can  be  excited  by  rates  at  which  our  most  deli- 
cate chronographs  fail  to  act.  The  muscle  ceases  to  be  tetanized  by  direct  exci- 
tation at  a  rate  by  which  it  can  still  be  indirectly  excited  through  its  nerve. 
The  highest  rate  for  the  nerve  has  been  placed  at  from  3000  to  22,000  by  differ- 
ent observers,2  and  this  wide  difference  is  probably  attributable  to  the  methods 
of  excitation  employed.  That  such  different  results  should  have  been  reached 
is  not  strange,  if  we  recall  the  many  conditions  upon  which  the  exciting  power 
of  the  irritant  depends.  That  tetanus  should  be  obtained  by  such  high  rates 
does  not  show  that  the  nerve  responds  to  each  of  the  separate  shocks.  As  a 
rule,  when  the  rate  of  excitation  is  so  high  that  tetanus  fails  a  contraction  is 
observed  when  the  current  is  thrown  into  the  nerve,  and  often  another  when 
it  is  withdrawn  from  the  nerve — that  is,  the  muscle  behaves  as  if  it  were  sub- 
jected to  a  continuous  battery  current.  A  satisfactory  explanation  for  this,  as 
well  as  for  the  failure  of  the  tetanus,  is  at  present  lacking. 

9.  Relative  Intensity  of  Tetanus  and  Single  Contractions. — The  amount  that 
a  muscle  is  capacle  of  shortening,  when  tetanized  by  maximal  excitations,  and 

1  C.  C.  Stewart:  American  Journal  of  Physiology,  1900,  iii.  p.  25. 

2  Kronecker  and  Sterling:  Archivfiir  Anatomie  nnd  Physiologie,  1878,  and  Journal  of  Physi- 
ology, 1880,  vol.  i.     Von  Frey  und  Wiedermann  :  Berichte  der  sachsischen  Gesdlschaft  der  Wissen- 
schaft,  1885.     Both  :  Pfluga>8  Archiv,  1888. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     127 

the  strength  of  the  tetanic  contraction,  depends  very  largely  on  the  kind  of 
muscle.  For  example,  pale  striated  muscles,  although  capable  of  higher  and 
more  rapid  single  contractions  than  the  red  striated,  do  not  show  as  great  an 
increase  in  the  height  and  strength  of  contractions  when  tetanized  as  do  thf 
red;  the  latter,  which  are  very  rich  in  sarcoplasma,  have  likewise  the  givatoi 
endurance.  Gruetzner  has  called  them  "  tetanus  muscles,"  since  they  seem  to 
be  particularly  adapted  to  this  form  of  contraction.  Fick  found  that  human 
muscles  when  tetanized  develop  ten  times  the  amount  of  tension,  by  isonirtric 
contractions,  that  they  give  by  single  contractions ;  and  in  this  respect  they 
can  be  said  to  resemble  red  striated  muscles.  The  following  relations  have 
been  found  to  exist  between  the  strength  of  separate  contractions  and  tetanus 
in  certain  muscles  :  triceps  and  gastrocnemius  of  the  frog,  1  :  2  or  3  ;  the  cor- 
responding muscles  of  the  turtle,  1:5;  hyoglossus  and  rectus  abdorainalis  of 
the  frog,  1  :  8  or  9.1  It  is  evident  that  no  just  estimate  of  the  part  played 
by  different  groups  of  muscles  in  the  movement  of  the  body  can  be  reached 
without  a  careful  analysis  of  the  nature  of  the  contractions  peculiar  to  each 
of  the  muscles  participating  in  the  movement. 

Both  the  height  and  strength  of  the  tetanus  is  controlled  by  the  intensity 
of  the  stimulus.  A  strong  stimulus  not  only  causes  the  separate  contractions 
of  which  the  tetanus  is  composed  to  be  higher,  but  is  favorable  to  the  develop- 
ment of  all  the  other  factors  which  have  been  described  as  entering  into  the  pro- 
duction of  tetanus.  All  normal  physiological  contractions  are  supposed  to  be 
tetani,  and  everyone  is  conscious  of  the  wonderful  accuracy  with  which  he  can 
grade  the  extent  and  strength  of  his  voluntary  movements.  The  remarkable 
shading  of  the  intensity  of  action  observable  in  co-ordinated  movements  must 
find  its  explanation  in  the  adjustment  of  protoplasmic  activity  in  the  nerve- 
cells  of  the  central  nervous  system. 

10.  Continuous  Contractions  and  Contractures. — Under  ordinary  circum- 
stances a  striated  muscle,  %if  excited  by  a  single  stimulus,  gives  a  rapid  con- 
traction, followed  almost  immediately  by  a  nearly  equally  rapid  relaxation. 
The  duration  and  character  of  the  period  of  relaxation  are,  however,  subject  to 
great  variation.  In  certain  conditions  the  muscle  may  remain  in  a  state  of 
continuous  contraction  for  a  considerable  time,  and  then  relax  either  slowly 
or  quite  suddenly ;  or  it  may  begin  to  relax  quickly  and  then  suddenly  stop^ 
as  if  the  relaxation  process  had  received  a  sudden  check  ;  or,  after  relaxing 
quite  rapidly  for  a  short  time,  it  may,  without  having  received  any  visible 
stimulus,  contract  again  for  a  short  distance  and  remain  so  contracted  for  a 
considerable  time.  In  any  case  when  the  relaxation  period  is  unusually  long, 
the  condition  of  prolonged  contraction  is  termed  "  contracture."  The  form 
of  contracture  which  we  are  considering  at  present  originates  in  the  muscle 
itself,  and  is  to  be  sharply  distinguished  from  a  form  of  pathological  contract- 
ure, which  originates  in  the  central  nervous  system  and  in  which  the  muscle 
is  kept  continuously  contracted  by  impulses  coming  from  the  spinal  cord. 

There  are  a  great  variety  of  conditions  under  which  muscles   respond  to 

1  Biedermann  :  Elektrophysiologie,  S.  109. 


128  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

excitation  by  prolonged  contractions.  If  a  muscle  be  excited  by  frequent 
induction  shocks,  even  at  a  rate  insufficient  to  produce  tetanus,  after  a  time 
it  will  take  on  a  condition  of  continuous  contraction,  which  may  be  main- 
tained for  some  time  after  the  excitations  have  ceased  (see  Fig.  50).  If  the 
muscle  be  very  irritable,  the  contraction  caused  by  a  single  irritation  may  be 
long  drawn  out.  A  muscle  poisoned  by  veratria — and  the  same  is  true  of 
some  other  drugs  (see  p.  137) — may  show  a  remarkable  degree  of  contracture 
as  a  result  of  a  single  excitation.  The  contractions  of  fatigued  muscles  tend 
to  be  greatly  prolonged ;  and  this  is  very  markedly  the  case  with  a  dying 
muscle,  which  gives  well-defined,  long-continued  contractions,  localized  at 
the  point  excited,  called  by  Schiff  the  "  idio-muscular  contraction."  The 
contractions  caused  by  the  making  and  breaking  of  a  strong  battery  current 
applied  to  a  muscle  may  likewise  be  followed  by  localized  contractions  which 
last  a  considerable  time. 

In  this  connection  one  must  bear  in  mind  that  the  length  of  muscles 
varies  with  their  elasticity  (see  p.  105),  and  that  this  changes  not  a  little 
under  varying  conditions.  Finally,  it  is  necessary  to  recall  that  muscles  when 
entering  into  rigor  mortis  or  rigor  caloris  take  on  a  condition  of  contraction 
which  may  last  for  days  (see  p.  159). 

Contracture  in  Normal  Muscles  following  Frequent  Excitations. — The  con- 
dition of  prolonged  after-contraction  which  results  from  frequent  excitations 
was  first  studied  with  care  on  the  muscles  of  the  frog,  by  Tiegel,1  who  gave 
it  the  name  of  "  contracture." 

Richet  found  that  the  claw-muscles  of  the  crab  are  particularly  subject  to 
this  form  of  contraction,  Rossbach  observed  it  in  the  muscles  of  the  cat,  and 
Mosso  2  saw  it  in  the  muscles  of  man  when  vigorously  excited  either  volun- 
tarily or  electrically.  Mosso  finds  a  teleological  reason  for  its  existence  in 
that  it  appears  most  marked  under  conditions  when  prolonged  contractions 
are  desirable,  and  might  offer  a  certain  economy  in^the  innervation  of  muscle 
by  lessening  the  work  of  the  nerve-cell.  Richet3  writes  that  normal  con- 
tracture is  not  to  be  confused  with  the  prolonged  relaxation  of  fatigued  and 
dying  muscles,  nor  with  the  contraction  of  muscle  substance  in  rigor  mortis ; 
it  is  best  seen  on  muscles  which  are  fresh  and  excitable.  Although  most 
readily  called  out  by  strong  direct  electrical  excitation  of  the  muscle,  it  is 
not  due  to  the  effect  of  the  current  as  such,  because  it  may  be  produced  by 
exciting  the  muscle  indirectly  through  its  nerve,  and  by  voluntary  muscular 
contractions  of  man.  On  the  other  hand,  the  presence  of  the  nerve  is  not 
essential,  for  curarized  muscles  may  exhibit  contracture. 

That  a  condition  of  increased  excitability  is  favorable  to  the  development 
of  contracture  is  made  evident  by  the  curve  reproduced  in  Figure  57.  In  this 
experiment  the  muscle  was  subjected  to  a  tetanizing  induction  current  for 
nine  seconds,  the  stimulation  being  interrupted  for  an  instant  every  two 

1  Tiegel :  Pftiiger's  Archiv,  1876,  xiii.  S.  71-84. 

8  Mosso :  Archives  italiennes  de  Biologic,  1890,  xiii.  pp.  165-179. 

8  Kichet:  Dictionnaire  de  Physiologic,  1899,  t.  iv.  pp.  391-393. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND   NERVE.     129 

seconds,  to  permit  the  contracture  which  was  present  at  these  times  to  show 
itself.  The  effect  was  to  increase  the  excitability  of  the  muscle,  as  shown 
by  the  increased  height  of  the  contractions  recorded  after  the  tetanus,  and  t<> 
produce  a  marked  contracture,  as  was  shown  by  the  fact  that  the  muscle  only 
partially  relaxed  after  the  tetanizing  current  had  ceased,  and  kept  partially 
contracted  in  the  intervals  between  the  succeeding  separate  contractions.  The 
fact  that  contracture  can  develop  hand  in  hand  with  increasing  excitability 
shows  that  it  may  occur  in  the  absence  of  fatigue.  It  is  interesting  to  note 
that  the  muscle  made  contraction  and  relaxation  movements  at  the  same 
time  that  it  remained  continually,  although  incompletely,  contracted  ;  and 
finally,  that  the  contracture  offered  a  firm,  elastic  support  to  the  separate 
contraction  movements,  and  that  the  relaxation  movements  following  these 
separate  contractions  were  rapid,  as  is  made  evident  by  the  character  of  the 
elastic  oscillations  resulting  from  the  rapid  fall  of  the  lever. 

The  fact  that  a  muscle  can  remain  continuously,  though  incompletely,  con- 
tracted, at  the  same  time  that  it  makes  rapid  contraction  and  relaxation  move- 
ments, suggests  that  it  may  at  the  same  time  be  the  seat  of  two  independent 
contraction  processes.  The  observation  recalls  the  action  of  the  heart  mus- 
cle, for  the  ventricle  maintains  a  condition  of  greater  or  less  tonus,  at  the 
same  time  that  it  makes  separate  beats ;  it  is  therefore  in  harmony  with  a 
well-known  physiological  process. 

Contracture  following  Single  Excitations. — An  examination  of  the  contract- 
ure effects  sometimes  seen  to  follow  single  excitations  of  irritable  muscles 
throws  some  light  on  the  nature  of  the  process.  Richet  observed  on  the 
closing-muscle  of  the  claw  of  the  crab  that  a  single  excitation  caused  a  rapid 
contraction,  which  was  followed  by  a  rapid  relaxation,  and  this  in  turn  by  a 
second  contraction  movement  which  lasted  a  considerable  time. 

A  similar  curve  may  be  obtained  from  the  striated  muscle  of  a  frog  incom- 
pletely poisoned  with  veratria ;  if  a  single  shock  be  given,  the  curve  rises 
suddenly,  and  this  quick  rise  is  followed  by  an  immediate  fall,  which  is  inter- 
rupted by  a  second  and  slower  rise,  which  is  continued  as  a  prolonged  con- 
traction. In  both  cases  the  curve  suggests  that  the  single  excitation  called 
out  two  contraction  movements,  the  first  a  rapid,  short-lived  contraction,  the 
second  a  slower,  prolonged  contraction.  It  has  been  suggested  that  the  mus- 
cle contains  two  kinds  of  muscle-fibres,  which,  like  the  pale  (rapidly  con- 
tracting radialis  externus)  and  red  (slowly  contracting  radialis  internus) 
muscles  of  the  rabbit,  have  two  different  rates  of  contraction.1  This  expla- 
nation is  not  very  satisfactory,  because  it  has  been  found  that  both  the  pale 
and  the  red  muscles  of  the  rabbit  can  give  typical  veratria  contracture  curves.2 
Moreover,  both  heart-muscle  and  non-striated  muscles  show  independent  tonus 
and  contraction  movements  though  containing  only  one  kind  of  muscle-fibre.3 

1  Griitzner:  PJluger's  Archiv,  1887,  Bd.  41,  8.  256. 

2  Carvallo  and  Weiss:  Journal  de  Physiologic  et  Pathologic  generale,  1899,  t.  i.  p.  1.     Bu- 
cannan  :  Journal  of  Physiology,  1899,  vol.  xxv.  p.  145. 

3  Bottazzi  :  Journal  of  Physiology,  1897,  vol.  xxi.  p.  1 

VOL.  II.— 9 


130  AN  AMERICAN    TEXT-BOOK    OF   PHYSIOLOGY. 

It  is  hard  to  think*  of  one  and  the  same  kind  of  muscle  substance  con- 
tracting and  relaxing  quickly  at  the  same  time  that  it  is  continuously  con- 
tracted, and  the  attempt  has  been  made  to  explain  the  phenomenon  on  the 
assumption  that  every  muscle-fibre  contains  two  kinds  of  contractile  sub- 
stance, and  that  the  anisotropic  fibrillary  structures  of  the  fibre  are  capable 
of  rapid  contractions  and  the  isotropic  sarcoplasma  of  slow  contractions. 
According  to  this,  the  first  quick  rise  of  the  contraction  of  the  veratrinized 
muscle,  etc.,  is  due  to  the  anisotropic  substance,  and  the  prolonged  after- 
contraction  to  the  sarcoplasma.  This  explanation  offers  much  that  is  satis- 
factory, but  can  scarcely  be  accepted  until  we  are  sure  that  anisotropic  and 
isotropic  substances  are  capable  of  independent  contractions. 

The  prolonged  contraction  of  a  muscle  treated  by  veratria  is  an  active 
process,  and  not  merely  the  result  of  a  change  in  its  physical  condition,  such 
as  an  increase  in  elasticity.  This  is  shown  by  the  fact  that  during  the  stage 
of  contracture  the  muscle  liberates  more  heat  than  when  at  rest.  The  heat 
developed  during  a  single  veratria  contraction  may  be  as  much  as  is  given 
off  by  a  normal  muscle  excited  to  tetanus  for  two  seconds.1  The  fact  that 
the  prolonged  contraction  of  the  veratrinized  muscle  disappears  on  etheriza- 
tion, and  returns  as  the  effect  of  the  ether  passes  off,  also  favors  the  view 
that  it  is  dependent  on  physiological  activity  of  the  muscle  protoplasm.2 

This  conclusion  is  likewise  indicated  by  the  observation  that  the  prolonga- 
tion of  the  contraction  is  most  marked  at  a  moderate  temperature,  and  fails 
at  very  low  or  very  high  temperatures.3  It  has  sometimes  been  thought  that 
it  was  an  expression  of  fatigue,  but  this  can  hardly  be  the  case,  because  it  is 
seen  when  the  rate  of  the  rise  and  the  height  of  the  curve  of  contraction  are 
normal,  and  it  ceases  in  the  case  of  the  veratrinized  muscle  if  the  muscle  is 
worked  for  a  time,  and  reappears  when  it  has  become  rested.  Moreover, 
veratria  in  small  doses  strengthens  the  contractions  of  fatigued  muscle  and 
increases  its  irritability,  so  that  it  responds  to  smaller  stimuli  by  more  work. 
It  would  appear  that  we  may  conclude  that  the  contracture  of  the  veratrin- 
ized muscle,  like  that  of  the  normal  muscle,  is  a  true  contraction  process,  but 
that  we  must  await  further  evidence  before  deciding  as  to  the  exact  nature  of 
the  contracture. 

Effect  of  Fatigue. — If  a  muscle  be  excited  to  contraction  by  frequent  exci- 
tations, its  irritability  for  a  time  will  be  increased,  the  contractions  will 
become  stronger,  higher,  and  more  prolonged.  If  a  muscle  be  excited  to 
contraction  at  too  slow  a  rate  to  cause  an  increase  of  irritability,  it  will 
gradually  fatigue,  and,  as  it  does  so,  its  contractions  will  become  weaker, 
lower,  and  more  prolonged.  The  prolongation  of  the  contractions  seen  in 
these  two  cases  is  probably  due  to  quite  different  causes. 

In  the  first  experiment  it  was  a  true  contraction  process ;   in  the  second  it 

1  Fick  and  Boehm :    Verhand.  der  physikal-med.  Gesellschaft  in  Wiirzburg,  1872,  Bd.  iii.,  N. 
F.,  S.  198. 

2  Locke :  Journal  of  Experimental  Medicine,  1896,  vol.  i.  p.  630. 

3  Brunton  and  Cash :  Journal  of  Physiology,  1883,  vol.  iv.  p.  237. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND    NERVE.     131 

was  the  result  of  inability  to  relax.  Relaxation  as  well  as  contraction  is  to  be 
regarded  as  an  active  process,  and  in  fatigue  the  power  both  to  contract  ami 
relax  is  lessened. 

The  prolonged  contraction  of  the  fatigued  muscle  is  chiefly  caused  by  the 
injurious  effects  of  the  waste  products  produced  within  it  as  a  result  of  the 
chemical  changes  accompanying  its  activity.  One  of  these  waste  products, 
sarcolactic  acid,  is  known  to  have  the  effect  to  prolong  muscular  contractions,1 
and  it  is  not  unlikely  that  others  may  exert  a  similar  influence. 

In  case  the  muscle  be  excited  frequently  and  for  a  considerable  time, 
the  contraction  effect  and  the  decreased  power  of  relaxation  due  to  fatigue, 
toward  the  end  of  the  experiment,  may  both  be  present  at  the  same  time,  and 
both  act  to  prolong  the  curve  of  contraction.  This  was  probably  the  case  in 
the  experiments  the  records  of  which  are  given  in  Figures  47  and  48,  and 
many  of  the  figures  employed  to  illustrate  the  development  of  tetanus. 

An  example  of  this  is  to  be  seen  in  the  effect  of  certain  chemical  sub- 
stances on  the  muscle.  For  example,  the  withdrawal  of  water  by  drying,  by 
the  application  of  glycerin,  or  by  ^ strong  solution  of  sodium  chloride,  may, 
by  rapidly  altering  the  constitution  of  the  protoplasm,  cause  an  increase  of 
excitability  which  may  pass  over  to  a  state  of  excitation,  which  will  be  man- 
ifested by  irregular  but  more  or  less  continuous  contractions.  Such  contrac- 
tions are  of  the  type  of  an  incomplete  tetanus. 

Effect  of  Constant  Battery  Current. — Attention  has  already  been  called  to 
the  fact  that  under  certain  circumstances  a  form  of  continuous  contraction 
may  be  excited  by  a  continuous  constant  electric  current.  If  the  current  be 
very  strong,  the  short  closing  contraction  may  be  followed  by  a  more  or  less 
continuous  contraction — the  closing  (or  Wundt's)  tetanus  ;  and  the  short  open- 
ing contraction  may  be  followed  by  another  continuous  contraction,  which 
only  gradually  passes  off — the  opening  (or  Hitter's)  tetanus.  This  form  of 
contraction  is  quite  readily  excited  in  normal  human  muscles  by  both  direct 
and  indirect  excitation.  The  term  "  galvanotonus  "  is  sometimes  employed 
for  the  continuous  contraction  of  human  muscles  excited  by  the  continuous 
flow  of  a  constant  current. 

Although  a  continuous  contraction  caused  by  the  constant  current  is 
spoken  of  as  tetanus,  it  is  a  matter  of  doubt  whether  it  is  a  true  tetanic  con- 
dition, for  the  term  tetanus  is  limited  to  a  form  of  contraction  which,  though 
apparently  continuous,  is  really  an  interrupted  process,  and  results  from 
many  frequently  repeated  stimuli.  Von  Frey 2  expresses  the  view  that  the 
continuous  contraction  which  follows  the  closing  of  the  continuous  constant 
current  is  a  form  of  tetanus.  It  is  certainly  true  that  the  closing  tetanus 
often  shows  irregular  oscillations,  suggestive  of  a  more  or  less  intermittent 
excitation  which  might  be  explained  on  the  supposition  that  the  flow  of  the 
current  produces  electrolytic  decompositions  within  the  tissue,  and  that  the 
liberated  ions  exciting  the  protoplasm  of  the  different  fibres  irregularly  lead 

1  Lee:  American  Journal  of  Physiology,  1899,  vol.  ii.  p.  11. 
3  Archiv  Jiir  Anatomic  und  Physiologic,  1885,  S.  55. 


132  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

to  irregular  contractions  of  the  separate  fibres,  the  combined  action  of  which 

produces  more  or  less  regular  continued  contraction.     Another  view  would  be 

that  contracture  might  be  produced  under  the  influence  of  the  changes  caused 

-  by  the  electric  current,  and  a  condition  result  similar  to  that  which  causes 

.   the  prolonged  contractions  characteristic  of  poisoning  with  veratria,  etc. 

Effect  of  Death  Processes. — If  a  muscle  be  dying,  it  responds  to  excitations 
by  very  slow,  weak,  and  prolonged  contractions,  definitely  localized  at  the 
place  excited.  Such  a  form  of  contraction  is  often  classed  as  contracture,  in 
spite  of  the  fact  that  the  irritability  is  greatly  lessened.  This  form  of  con- 
traction may  be  seen  toward  the  end  of  prolonged  wasting  diseases  in  the  case 
of  the  muscles  of  men.  They  respond  to  mechanical  stimulations  by  local- 
ized, slowly  developing  contractions. 

Pathological  Contracture  of  Central  Nervous  Origin. — In  certain  patholog- 
ical conditions  there  may  be  contractures  which  do  not  depend  upon  the  con- 
dition of  the  muscle,  but  which  originate  in  the  central  nervous  system.  In 
these  cases  the  muscles  are  in  continuous  receipt  of  nerve  impulses  from  the 
spinal  cord  cells,  and  are  kept  in  continuous  contraction,  which  varies  in  degree 
from  the  amount  observed  during  ordinary  reflex  muscle  ton  us  to  a  state  of 
intense  rigidity.  The  peculiarity  of  the  condition  is  its  endurance.  The 
muscle  does  not  appear  to  fatigue ;  moreover,  it  is  said  that  it  does  not 
develop  the  large  amount  of  heat  (Brissand  et  Regnard)  which  is  always 
formed  as  a  result  of  the  chemical  changes  which  take  place  during  the  ordi- 
nary contractions. 

For  these  reasons,  Richet  *  considers  the  shortening  of  the  muscle  to  be 
not  a  true  contraction,  but  the  result  of  an  increase  of  elasticity.  It  is  possi- 
ble that  some  pathological  contractions  may  be  of  different  nature  from  those 
which  we  have  been  considering,  but  they  have  not  been  studied  sufficiently 
to  enable  us  to  draw  definite  conclusions  from  them. 

(d)  Normal  Physiological  Contractions. — All  normal  physiological  con- 
tractions of  muscles  are  regarded  as  tetani.  Even  the  shortest  possible  vol- 
untary or  reflex  movements  are  considered  to  be  too  long  to  be  single  contrac- 
tions. Inasmuch  as  we  can  artificially  excite  normal  muscles  to  continuous 
contraction  only  by  means  of  a  series  of  rapidly  following  stimuli,  we  find  it 
hard  to  explain  continuous  physiological  contractions  on  any  other  basis,  and 
hence  the  view  that  the  excitation  sent  by  the  nerve-cells  to  muscles  has 
always  a  rhythmic  character,  and  that  the  normal  motor-nerve  impulse  is  a 
discontinuous  rather  than  continuous  form  of  excitation.  The  view  is  prob- 
ably correct,  but  cannot  be  considered  as  proved.  The  evidence  in  favor  of 
it  is  as  follows  : 

Muscle-sounds,  Tremors,  etc. — During  voluntary  muscular  contractions  the 
muscle  gives  out  a  sound,  which  would  imply  that  its  finest  particles  are  not 
in  a  state  of  equilibrium,  but  vibrating.  By  delicate  mechanisms  it  has  been 
possible  to  obtain  records  of  voluntary  and  reflex  contractions  which  showed 
oscillations,  although  the  contraction  of  the  muscle  appeared  to  the  eye  to  be 

1  Dictionnaire  de  Physiologic,  1899,  iv.  p.  393. 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND    NERVE.     133 

•  continuous.  If  the  surface  of  a  muscle  be  exposed  and  be  wet  and  glistening, 
the  light  reflected  from  it  during  continued  contractions  is  seen  to  flicker,  as 
if  the  surface  were  shaken  by  fine  oscillations.  In  fatigue  the  muscle  passes 
from  apparently  continuous  contraction  to  one  exhibiting  tremors,  and  mus- 
cular tremors  are  observed  under  a  variety  of  pathological  conditions. 

With  these  facts  in  mind,  a  number  of  observers  have  endeavored  to  dis- 
cover the  rate  at  which  the  muscle  is  normally  stimulated.  Experiments  in 
which  muscles  have  been  excited  t6  incomplete  tetanic  contractions  by  induced 
currents,  interrupted  at  different  rates,  have  shown  that  the  muscle  follows  the 
rate  of  excitation  with  a  corresponding  number  of  vibrations,  and  does  not 
show  a  rate  of  vibration  peculiar  to  itself.  Further,  it  has  been  ascertained 
that  the  sound  given  out  by  a  muscle  excited  to  complete  tetanus,  I.  e.  an 
apparently  continuous  contraction,  corresponds  to  the  rate  at  which  it  is  ex- 
cited. Apparently,  any  rate  of  oscillations  detected  in  a  muscle  during  normal 
physiological  excitation  would  be  an  indication  of  the  rate  of  discharge  of 
impulses  from  the  central  nerve-cells. 

Wollaston  was  the  first  to  observe  that  a  muscle  gives  a  low  dull  sound 
when  it  is  voluntarily  contracted,  and  that  this  sound  corresponds  to  a  rate  of 
vibration  of  36  to  40  per  second.  It  may  be  heard  with  a  stethoscope  placed 
over  the  contracting  biceps  muscle,  for  instance,  or  if,  when  all  is  still  and  the 
ears  are  stopped,  one  vigorously  contracts  his  masseter  muscles.  Helmholtz 
placed  vibrating  reeds  consisting  of  little  strips  of  paper,  etc.,  on  the  muscle, 
and  found  that  only  those  which  had  a  rate  of  vibration  of  18  to  20  per 
second  were  thrown  into  oscillation  when  the  muscle  was  voluntarily  contracted. 
This  observation  indicated  that  the  muscle  had  a  rate  of  vibration  of  18  to  20 
per  second,  a  rate  too  slow  to  be  recognized  as  a  tone.  He  concluded  that  the 
tone  heard  from  the  voluntarily  contracted  muscle  was  the  overtone,  instead 
of  the  true  muscle-tone.  The  consideration  that  the  resonance  tone  of  the 
ear  itself  corresponds  to  36  to  40  vibrations  per  second,  makes  it  question- 
able whether  the  muscle-sound  should  be  accepted  as  evidence  of  the  rate  of 
normal  physiological  excitation ;  nervetheless,  the  experiments  with  the 
vibrating  reeds  remain  to  indicate  18  to  20  per  second  to  be  the  normal 
rate. 

Within  the  last  few  years  a  number  of  researches  bearing  upon  this  question 
have  been  published,  and  the  results  of  these  point  to  a  still  slower  rate  of  vol- 
untary excitation,  varying  from  8  to  12  per  second  according  to  the  muscle  on 
which  the  experiment  is  made.  Lov&i1  discovered  in  the  tetanus  excited  in 
frogs  poisoned  with  strychnia,  and  in  voluntary  contractions,  both  by  mechani- 
cal methods  and  by  recording  the  electrical  changes  occurring  during  action 
with  the  capillary  electrometer,  rates  of  7  to  9  per  second.  Horsley  and 
Schafer2  ex  cited  the  brain  cortex  and  motor  tracts  in  the  corona  radiata  and  the 
spinal  cord  of  mammals  by  induction  shocks,  at  widely  differing  rates,  and 
recorded  the  resulting  muscular  contractions  by  tambours  placed  over  the 
muscles.  They  observed  oscillations  in  the  myograms  obtained  which  had  a 

1  Central blatt  ftir  die  medicinischen  Wissenscho/ten,  1881. 

2  Journal  of  Physiology,  1886,  vii.  p.  96. 


134  AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 

rate  of  8  to  12  per  second,  the  average  being  10.  The  rate  of  oscillations  was 
quite  independent  of  the  rate  of  excitation,  and  oscillations  of  the  same  rate 
were  seen  by  voluntary  and  by  reflex  contractions.  Tunstall l  found  by  the  use 
of  tambours,  in  experiments  on  voluntary  contractions  of  men,  a  rate  of  8  to  13 
per  second,  with  an  average  of  10.  Griffiths2  likewise  used  the  tambour 
method,  and  studied  the  effect  of  tension  on  the  rate  of  oscillations  in  voluntarily 
contracted  human  muscles.  He  observed  rates  varying  from  8  to  19,  the  rate 
being  increased  with  an  increase  of  weight  up  to  a  certain  point,  and  beyond  this 
decreased.  The  oscillations  became  more  extensive  as  fatigue  developed.  Von 
Kries  by  a  similar  method  found  rates  varying  with  different  muscles,  but 
averaging  about  10. 

It  is  not  easy  to  harmonize  the  view  that  8  to  13  excitations  per  second 
can  cause  voluntary  tetani,  when  it  is  possible  for  the  expert  pianist  to  make 
as  many  as  10  or  11  separate  movements  of  the  finger  in  a  second.  It  is, 
indeed,  a  common  observation  that  a  muscle  can  be  slightly  and  continuously 
voluntarily  contracted,  and,  at  the  same  time,  be  capable  of  making  additional 
short  rapid  movements.  Von  Kries  would  explain  this  as  due  to  a  peculiar 
method  of  innervation,  while  Biedermann  favors  Gruetzner's8  view  that  the 
muscle  may  contain  two  forms  of  muscle-substance,  one  of  which  is  slow  to 
react,  resembling  red  muscle-tissue,  and  maintains  the  continuous  contraction, 
the  other,  of  more  rapid  action,  being  responsible  for  the  quicker  movements. 
Although  the  evidence  is,  on  the  whole,  in  favor  of  the  view  that  all  normal 
contractions  of  voluntary  muscles  are  tetanic  in  character,  there  is  a  great  deal 
which  remains  to  be  explained. 

Effect  of  Artificial  compared  with  Normal  Stimulation. — Experiment  shows 
that,  with  the  same  strength  of  irritant,  a  muscle  contracts  more  vigorously 
when  irritated  indirectly,  through  its  nerve,  than  when  it  is  directly  stimulated. 
Rosenthal  describes  the  following  experiment :  If  the  nerve  of  muscle  A  be 
allowed  to  rest  on  a  curarized  muscle  B,  and  an  electric  shock  be  applied  in 
such  a  way  as  to  excite  nerve  A  and  muscle  B  to  the  same  amount,  muscle  A 
will  be  found  to  contract  more  than  muscle  B. 

Further,  it  has  been  found  that  muscles  respond  more  vigorously  to  volun- 
tary excitations  than  to  any  artificial  stimulus  which  can  be  applied  to  either 
the  nerve  or  muscle.  This  shows  itself,  not  only  in  the  fact  that  a  muscle  can 
by  voluntary  stimulation  lift  much  larger  weights  than  by  electrical  excitation, 
but  that  after  a  human  muscle  has  been  fatigued  by  electrical  excitations  it 
can  still  respond  vigorously  to  the  will.  An  illustration  of  this  is  given  in 
Figure  58. 

Fatigue  of  Voluntary  Muscular  Contractions. — Mosso  and  his  pupils  have 
done  a  large  amount  of  work  upon  the  fatigue  of  human  muscles  when  excited 
by  voluntary  and  artificial  stimuli  under  varying  conditions  (see  p.  72).  The 
results  at  which  they  arrived  all  favor  the  view  that  human  muscles  differ 
but  little  from  those  of  warm-blooded  animals,  and  that  the  facts  which  have 


Journal  of  Physiology,  1886,  vii.  p.  114.  2  Journal  of  Physiology,  1888,  ix.  p.  39. 

8  P/Higer**  Archiv,  1887,  Bd.  41,  S.  277. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.      1  :'.."> 

been  ascertained  by  experiments  upon  cold-blooded  animals,  such  as  the 
frog,  can  be  accepted  with  but  slight  modifications  for  the  muscles  <•!'  man. 
In  the  experiment  recorded  in  Figure  58  we  see  the  effect  of  repeated  tetanic 


FIG.  58.— Voluntary  excitations  are  more  effective  than  electrical.  The  flexor  muscles  of  the  second 
finger  of  the  left  hand  of  a  man  were  excited  first  voluntarily,  a,  then  electrically,  a-b,  and  then  volun- 
tarily, b.  The  electrical  excitation  consisted  of  series  of  induction  shocks,  which  were  applied  once 
every  two  seconds,  during  about  half  a  second,  the  spring  interrupter  of  the  induction  coil  vibrating 
23  times  per  second.  Each  time  the  muscle  contracted  it  raised  a  weight  of  one  kilogram.  Each  of  the 
contractions  recorded,  whether  the  result  of  electrical  or  voluntary  excitation,  was  a  short  tetanus. 

contractions,  excited  by  electricity,  to  fatigue  a  human  muscle.  Normal 
voluntary  contractions,  if  frequently  repeated,  provided  the  muscle  has  to 
raise  a  considerable  weight,  likewise  cause  fatigue.  This  was  illustrated  in 
the  experiment  recorded  in  Figure  59. 


FIG.  59.— Effect  of  fatigue  on  voluntary  muscular  contractions.  The  flexor  muscles  of  the  second 
finger  of  left  hand  were  voluntarily  contracted  once  every  two  seconds,  and  always  with  the  utmost 
force.  The  weight  raised  was  four  kilograms. 

It  is  doubtful  whether,  in  an  experiment  such  as  is  shown  in  Figure  59,  the 
loss  of  the  power  to  raise  the  weight  is  due  to  fatigue  of  the  muscles.  It  is 
more  likely  that  the  decline  in  power  is  due  to  fatigue  of  the  central  nerve- 


136  AN  AMERICAN   TEX7-BOOK   OF  PHYSIOLOGY. 

cells  by  which  the  muscles  are  excited  to  action  during  the  voluntary  mus- 
cular work.1  This  fact,  that  the  nerve-cells  give  out  before  the  muscles,  ex- 
plains the  apparent  contradiction,  that  a  muscle  fatigued  by  electric  excitations 
can  be  voluntarily  contracted,  and  when  the  power  to  voluntarily  contract  the 
muscles  has  been  stopped  by  fatiguing  voluntary  work  the  muscles  will  respond 
to  electrical  excitation.  It  is  undoubtedly  of  advantage  to  the  body  that  the 
nerve-cells  should  fatigue  before  the  muscles,  for  the  muscles  are  thereby  pro- 
tected from  the  injurious  effects  of  overwork,  and  are  always  ready  to  serve  the 
brain.2  It  may  be  added  that  nerve-cells  not  only  fatigue  more  quickly,  but 
recover  from  fatigue  more  rapidly  than  the  muscles. 

(e)  Effect  of  Temperature  upon  Muscular  Contraction. — Heat,  within  certain 
limits,  increases  the  irritability  and  conductivity  of  muscle-tissue,  and  at  the 
same  time  has  a  favoring  influence  upon  those  forms  of  chemical  change  which 
liberate  energy.  The  effect  of  a  rise  of  temperature,  as  shown  by  the  myo- 
gram,  is  a  shortening  of  the  latent  period,  an  increase  in  the  height  of  contrac- 
tion, and  a  quickening  of  the  contraction  and  relaxation,  the  whole  curve  being 
shortened.  Of  course  there  is  an  upper  limit  to  this  favoring  action,  since,  at  a 


FIG.  60.— Schema  of  effect  of  temperature  on  height  and  form  of  contraction  curve :  a,  contraction  at 
19°  C. ;  b,  c,  d,  e,f,  contractions  made  at  intervals,  each  one  at  a  lower  temperature;  g,  h,  contractions 
at  higher  temperatures  than  19°  C.,  h  being  made  when  the  temperature  was  30°  C. ;  i,  k,  I,  show  a  different 
series  of  contractions,  made  as  the  temperature  was  Increased  from  30°  C.  toward  the  point  at  which  the 
muscle-substance  coagulates  (after  Gad  and  Heymans). 

certain  temperature,  about  45°  C.  for  frog's  muscle  and  about  50°  C.  for  the 
striated  muscles  of  warm-blooded  animals,  53°-58°  C.  for  the  non-striated 
muscles  of  the  bladder  of  the  cat,3  heat-rigor  begins,  and  this  change  is  accom- 
panied by  a  loss  of  all  vital  properties.  Cold  can  be  said,  in  general,  to  pro- 
duce effects  the  opposite  of  those  of  heat;  as  the  muscle  is  cooled,  the  latent 
period,  the  contraction,  and  the  relaxation  are  all  prolonged. 

Nevertheless,  the  effect  of  temperature  is  not  a  simple  one  (see  Fig.  60).  If 
during  the  cooling  process  a  striated  muscle  of  a  frog  be  irritated  from  time  to 
time  with  single  induction  shocks,  the  height  of  the  contractions  does  not  con- 
tinually grow  less  as  one  would  expect.4  The  maximal  height  is  obtained  at 
30°  C.,  the  height  above  this  point  being  somewhat  less,  the  irritability  les- 
sening as  the  coagulation- point  is  approached;  from  30°  C.  to  19°  C.  the 
height  continually  decreases,  but  from  19°  to  0°  C.  the  height  increases,  while 

1  Lombard :   Archives  italiennes  de  Biologie,  xiii.  p.  1 ;  of  American  Journal  of  Psychology, 
1890,  p.  1  ;  Journal  of  Physiology,  1892,  p.  1 ;  1893,  p.  97. 
'Waller:  Brain,  1891,  p.  179. 

SC.  C.  Stewart:  American  Journal  of  Physiology,  1900,  iii.  p.  25. 
4  Gad  und  Heymans  :  Archivfur  Analomie  und  Physiologie,  1890,  S.  73. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.     137 

below  0°  0.  it  again  becomes  less,  until  at  the  freezing-point  of  muscle  no  con- 
traction is  obtained.  The  cause  of  these  peculiar  phenomena  is  not  definitely 
understood. 

(/)  Effect  of  Drugs  and  Chemicals  upon  Muscular  Contraction. — Certain  drugs 
and  chemicals  have  a  marked  effect  upon  the  irritability  (see  p.  58)  and  con- 
ductivity (see  p.  93)  of  muscles,  and  these  effects  must  necessarily  find  expres- 
sion in  the  amount  of  contraction  which  would  be  excited  by  a  given  irri- 
tant. In  addition  to  this,  it  is  worthy  of  notice  that  the  character  of  the  con- 
traction may  be  altered. 

The  drug  which  has  the  most  striking  effect  upon  the  form  of  contraction  is 
veratria.  A  few  drops  of  a  1  per  cent,  solution  of  the  acetate  of  veratria, 
injected  into  the  dorsal  lymph  sac  of  a  frog  whose  brain  has  first  been 
destroyed,  in  a  few  minutes  alter  completely  the  character  of  the  reflex 
movements  :  the  muscles  are  still  capable  of  rapidly  contracting,  but  the  con- 
tractions are  cramp-like,  the  power  to  relax  being  greatly  lessened.  The 
poison  acts  upon  the  muscle-substance,  and  even  a  very  small  dose  applied 


FIG.  61.— Myogram  of  muscle  poisoned  with  veratria  and  that  of  a  normal  muscle :  a,  myogram  from  a 
normal  gastrocnemius  muscle  of  a  frog— the  waves  at  the  close  are  due  to  the  recoil  of  the  recording  lever ; 
b,  myogram  from  a  gastrocnemius  muscle  poisoned  with  veratria,  recorded  at  the  same  part  of  the  drum. 

directly  to  the  muscle  for  a  few  hours — e.  g.,  a  solution  containing  1  part  to 
100,000  of  0.6  per  cent,  solution  of  sodium  chloride — suffices  to  greatly  alter 
the  character  of  the  contraction  called  out  by  various  irritants.1  If  a  muscle 
poisoned  with  veratria  be  isolated  and  connected  with  a  myograph,  a  contrac- 
tion excited  by  a  single  induction  shock  will  show  a  rise  as  rapid,  as  high, 
and  as  strong  as  normal,  but  the  fall  of  the  curve  will  be  greatly  prolonged 
(see  Fig.  61).  Often  the  crest  of  the  curve  will  exhibit  a  notch,  which  shows 
that  relaxation  may  begin  and  be  checked  by  a  second  contraction  process 
which  carries  the  curve  up  again  and  holds  it  there  for  a  considerable  time. 
In  the  above  experiment  the  contracture  effect  followed  the  primary  contrac- 
tion immediately.  The  nature  of  the  contracture  of  a  muscle  poisoned  with 
veratria  has  been  considered  (see  p.  130). 

There  are  a  number  of  drugs  which  have  an  action  on  muscle-tissue  simi- 
lar to  that  of  veratria — e.  g.,  cornutine 2  produces  a  similar  effect  on  striated 
muscles ;  digitaline  increases  the  tonus  of  heart  muscle  and  of  the  smooth 

^ucannan:  Journal  of  Physiology,  1899,  xxv.  p.  137. 
*  Cushny  :  Pharmacology  and  Therapeutics,  1899. 


138  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

muscle-tissue  of  the  walls  of  the  blood-vessels ;  epinephrin,1  the  active  prin- 
ciple of  the  extracts  obtained  from  the  medullary  part  of  the  suprarenal 
capsules,  may  be  mentioned  here,  and  is  of  especial  interest  because  derived 
from  the  animal  body.  If  injected  into  the  blood,  it  increases  the  strength 
and  prolongs  the  contraction  of  the  muscles  generally,  and  causes  through  its 
effect  on  the  muscle  of  the  heart  and  the  non-striated  muscles  of  the  blood- 
vessels a  marked  rise  of  blood-pressure.2 

Barium  salts,  and  to  a  less  degree  calcium  and  strontium,  act  similarly 
to  veratria  to  prolong  the  relaxation  of  the  muscle,  without  lessening  the 
rapidity  and  height  of  contraction. 

Potassium  and  ammonium  salts  and  a  large  number  of  other  chemical 
substances  and  drugs  act  to  kill  the  muscle,  and  as  the  death  process  develops 
excitation  produces  prolonged  localized  contractions.  This  effect  seems  to  be 
quite  different  from  that  of  veratria,  for  it  is  accompanied  by  a  rapid  lessen- 
ing of  the  muscular  power. 

5.  Liberation  of  Energy  by  the  Contracting  Muscle. — The  law  of  con- 
servation of  energy  applies  no  less  to  the  living  body  than  to  the  inanimate 
world  in  which  it  dwells.  Every  manifestation  of  life  is  the  result  of  the 
liberation  of  energy  which  was  stored  in  the  body  in  the  form  of  chemical 
compounds.  When  a  muscle  is  excited  to  action  it  undergoes  chemical 
changes,  which  are  accompanied  by  the  conversion  of  potential  into  kinetic  en- 
ergy. This  active  energy  leaves  the  muscle  in  part  as  thermal  energy,  in  part 
as  mechanical  energy,  and,  to  a  slight  extent,  under  certain  conditions,  as^glec- 
trical  energy.  In  general,  the  sum  of  the  liberated  energy  is  given  off  as  heat 
or  motion.  The  proportion  in  which  these  two  forms  of  energy  shall  be  pro- 
duced by  a  muscle  may  vary  within  wide  limits,  according  to  the  state  of  the 
muscle  and  the  conditions  under  which  the  work  is  done.  Fick 3  states  that 
if  the  muscle  works  against  a  very  heavy  weight,  possibly  one-fourth  of  the 
liberated  energy  may  be  obtained  as  mechanical  work  ;  but  if  the  weight  be 
light  not  more  than  one-twentieth  of  the  chemical  energy  is  given  off  in  this 
form,  the  muscle  working  no  more  economically  than  a  steam  engine.  Zuntz 4 
studied  the  work  that  the  body  as  a  whole  could  accomplish,  and  found  that 
somewhat  more  than  one-third  of  the  energy  liberated  can  be  obtained  as 
external  mechanical  work.  The  fact  that  always  a  part,  and  often  the  whole, 
of  the  mechanical  energy  developed  by  the  muscle  is  converted  to  thermal 
energy  within  the  muscle,  and  leaves  it  as  heat,  makes  it  the  more  difficult  to 
determine  in  what  proportion  these  two  forms  of  energy  were  originally  pro- 
duced. Moreover,  if  Engelmann's  view  be  correct,  that  the  change  of  form 
exhibited  by  the  muscle  is  the  result  of  the  imbibition  of  the  fluid  of  the 
isotropic  substance  by  the  anisotropic  material,  this  change  being  brought 
about  by  the  heat  which  is  liberated  within  the  muscle,  we  must  consider 
potential  energy  to  be  set  free  first  as  heat,  a  part  of  which  is  afterward 

»Abel:  Zeitschrift  fur  physiologische  Chemie,  1899,  Bd.  xxviii.  S.  354. 
*  Oliver  and  Schafer :  Journal  of  Physiology,  1895,  xviii.  pp.  230-276. 
3 Fick:  Pfliiger's  Archiv,  1878,  xvi.  S.  85. 
*/6rrf.,  1897,  Bd.  Ixviii.,  S.  191. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     139 

changed  to  mechanical  energy,  which  in  part,  at  least,  is  again  changed  to 
heat. 

Liberation  of  Mechanical  Energy. — The  amount  of  work  which  a  muscle 
can  do  depends  on  the  following  conditions : 

(a)  The  kind  of  muscle.  The  muscles  of  warm-blooded  animals  are  stronger 
than  those  of  cold-blooded  animals  ;  a  human  muscle  can  do  twice  the  amount 
of  work  of  an  equal  amount  of  frog's  muscle.  The  muscles  of  certain  insects 
have  even  greater  strength.1  Within  the  same  animal  there  are  great  differ- 
ences in  the  capacity  of  different  forms  of  muscle  tissue  (see  p.  107).  Pale 
striated  muscle  tissue,  although  more  capable  of  rapid  liberation  of  energy, 
has  not  the  endurance  or  the  strength  of  the  red  striated  muscle  tissue ;  and 
different  forms  of  non-striated  muscle  differ  among  themselves  as  well  as 
from  striated  in  their  capacity  for  work. 

(6)  The  condition  of  the  musck.  Any  of  the  influences  which  lessen  the 
irritability  of  the  muscle — lack  of  blood,  fatigue,  cold,  etc. — decreases  the  power 
to  liberate  energy,  and  any  influence  which  heightens  the  irritability  is  favora- 
ble to  the  work.  The  effect  of  tension  to  heighten  irritability  has  already  been 
referred  to  and  is  of  especial  interest  in  this  connection,  since  the  very  re- 
sistance of  the  weight  is,  within  limits,  a  condition  favorable  to  the  liberation 
of  the  energy  required  to  overcome  the  resistance.  This  will  be  referred  to 
again. 

(c)  The  strength  and  character  of  the  stimulus.  The  liberation  of  energy  is, 
up  to  a  certain  point,  the  greater,  the  stronger  the  excitation.  Furthermore, 
rapidly  repeated  excitations  are  much  more  effective  than  single  excitations, 
because  a  series  of  rapidly  following  stimuli,  both  by  altering  the  irritability  and 
by  inducing  the  form  of  contraction  known  as  tetanus,  act  to  produce  powerful 
and  high  contractions.  Bernstein  states  that  the  energy  developed  by  the 
muscle  increases  with  the  increase  of  the  rate  of  excitation  from  10  to  50  per 
second,  at  which  rate  the  contraction  power  may  be  double  that  called  out  by  a 
single  excitation. 

(d  )  The  method,  of  contraction  and  the  mechanical  conditions  under  which 
the  work  is  done.  In  estimating  the  amount  of  mechanical  energy  liberated 
by  a  muscle,  we  observe  the  amount  of  external  work  which  it  accomplishes, 
i.  e.  the  amount  of  mechanical  energy  which  it  imparts  to  external  objects. 
If  a  muscle  by  contracting  raises  a  weight,  it  gives  energy  to  the  weight,  the 
amount  being  exactly  that  which  the  weight  in  falling  through  the  distance 
which  it  was  raised  by  the  muscle  can  impart  as  motion,  heat,  etc.,  to  the 
objects  with  which  it  comes  in  contact.  The  measure  of  the  mechanical 
work  done  by  the  contracting  muscle  is  the  product  of  the  weight  into  the 
height  to  which  it  is  lifted.  For  example,  if  a  muscle  raises  a  weight  of  5 
grams  10  millimeters,  it  does  50  grammillimeters  of  work.  An  unweighted 
muscle  in  contracting  does  no  external  work ;  a  muscle,  however  vigorously 
it  may  contract,  if  it  be  prevented  from  shortening,  does  no  external  work  ; 
finally,  a  muscle  which  raises  a  weight  and  then  lowers  it  again  when  it 
relaxes,  does  not  alter  its  surroundings  as  the  total  result  of  its  activity,  and 
1  Hermann  :  Handbuch  der  Physiologic,  1879,  Bd.  i.  S.  64. 


140  ^4^  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

hence  does  no  external  work.  Although  no  external  work  is  accomplished 
under  these  circumstances,  internal  work  is  being  done,  as  is  evidenced  by 
the  heat  evolved  by  the  muscle  and  the  fatigue  produced.  Unquestionably 
mechanical  energy  is  developed  within  the  muscle  in  all  these  cases,  but  it 
is  all  converted  to  heat  before  it  leaves  the  muscle. 

The  amount  of  weight  is  an  important  factor  in  determining  the  extent  to 
which  a  muscle  will  shorten  when  excited  by  a  given  stimulus,  and,  therefore, 
the  quantity  of  work  which  it  will  accomplish.  If  a  muscle  be  after-loaded, 
i.  e.  if  the  weight  be  supported  at  the  normal  resting  length  of  the  muscle,  and 
the  muscle  be  excited  to  a  series  of  maximal  contractions,  the  weight  being  in- 
creased to  a  like  amount  before  each  of  the  succeeding  excitations,  there  is,  in 
general,  a  gradual  lessening  in  the  height  of  the  contractions,  but  the  de- 
crease in  height  is  not  proportional  to  the  increase  of  the  weight.  The 
decrease  in  the  height  of  contractions  is,  as  a  rule,  more  rapid  at  the  beginning 
of  the  series  than  later,  though  at  times  an  opposite  tendency  may  show  itself 
and  the  increasing  weights  temporarily  increase  the  irritability  and  therefore 
increase  the  amount  of  shortening.  The  effect  of  tension  to  increase  the  activ- 
ity of  the  contraction  process  is  seen  if  a  muscle  which  is  connected  with  a 
strong  spring  or  heavy  weight  be  excited  to  isometric  contractions  and  in 
the  midst  of  a  contraction  be  suddenly  released ;  the  muscle  under  such  cir- 
cumstances is  found  to  contract  higher  than  when  excited  by  the  same  stimulus 
without  being  subjected  to  tension.1  The  effect  of  tension  on  the  activity  of 
muscular  contractions  is  to  be  clearly  seen  in  the  case  of  the  heart  muscle. 
A  rise  of  pressure  of  the  fluid  within  the  isolated  heart  of  a  frog  increases 
the  strength  as  well  as  the  rate  of  the  beat. 

If  the  weight  be  gradually  increased,  although  the  height  of  the  contrac- 
tions is  lessened,  the  work  will  for  a  time  increase,  and  a  curve  of  work  (con- 
structed by  raising  ordinates  of  a  length  corresponding  to  the  work  done, 
from  points  on  an  abscissa  at  distances  proportional  to  the  weights  em- 
ployed), will  be  seen  to  rise.  After  the  weight  has  been  increased  to  a  cer- 
tain amount  the  decline  in  the  height  of  contractions  will  be  so  great  that  the 
product  of  the  weight  into  the  height  will  begin  to  decrease,  and  the  curve  of 
work  will  fall,  until  finally  a  weight  will  be  reached  which  the  contrasting 
muscle  can  just  support  at,  but  not  rajse  above,  its  normal  resting  length 
This  weight  will  be  a  measure  of  the  absolute  muscular  force. 


Load 
(grams). 
0                    ... 

Example. 

Height  of  lift 
(millimeters). 
13  

Work 
(grammillimeters). 
0 

30 

11      

330 

60 

9 

540 

90  

.7  

630 

120      

....            5 

.    .  600 

150 

3 

450 

180  

2  

360 

210  . 

.    0  . 

0 

1  Ffck  :  Mechanische  Arbeit,  etc.,  S.  132.     Santesson  :  Skandinavisches  Archiv  fur  Physiologie, 
1889,  i.  S.  56. 


GENERAL    PHYSIOLOGY  OF  MUSCLE   AND    NERVE.      141 

In  the  above  experiment  30  grams  were  added  to  the  muscle  after  each 
contraction ;  as  the  weight  was  increased  up  to  90  grams  the  amount  of 
work  was  increased,  with  greater  weights  the  amount  of  work  was  lessened. 

It  is  evident  that  the  absolute  force  of  a  muscle  of  a  given  type  will  depend 
not  only  on  the  quantity,  but  also  on  the  arrangement  of  the  microscopic  ele- 
ments of  which  the  muscle  is  composed.  Each  element  of  a  fibre  has  to  stand 
the  strain  of  the  whole  fibre ;  so  the  force  to  be  developed  depends  not  on  the 
length  of  the  fibres,  but  on  the  number  of  muscle  elements  which  are  arranged 
side  by  side,  i.  e.  the  absolute  force  of  a  muscle  will  be  proportionate  to  the 
number  of  fibres.  This  can  be  stated  for  a  muscle  with  parallel  fibres  in 
terms  of  the  cross  section  of  the  muscle.  In  the  case  of  a  muscle  like  the 
gastrocnemius,  where  the  fibres  take  an  oblique  course  and  are  inserted  into 
a  common  tendon  in  the  middle,  the  "  physiological  cross-section  "  has  to  be 
estimated,  i.  e.  the  total  section  taken  at  right  angles  to  the  fibres.  Such  a 
muscle  is  very  strong  in  proportion  to  its  thickness.  Rosenthal  estimated 
the  absolute  force  of  striated  muscles  of  the  frog  to  be  about  3  kilograms  per 
square  centimeter,  and  Hermann l  found  the  absolute  force  of  striated  muscle 
of  man  to  be  6.24  kilograms  per  square  centimeter. 

The  physiological  work  of  which  a  muscle  is  capable,  on  the  other  hand, 
is  dependent  not  only  on  the  weight  which  it  can  lift,  but  also  the  height  to 
which  the  weight  can  be  lifted.  All  the  muscle  elements,  whether  arranged 
side  by  side  or  in  chains,  influence  the  result,  and  for  purposes  of  comparison 
one  can  state  the  capacity  of  the  muscle  for  work  in  terms  of  the  unit  of 
bulk,  the  cubic  centimeter,  or  the  unit  of  weight,  the  gram.  Thus,  Fick 
states  the  maximal  amount  of  external  work  of  which  frog's  muscle  is 
capable,  as  one  grammeter  per  gram  of  muscle  substance. 

From  what  has  been  said  it  is  evident  that  the  amount  of  muscle  sub- 
stance determines  the  amount  of  work  of  which  the  muscle  is  capable,  while 
the  arrangement  of  the  muscle  substance  decides  the  character  of  the  work 
which  it  is  best  fitted  to  perform.  Muscles  with  long  parallel  fibres,  even 
though  of  small  sectional  area,  such  as  the  sartorius,  are  specially  fitted  to 
produce  extensive  movements  of  the  parts  to  which  they  are  attached ;  and 
muscles  which  have  a  large  number  of  fibres,  even  though  these  be  short,  as 
in  the  case  of  the  gastrocnemius,  are  adapted  to  move  great  weights. 

Carvallo  and  Weiss2  state  that  the  gastrocnemius  muscle  of  the  frog 
when  at  rest  tears  if  subjected  to  a  weight  of  2  kilos.  Its  contraction  power 
is  estimated  to  be  half  a  kilo,  and  when  it  is  contracting  its  resistance  is  cor- 
respondingly increased,  so  that  a  weight  of  2J  kilos  is  required  to  rupture  it. 
The  increased  resistance  can  be  best  explained  on  the  idea  that,  as  Pfluger 
thinks,  a  new  chemical  attraction  force  is  developed  in  contraction. 

Liberation  of  Thermal  Energy. — Energy  leaves  the  body  as  mechanical 
energy  only  when  by  its  movements  the  body  imparts  energy  to  surrounding 
objects.  Most  of  the  energy  liberated  within  the  body  leaves  it  as  heat ; 

1  Fyiuger's  Archiv,  1898,  Bd.  73,  S.  429. 

1  Carvallo  tvnd  Weiss :  Comptes  rendus  Societe  de  Biologie,  1899,  p.  122. 


142  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

even  during  violent  muscular  exercise  five  times  more  energy  may  be  ex- 
pended as  heat  than  as  mechanical  energy,  and  the  disproportion  may  be 
even  greater  than  this.  Rosenthal  says  that  at  the  most  not  more  than  30 
per  cent,  of  the  energy  developed  in  the  muscle  by  oxidation  and  splitting 
processes  is  to  be  got  as  available  mechanical  energy.  So  great  is  the  pro- 
duction of  heat  during  exercise  that,  in  spite  of  the  great  amount  leaving  the 
body,  the  temperature  of  an  oarsman  has  been  found  to  be  increased,  during 
a  race  of  2000  meters,  from  37.5°  C.  to  39°  or  40°  C.1 

It  is  exceedingly  difficult  to  ascertain  with  accuracy  on  the  warm-blooded 
animal  the  exact  relation  of  heat-produccion  to  muscular  contraction.  The 
best  results  have  been  obtained  by  experiments  on  isolated  muscles  of  cold- 
blooded animals.  Helmholtz  observed  the  temperature  of  a  muscle  of  a 
frog  to  be  increased  by  tetanus  lasting  a  couple  of  minutes  0.14°  to  0.18° 
C. ;  Heidenhain  saw  a  change  of  0.005°  C.  result  from  a  single  contraction ; 
and  Fick  ascertained  that  a  fresh,  isolated  muscle  of  a  frog  can  by  a  single 
contraction  produce  per  gram  of  muscle-substance  enough  heat  to  raise 
3  milligrams  of  water  1°  C.2  To  obtain  evidence  of  the  slight  changes 
of  temperature  which  occur  in  such  small  masses  of  muscle-tissue  it  is 
necessary  to  employ  a  very  delicate  instrument,  such  as  a  thermopile  or  a 
bolometer. 

The  thermopile  consists  of  strips  of  two  dissimilar  metals,  united  at  their  extremities, 
so  as  to  form  a  series  of  thermo-electric  junctions.  If  there  be  a  difference  of  temperature 
at  two  such  junctions,  a  difference  of  electric  potential  is  developed,  -which  causes  the 
flow  of  an  electric  current.  If  the  current  be  passed  through  the  coils  of  wire  of  a 
galvanometer  its  amount  can  be  measured,  and  the  extent  of  the  change  in  tempera- 
ture at  one  of  the  junctions,  the  other  remaining  constant,  can  be  estimated.  In  the 
more  sensitive  instruments,  several  thermo-electric  junctions  are  used.  The  amount  of 
current  depends  largely  on  the  metals  employed,  antimony  and  bismuth  being  a  very 
sensitive  combination. 

The  action  of  the  bolometer  is  based  on  the  fact  that  the  resistance  of  a  wire  to  the 
passage  of  an  electric  current  changes  with  its  temperature. 

The  amount  of  heat  developed  within  the  muscle  by  direct  conversion  of 
potential  to  thermal  energy,  and  the  amount  formed  indirectly,  through  con- 
version of  mechanical  to  thermal  energy,  has  been  made  a  subject  of  careful 
study  by  Heidenhain,3  Fick  and  his  pupils,4  and  others,  the  experiments  being 
made  chiefly  with  isolated  muscles  of  frogs. 

In  general,  the  stronger  the  stimulus  and  the  greater  the  irritability  of  the 
muscle — in  other  words,  the  more  extensive  the  chemical  changes  excited  in 
the  muscle — the  greater  the  amount,  not  only  of  mechanical,  but  of  thermal 
energy  liberated.  Increase  of  tension,  which  is  very  favorable  to  muscular 

1  George  Kolb :  Physiolog  i  of  Sport,  translated  from  the  German,  second  edition,  London, 
1892. 

3  Fick  :  Pfliiger's  Archiv,  1878,  xvi.  S.  80. 

3  Mechanische  Leistung,  Warmeentwicklung  and  Stoffumsatz  bei  der  Muskelthatigkeit,  Leipzig, 
1864. 

*  Myothermische  Untersuchunyen  aus  den  ph  riologischen  Laboratorium  zv  Zurich  und  Wurzburg, 
Wiesbaden,  1889. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     143 

activity,  greatly  increases  the  heat-production.  For  this  reason  isometric 
contractions,  that  is,  those  in  which  the  muscle  works  against  a  resistance 
which  is  sufficient  to  prevent  it  from  shortening,  are  accompanied  by  a 
greater  liberation  of  heat  than  isotonic  contractions,  in  which  the  contracting 
muscle  raises  a  constant  weight.  As  the  weight  is  increased,  both  the 
amount  of  heat  developed  and  the  work  are  increased,  but  the  liberation  of 
heat  reaches  its  maximum  and  begins  to  decline  sooner  than  the  amount  of 
work — i.  e.,  with  large  weights  the  muscle  works  more  economically;  simi- 
larly, as  the  muscle  is  weakened  by  fatigue  the  heat-production  lessens  sooner 
than  the  work. 

Muscle-tonus  and  Chemical  Tonus. — During  waking  hours,  the  cells 
of  the  central  nervous  system  are  continually  under  the  influence  of  a  shower 
of  weak  nervous  impulses,  coming  from  the  sensory  organs  all  over  the  body ; l 
moreover,  activity  of  brain-cells,  especially  emotional  forms  of  activity,  leads 
to  an  overflow  of  nervous  impulses  to  the  spinal  cord  and  an  increased  irrita- 
bility, or,  if  stronger,  excitation  of  motor  nerve-cells.  If,  when  one  is  quietly 
sitting  and  reading,  he  turns  his  attention  to  the  sensory  impressions  which 
are  coming  at  every  moment  from  all  over  the  body  to  the  brain,  notes  the 
temperature  of  different  parts  of  the  skin,  the  pressure  of  the  clothes,  etc., 
upon  different  parts,  the  light  reflected  from  neighboring  objects,  and  the  slight 
sounds  about  him,  he  will  recognize  that  the  central  nervous  system  is  all  the 
time  subject  to  a  vast  number  of  excitations,  which,  because  of  their  very 
repetition,  are  ordinarily  disregarded  by  the  mind,  but  which  are,  nevertheless, 
all  the  time  influencing  the  nerve-cells.  The  effect  of  this  multitude  of  affer- 
ent stimuli,  in  spite  of  their  feebleness,  is  to  cause  the  motor  cells  of  the  cord 
to  continually  send  delicate  motor  stimuli  to  the  muscles.  These  cause  the 
muscle  to  keep  in  the  state  of  slight  but  continued  contraction  which  gives  the 
tension  peculiar  to  waking  hours,  and  which  is  called  muscle-tonus.  That 
such  a  tension  exists  is  made  evident  by  the  change  in  attitude  which  occurs 
when  the  relaxation  accompanying  sleep  comes  on.  The  effect  of  brain  activ- 
ity to  cause  muscular  tension  is,  likewise,  most  easily  recognized  by  observing 
the  relaxation  of  the  muscles  which  occurs  when  mental  excitement  ceases. 

Muscle-tonus,  like  every  form  of  muscular  contraction,  is  the  result  of  chem- 
ical change,  and  the  liberation  of  energy.  But  little  of  this  energy  leaves  the 
body  as  mechanical  energy,  most  of  it  being  given  off  as  heat. 

This  view  is  by  no  means  universally  accepted,  and  many  physiologists 
believe  in  a  production  of  heat  by  the  muscles,  as  a  result  of  nervous  influences, 
independent  of  contraction.  It  is  thought  that  a  condition  of  slight  but  con- 
tinuous chemical  activity  resulting  in  the  production  of  heat  maybe  maintained 
in  the  muscles  by  intermittent  but  frequent  reflex  excitations,  a  condition  which 
has  been  called  chemical  tonus.2  That  the  chemical  activity  of  muscles  is  kept 

1  Brondgeest:  Archiv  fiir  Anatomie  und  Physiologic,  1860,  S.  703;  Hermann,  Ibid.,  1861, 
S.  350. 

2  Koehrig  und  Zuntz :  Pfliiger's  Archiv,  1871,  Bd.  iv ;  Pfliiger:  Pfliiger's  Archiv,  1878,  xviii. 
S.  247. 


144 


AN   AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


up  by  small  stimuli  from  the  spinal  cord  is  shown  by  the  fact  that  if  the  nerves 
be  severed,  or  the  nerve-ends  be  poisoned  by  curare,  the  muscle  absorbs  less 
oxygen  and  gives  off  less  carbon  dioxide  than  when  at  rest  under  normal 
conditions.1 

The  theory  of  a  reflex  chemical  tonus  independent  of  contraction  implies 
the  existence  of  special  nervous  mechanisms  for  the  exciting  of  chemical 
changes  in  the  muscles  which  shall  result  in  the  liberation  of  energy  as  heat, 
independent  of  the  change  of  form  of  the  muscle.  The  question  of  the  exist- 
ence of  special  nervous  mechanisms  controlling  heat-production — heat-centres, 
as  they  are  called — will  be  considered  in  another  part  of  this  book. 

E.  ELECTRICAL  PHENOMENA  IN  MUSCLE  AND  NERVE.  2 

The  active  muscle  liberates  three  forms  of  energy  :  mechanical  work,  heat, 
and  electricity.  The  active  nerve  makes  no  visible  movements,  gives  off  no 

recognizable  quantity  of  heat,  but 
exhibits  changes  in  electrical  condition 
quite  comparable  to  those  observed  in 

^ - — ^^^^  the     active    muscle.      The    electrical 

<\^' """-^  changes  in  nerves  are  the  only  evidence 

of  activity  which  we  can  observe,  aside 
from  the  effect  of  the  nerve  on  the 
organ  which  it  excites  ;  they  are  there- 
fore of  great  interest  to  us. 

Electrical  energy,  like  all  forms  of 
active  energy,  is  the  result  of  a  trans- 
formation of  potential  or  some  form  of 
kinetic  energy.  In  the  case  of  the 
muscle,  as  of  an  electric  battery,  we 
find  electricity  to  be  associated  with 
chemical  change,  and  believe  it  to  be 
liberated  from  stored  potential  energy. 
In  the  case  of  nerves  no  chemical 
change  can  be  detected  during  action, 
and  hence  we  are  at  a  loss  to  explain 
the  development  of  electricity.  We 
can  only  say  that  it  is  the  result  of 
some  chemical  or  physical  process 
which  we  have  as  yet  failed  to  discover. 
Although  activity  of  nerve  and 
muscle  is  found  to  be  associated  with  electrical  change,  we  must  not  suppose 
functional  activity  to  be  in  any  sense  an  electrical  process.  The  movements 

1  Zuntz  :   Pfliiger's  Archiv,  1876,  xii.  522 ;  Colasanti,  Ibid.,  1878,  xvi.  S.  57. 
2Biederraann:    Electrophysiologie,  Jena,  1895,  Bd.  ii.;  translation  by  F.  A.  Wei  by,  1898; 
Waller:  Lectures  on  Animal  Electricity,  London,  1897. 


FIG.  62.— Schema  of  galvanometer :  n,  s,  north 
and  south  poles  of  astatic  pair  of  magnets ;  m, 
compensating  magnet,  held  by  friction  on  the 
staff,  and  capable  of  being  approached  to,  or  ro- 
tated with  reference  to,  the  suspended  magnet ; 
X,  mirror;  /,  fibre  supporting  the  magnets;  c,  c, 
c,  c,  coils  of  wire  to  carry  the  electric  current 
near  to  the  magnets,  the  upper  coils  being  wound 
in  the  opposite  direction  to  the  lower ;  e,  e,  non- 
polarizable  electrodes  applied  to  the  longitudinal 
surface  and  cross  section  of  a  muscle. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     145 

of  a  man  may  he  interpreted  from  the  movements  of  his  shadow,  but  they 
are  very  different  phenomena;  the  activity  of  tin-  nerve  and  muscle  is  indi- 
cated by  the  electrical  changes  accompanying  it,  but  they  may  be  independent 
processes.  Certainly  the  irritating  change  which  is  transmitted  along  the 
nerve  and  which  excites  the  muscle  to  action,  although  accompanied  by  elec- 
trical changes,  is  not  itself  an  electric  current. 

Electrical  energy  is  exhibited  not  only  by  active  nerve  and  muscle,  but 
during  the  activity  of  a  great  variety  of  forms  of  living  matter.  It  may  be 
detected  in  gland-cells,  in  the  cells  of  many  of  the  lower  animal  organisms, 
and  even  in  plant-cells.  The  amount  of  electrical  energy  developed  in  animal 
tissues  may  be  far  from  trivial.  Although  delicate  instruments  are  necessary 
to  observe  the  electrical  changes  in  nerve  and  muscle,  as  the  great  internal 
resistance  of  the  tissues  causes  the  currents  to  be  small,  we  find  in  certain 
fish  special  electric  organs,  which  appear  to  be  modified  muscle  tissue,  and 
which  are  capable  of  discharging  a  great  amount  of  electrical  energy  when 
excited  through  their  nerves.  So  intense  is  the  action  of  this  electrical 
apparatus  that  it  can  be  used  as  a  weapon  of  defence  and  offence.  Gotch 
and  Burch  state  that  the  electric  organ  of  the  malapterurus  electricus  can 
give  a  shock  having  an  electric  potential  of  200  volts.1 

1 .  Methods  of  Ascertaining  the  Electrical  Condition  of  a  Muscle  or  a  Nerve, — 

If  the  electric  tension  of  any  two  parts  of  an  object  differs,  the  instant  they  are  joined  an 
electric  current  will  flow  from  the  point  where  the  tension  is  greater  to  that  where  it  is  less. 
The  presence,  direction  of  flow,  and  strength  of  an  electric  current  can  be  detected  by  an 
instrument  called  a  galvanometer.  If  any  two  parts  of  a  muscle  or  nerve,  as  e,  e,  Figure 
62,  be  connected  by  suitable  conductors  with  the  coils,  c,  c,  of  a  galvanometer,  and  if  there 
be  a  difference  in  the  electric  potential  of  the  two  parts  examined,  an  electric  current  will 
be  indicated  by  the  instrument.  In  such  tests  all  extra  sources  of  electricity  are  to  be 
avoided,  therefore  the  electrodes  applied  to  the  muscle  must  be  non-polarizable. 

The  Galvanometer — An  ordinary  form  of  galvanometer  consists  of  a  magnet  suspended 
by  an  exceedingly  delicate  fibre  of  silk,  or  quartz,  and  one  or  more  coils,  composed  of  many 
windings  of  pure  copper  wire,  placed  vertically  near  the  magnet  and  in  the  plane  of  the  mag^ 
netic  meridian.  If  an  electric  current  be  allowed  to  flow  through  the  wire,  it  influences  the 
magnetic  field  about  it,  and,  if  the  coils  be  close  to  the  suspended  magnet,  causes  the 
magnet  to  deviate  from  the  plane  of  the  magnetic  meridian  in  one  or  the  other  direction, 
according  to  the  direction  of  the  flow  of  the  current.  In  the  more  delicate  instruments  the 
influence  of  the  earth's  magnetism  is  lessened  by  the  use  of  two  magnets  of  as  nearly  as  pos- 
sible the  same  strength,  placed  so  as  to  point  in  opposite  directions,  and  fastened  at  the 
extremities  of  a  light  rod.  As  each  magnet  tends  to  point  toward  the  north,  they  mutually 
oppose  each  other,  and  therefore  the  effect  of  the  earth's  magnetism  is  partly  compensated. 
Still  another  magnet  may  be  brought  near  this  "astatic"  combination,  and  by  opposing  the 
action  of  the  earth's  magnetism  make  the  arrangement  even  more  delicate.  In  the  Thomp- 
son galvanometer,  the  rod  connecting  the  needles  bears  a  slightly  concave  mirror,  from 
which  a  beam  of  light  can  be  reflected  on  a  scale.  Or  a  scale  may  be  placed  so  that  its 
image  falls  on  the  mirror,  and  the  slightest  movement  of  the  magnet  may  be  read  in  the 
mirror  by  a  telescope. 

The  ordinary  galvanometer  is  influenced  by  changes  in  the  magnetism  of  the  earthr 
and  by  earth  currents  which  may  be  due  to  an  escape  of  electricity  from  neighboring 
street-car  circuits,  etc.  These  disturbances  may  interfere  with  the  use  of  the  instrument, 

1  Proceedings  of  the  Royal  Society,  1900,  vol.  Ixv.  p.  442. 
VOL.  II.— 10 


146 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


because  they  may  lead  to  uncontrollable  movements  of  the  magnet,  and  a  consequent  shift- 
ing of  the  0  point.  There  are  other  forms  of  instruments,  such  as  the  Deprez-d'  Arson val 
mirror  galvanometer,1  which  are  not  affected  by  such  influences. 

The  galvanometer  is  very  sensitive  to  the  presence  of  electric  currents.  Another  appa- 
ratus which  is  even  more  responsive  to  changes  in  electric  potential  of  short  duration  is 
the  capillary  electrometer. 

The  capillary  electrometer  (Fig.  63)  consists  of  a  glass  tube  (a)  drawn  out  to  form 
a  very  fine  capillary,  the  end  of  which  dips  into  a  glass  cup  with  parallel  sides  (/)  contain- 
ing a  10  per  cent,  solution  of  sulphuric  acid.  The 
upper  part  of  the  tube  is  connected  by  a  thick- 
walled  rubber  tube  with  a  pressure-bulb  containing 
mercury  (c).  As  the  pressure-bulb  is  raised,  the 
mercury  is  driven  into  the  capillary,  the  flow  being 
opposed  by  the  capillary  resistance.  By  a  suffi- 
ciently great  pressure,  mercury  may  be  driven  to  the 
extremity  of  the  capillary  and  all  the  air  expelled. 
When  the  pressure  is  relieved  the  mercury  rises 
again  in  the  tube,  drawing  the  sulphuric  acid  after 
it.  The  column  of  mercury  will  corne  to  rest  at  a 
point  where  the  pressure  and  the  capillary  force  just 
balance.  Seen  through  the  microscope  (e),  the  end 
of  the  column  of  mercury,  where  it  is  in  contact 
with  the  sulphuric  acid  appears  as  a  convex  menis- 
cus (d\  Any  alteration  of  the  surface  tension  of 
the  meniscus  causes  the  mercury  to  move  with 
great  rapidity  in  one  direction  or  the  other  along 

the  tube ;  and  a  very  slight  difference  of  electric 
FIG.  63. — Schema  of  capillary  electrometer.  ,  .  .  ,  „,  ••  .  n 

potential  suffices  to  cause  a  change  in  surface  ten- 
sion of  the  mercury-sulphuric  acid  meniscus.  A  platinum  wire  fused  into  the  glass  tube 
(a),  and  another  dipped  into  a  little  mercury  at  the  bottom  of  the  cup  holding  the  acid, 
permit  the  mercury  in  the  capillary  and  the  acid  to  be  connected  with  the  body  the  elec- 
tric condition  of  which  is  to  be  examined.  If  the  mercury  and  acid  be  connected  with 
two  points  of  different  electric  potential,  as  g  and  h  of  muscle  M,  the  mercury  will  instantly 
move  from  the  direction  of  greater  to  that  of  lesser  tension,  descending  deeper  into  the 
tube  if  the  tension  be  raised  on  the  mercury  side,  or  lowered  on  the  acid  side,  and  vice 
versa.  As  seen  through  the  microscope  the  picture  is  reversed  (tZ),  and  the  movements 
of  the  mercury  appear  to  be  in  the  opposite  direction  to  that  stated.  The  extent  of  the 
movements  of  the  mercury  column  can  be  estimated  by  a  scale  in  the  eyepiece.  More- 
over, the  movement  of  the  mercury  can  be  recorded  photographically,  by  placing  a  strong 
light  behind  the  column  of  mercury,  and  letting  its  shadow  fall  through  a  slit  in  the  wall 
of  a  dark  chamber,  upon  a  sheet  of  sensitized  paper  stretched  over  the  surface  of  a  revolv- 
ing drum  or  a  sensitized  plate  moved  by  clockwork  or  other  suitable  mechanism.  This 
instrument,  of  which  there  are  a  number  of  different  forms  besides  that  originally  devised 
by  Lippmann,  is  very  delicate,  recording  exceedingly  slight  differences  in  electrical  poten- 
tial. 

The  movements  of  a  galvanometer  may  be  recorded  photographically  by  letting  the 
beam  of  light  reflected  from  the  mirror  fall  through  a  horizontal  slit  on  a  photographic 
plate.  If  the  plate  be  arranged  to  descend  at  a  regular  rate  in  a  dark  chamber  behind  the 
screen  holding  the  slit,  the  movements  of  the  galvanometer  magnet  will  be  pictured  as 
black  lines  on  a  white  ground. 

The  movements  of  the  mercury  column  of  a  capillary  electrometer  may  be  recorded  in 
a  similar  manner,  by  placing  the  instrument  in  front  of  a  vertical  slit  behind  which  a  pho- 
tographic plate  or  sheet  of  sensitized  paper  moves  horizontally.     If  a  strong  light  falls  on 
1  Bernstein :  Pfliiger's  Archiv,  1898,  Bd.  Ixxiii.  S.  376. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     147 


the  slit,  it  will  influence  the  sensitized  surface  except  where  the  mercury  column  inter- 
venes ;    the  movements  of  the  mercury  will  be  photographed  as  a  silhouette. 

2.  Currents  of  Rest.— A  normal  resting  nerve  or  muscle  presents  no  dif- 
ferences in  electric  tension  and  gives  no  evidence  of  electric  currents,  wherefore 
we  say  it  is  iso-electric.  If  any  part  of  the  structure  be  injured,  its  electrical 
condition  is  forthwith  changed,  and  if  the  injured  portion  and  some  normal 
part  be  connected  with  a  galvanometer,  an  electric  current  is 'observed  to  flow 
from  the  normal  region  to  the  point  of  injury.  These  muscle-currents  were 
discovered  at  about  the  same  time  by  Matteucci  and  Du  Bois-Reymond,  and 
the  latter  wrote  a  now  celebrated  treatise  upon  the  electrical  phenomena  to  be 
observed  in  the  nerve  and  muscle  under  varying  conditions.1 

Directions  of  Currents  of  Rest. — If  a  striated  muscle,  with  long  parallel 
fibres,  such  as  the  sartorius  or  the  semimembranosus  of  a  frog,  be  prepared 
with  care  not  to  injure  the  surface,  and  then  be  given  a  cylindrical  shape  by 
cutting  off  the  two  ends  at  right  an- 
gles to  the  long  axis,  the  piece  will 
present  two  cross  sections  of  injured 
tissue  and  a  normal  longitudinal  sur- 
face (see  Fig.  64).  If  non-polarizable 
electrodes,  connected  with  the  coils  of 
wire  of  a  galvanometer,  be  applied  to 
various  parts  of  such  a  piece  of  mus- 
cle, it  will  be  found  that  all  points  on 
the  longitudinal  surfaces  are  positive 
in  relation  to  all  points  on  the  cross 
^sections,  but  that  the  differences  of 
tension  will  differ  according  to  the 
points  which  are  compared.  Suppose 
that  the  cylinder  be  divided  into 
equal  halves  by  a  plane  parallel  to  the 
cut  ends.  Points  on  the  line  bound- 
ing this  plane,  the  equator,  show  the 
greatest  positive  tension,  and  the  farther  other  points  on  the  longitudinal  sur- 
face are  from  the  equator  the  less  their  tension.  Points  on  the  cross  section 
show  a  negative  tension,  and  this  lessens  from  the  centre  to  the  periphery  of 
the  cross  section.  Points  on  the  cross  section  equidistant  from  the  centre,  or 
on  the  longitudinal  surface  equidistant  from  the  equator,  have  the  same  poten- 
tial and  give  no  current,  while  points  placed  unsym metrically  give  a  current. 
Splitting  the  cylinder  by  separation  of  the  parallel  fibres  gives  pieces  of  mus- 
cle which  show  the  same  electrical  peculiarities,  and  without  doubt  the  same 
would  be  true  of  separate  muscle-fibres  or  pieces  of  fibres. 

Samjloff 2  says  that  the  electro-motive  force  of  the  currents  ordinarily 

1  Untersuchungen  ilber  thierische  Elektricitat,  Berlin,  1849. 

2  Samjloff:  Pftiiger's  Archiv,  1899,  Bd.  Ixxviii.  S.  1. 


FIG.  64. — Schema  to  show  the  direction  of  cur- 
rents to  be  obtained  from  muscle.  The  schema 
represents  a  cylindrical  piece  of  muscle  with  nor- 
mal longitudinal  surface  (a,  c  and  6,  d),  and  two 
artificial  cross  sections  (a,  b  and  c,  d).  The  position 
of  the  equator  is  shown  by  line  e.  The  unbroken 
lines  connect  points  of  different  potential,  and  the 
arrows  show  the  direction  which  the  currents 
would  take  were  these  points  connected  with  a 
galvanometer.  The  broken  lines  connect  points 
of  equal  potential  from  which  no  current  would  be 
obtained. 


148  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

obtainable  from  muscle,  0.06—0.08  volt,  represents  only  about  80  per  cent,  of 
the  true  electro-motive  force  of  the  muscle-currents,  because  only  a  part  of 
the  current  is  led  off  to  the  galvanometer,  the  rest  being  short-circuited 
through  the  fluids  surrounding  the  muscle-fibres,  and  in  the  sheath  of  the 
muscle. 

The  fact  that  there  is  a  difference  in  electrical  potential  between  the 
normal  longitudinal  surface  and  the  injured  cross  section  of  the  muscle  can 
be  ascertained  by  the  use  of  the  "  physiological  rheoscope,"  as  the  nerve- 
muscle  preparation  is  called.  If  the  nerve  of  a  fresh  nerve-muscle  prepara- 
tion be  allowed  to  fall  so  as  to  suddenly  close  a  circuit  between  these  two 
parts  of  the  muscle,  an  electric  current  will  flow  through  it,  it  will  be  ex- 
cited, and  the  muscle  will  contract.  A  muscle  can  even  be  made  to  stimu- 
late itself  by  its  demarcation  current,  if  some  point  on  the  equator  be  sud- 
denly connected  with  a  fresh  cross  section  by  a  good  conductor. 

Theories  as  to  Cause  of  Currents  of  Rest. — Du  Bois-Reymond,  impressed  by 
the  facts  which  he  had  ascertained  as  to  the  direction  of  action  of  the  electro- 
motive forces  exhibited  by  the  muscle,  tried  to  explain  the  difference  in  elec- 
trical tension  of  the  surface  and  cross  section  on  the  supposition  that  the 
muscle  was  composed  of  electro-motive  molecules  which  presented  differences 
in  electric  tension  similar  to  those  shown  by  the  smallest  particles  of  muscle 
which  it  is  possible  to  study  experimentally.  Further,  he  considered  these  dif- 
ferences in  tension,  and  the  consequent  electric  currents,  to  exist  within  the 
normal  muscle — the  longitudinal  surface  and  normal  cross  section,  i.  e.  the 
point  where  the  muscle-fibre  joins  the  tendon,  having  the  same  sort  of  differ- 
ence in  electric  potential  as  the  normal  longitudinal  surface  and  the  artificial 
cross  section.  When  the  muscle  is  injured  the  balance  of  the  electro-motive 
forces  within  is  lost,  and  they  are  revealed.  It  is  difficult  to  refute  such  a 
theory  by  experiment,  because  our  instruments  only  record  differences  in  tension 
at  points  on  the  surface  of  the  muscle  to  which  we  can  apply  the  electrodes. 
We  cannot  say  that  there  is  an  absence  of  electric  tension  or  lack  of  electric 
currents  within  the  normal  resting  muscle ;  we  can  only  say  that  there  is  no 
direct  experimental  evidence  of  the  existence  of  such  currents. 

Another  theory  of  the  electrical  phenomena  observed  in  muscle,  and  one 
which  has  found  many  adherents,  was  advanced  by  Hermann.2  According  to 
Hermann's  view  there  are  no  differences  in  electric  potential  and  no  electric 
currents  within  the  normal  muscle  ;  the  "  current  of  rest "  is  a  "  current  of 
injury,"  a  "demarcation  current/'  i.  e.  it  is  due  to  chemical  changes  occurring 
in  the  dying  muscle- tissue  at  the  border  line  between  the  injured  and  living 
muscle-tissue. 

Although  the  greatest  differences  in  potential  are  observed  when  many  muscle- 
fibres  are  injured,  as  when  a  cut  is  made  completely  through  a  muscle,  injury 
to  any  part  causes  that  part  to  become  negative  as  compared  with  the  rest. 
Even  an  injury  to  a  tendon  causes  a  difference  in  potential.  It  is  exceedingly 

1  Du  Bois-Reyinond :  Gesammelte  Abhandlungen,  1877,  Bd.  ii.  S.  319. 

2  Hermann  :  Handbuch  der  Physiologic,  1879,  Bd.  i.  S.  235. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     149 

difficult,  therefore,  to  expose  a  muscle  without  injuring  it;  but  this  can  be  done 
in  the  case  of  the  heart  ventricle,  and  Engelmann  showed  that  this  gives  no  cur-' 
rent  when  at  iv>t,  although  a  current  is  found  as  soon  as  any  part  is  hurt,  the 
part  becoming  immediately  negative  in  relation  to  other  uninjured  parts.  In 
experimental  on  isolated,  long,  parallel-fibred  muscles,  the  current  which  is 
caused  by  the  injury  of  one  extremity  is  found  to  fade  away  only  very  gradu- 
ally (it  may  last  forty-eight  hours  or  more),  and  this  current  can  be  strength- 
ened but  little  by  new  injuries.  In  the  case  of  the  heart-muscle  the  current 
caused  by  cutting  off  a  piece  of  the  ventricle  soon  disappears,  but  another  cur- 
rent of  equal  strength  is  got  if  a  new  section  be  made  by  cutting  off  the  tissue 
injured  by  the  first  cut.  In  the  case  of  the  long-fibred  muscles  the  death 
process  gradually  progresses  the  length  of  the  injured  fibres,  while  in  the  case 
of  the  heart-muscle,  in  which  the  cells  are  very  short,  the  death  processes  are 
limited  to  the  injured  cells,  and  on  their  death  the  current  disappears;  when  a 
new  cut  is  made  other  cells  are  injured  and  again  a  strong  current  is  obtained. 

Dead  tissue  gives  no  current ;  normal  resting  living  tissue  gives  no  current ; 
dying  tissue  is  electrically  negative  as  compared  with  normal  living  tissue. 

Hering1  has  carried  Hermann's  view  that  electrical  change  is  the  result  of 
chemical  action  still  further.  He  considers  that  the  condition  of  negativity  is 
an  evidence  of  katabolic  (breaking-down)  chemical  processes  and  that  anabolic 
(building-up)  chemical  processes  are  accompanied  by  a  positive  electrical  change. 
Like  Du  Bois-Reymond,  he  believes  that  the  normal  resting  muscle  may  be 
the  seat  of  electro-motive  forces  which  do  not  manifest  themselves  as  long  as 
the  different  parts  are  in  like  condition. 

Current  of  Rest  of  a  Nerve. — Nerves  like  muscles  show  no  electric  currents 
if  normal  and  resting,  but  give  a  demarcation  current  if  injured,  the  dying  por- 
tion being  negative  to  normal  parts,  and  the  direction  of  the  currents  is  the 
same  as  injured  muscle.  The  current  of  injury  of  a  nerve  lasts  only  a  short 
time.  The  death  process  which  is  the  immediate  result  of  the  injury  pro- 
ceeds along  the  nerve  only  a  short  distance,  perhaps  to  the  first  node  of 
Ranvier,  and  when  it  has  ceased  to  advance  the  current  fails;  a  new  injury 
of  the  nerve  causes  another  demarcation  current  as  strong  as  the  first. 
Gotch  and  Horsley 2  ascertained  the  electro-motive  force  in  the  nerve  of  a 
cat  to  be  0.01  of  a  Daniell  cell  and  of  an  ape  only  0.005,  while  in  the  spinal 
nerve-roots  of  the  cat  it  was  0.025,  and  in  the  tracts  of  the  spinal  cord  of  the 
cat  0.046,  and  of  the  ape  0.029.  Larger  currents  are  obtained  from  non- 
medullated  nerves,  probably  because  a  non-medullated  nerve  contains  a 
larger  number  of  axis-cylinders  than  a  medullated  nerve  of  the  same  size. 
The  olfactory  nerve  of  the  pike  may  give  a  current  of  0.0215  to  0.0105 
Daniell,  while  a  piece  of  a  frog's  sciatic  of  equal  diameter  would  give  a  cur- 
rent of  only  0.006  Daniell.3 

Hering  found  that  an  irritable  nerve,  like  a  muscle,  could  be  excited  by 

1  Hering :  Lotos,  1888,  Bd.  ix. ;  translation  in  Brain,  1897. 

2  Philosophical  Transactions,  1891,  B.,  vol.  182,  pp.  267-526. 

'Kuehne  and  Steiner:  Heidelberger  Untersuchungen,  1880,  iii.  S.  149-169. 


150  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

its  own  current,  provided   the  longitudinal    and   fresh   cross  section   were 
united  suddenly  by  a  good  conductor. 

3.  Currents  of  Action  in  Muscle. — Just  as  the  dying  tissue  of  nerves 
and  muscles  is  electrically  negative  as  compared  with  normal  tissue,  so  active 
nerve-  and  muscle-tissue  is  electrically  negative  as  compared  with  resting 
tissue. 

Du  Bois-Reymond  discovered  that  if  the  normal  longitudinal  surface  and 
injured  cut  end  of  a  muscle  were  connected  with  a  galvanometer  and  the  muscle 
were  tetanized,  the  magnet  swung  back  in  the  opposite  direction  to  the  deflec- 
tion which  it  had  received  from  the  current  of  rest.  This  backward  swing  of 
the  magnet  was  not  due  to  a  lessening  of  the  current  of  rest,  for  if  the  effect 
of  the  current  of  rest  on  the  galvanometer  were  compensated  for  by  a  battery 
current  of  equal  strength  and  of  opposite  direction,  so  that  the  needle  stood  at 
0,  and  the  muscle  were  then  tetanized,  there  was  a  deviation  of  the  needle 
in  the  opposite  direction  to  that  given  it  by  the  current  of  rest.  Du  Bois- 
Reymond  called  this  current  of  action  the  negative  variation  current.  This 
negative  variation  current  was  found  to  last  as  long  as  the  muscle  continued  in 
tetanus.  On  the  cessation  of  the  stimulus  the  current  subsided  more  or  less 
rapidly  and  the  needle  returned  more  or  less  completely  to  the  position  given  it  \ 
by  the  current  of  rest  before  the  excitation.  The  return  was  rarely  complete,  ' 
and  by  repeated  excitations  there  was  a  gradual  lessening  of  the  current  of 
rest,  the  amount  varying  with  the  extent  of  the  preceding  irritation.  The 
strength  of  the  current  of  action  is  influenced  greatly  by  the  condition  of  the 
muscle,  the  temperature,  etc.  It  increases  with  increasing  strength  of 
excitation,  in  the  same  way  as  the  strength  of  the  contraction.1 

Secondary  Tetanus. — Matteucci  and  Du  Bois-Reymond  (1842)  both  dis- 
covered the  phenomenon  which  Du  Bois-Reymond  called  secondary  tetanus. 

If  two  nerve-muscle  preparations  be 
made,  and  the  nerve  of  preparation  B 
be  laid  on  the  muscle  of  preparation 
Ay  when  the  nerve  of  A  is  stimulated, 
not  only  the  muscle  of  A  but  the 
muscle  of  B  will  twitch  (see  Fig.  65). 
If  nerve  A  be  excited  by  many 


FIG.  65.— -Secondary  tetanus. 


rapidly  following  induction  shocks  so 
that  muscle  A  enters  into  tetanus, 
muscle  B  will  also  be  tetanized.  The  phenomenon  is  not  due  to  a  spread  of 
the  irritating  electric  current  through  nerve  and  muscle  A  to  nerve  J5,  for  the 
tetanus  of  both  muscles  stops  if  nerve  A  be  ligated  ;  moreover,  a  secondary 
tetanus  is  obtained  in  case  tetanus  of  muscle  A  is  called  out  by  mechanical 
stimuli,  such  as  a  series  of  rapid  light  blows,  applied  to  nerve  A. 

Du  Bois-Reymond  considered  "  secondary  tetanus  "  a  proof  of  the  discon- 
tinuity of  the  apparently  continuous  contraction  of  tetanus,  for  muscle  F>  could 
only  have  been  excited  to  tetanus  by  rhythmic  excitations  from  A.  Each  of 

1  Burdon-Sanderson :  Journal  of  Physiology,  1898,  xxiii.  p.  323. 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND   NERVE.        151 

the  rapidly  following  excitations  applied  to  A  was  the  cause  of  a  separate  con- 
traction process  and  a  separate  current  of  action  in  B  ;  the  separate  contractions 
combined  to  produce  the  tetanus  of  B,  but  the  separate  currents  of  action  did 
not  fuse,  although  they  caused  a  continuous  negative  variation  of  the  slowly 
moving  magnet  of  the  galvanometer. 

The  correctness  of  this  view  has  been  shown  by  experiments  with  the  capil- 
lary electrometer,  which  approaches  the  "  physiological  rheoscope,"  as  the 
nerve-muscle  preparation  is  called,  in  its  sensitiveness  to  rapid  changes  in  elec- 
trical potential. 

Burdon  Sanderson  1  has  obtained,  by  photographically  recording  the  move- 
ments of  the  column  of  mercury  of  the  capillary  electrometer  (see  Fig.  63, 
p.  146),  beautiful  records  of  the  changes  of  electric  potential  which  occur  when 
an  injured  muscle  is  tetanized. 

The  record  in  Figure  66  shows,  first,  a  series  of  negative  changes  resulting 
from  the  separate  stimuli.  It  is  these  which  cause  secondary  tetanus  and 
which  produced  the  negative  variation  current  disclosed  by  the  galvanometer 
in  the  experiments  of  Dti  Bois-Reymond.  Second,  there  is  a  more  permanent 
negative  change,  likewise  opposed  to  and  lessening  the  effect  of  the  negative 
change  at  the  part  where  the  tissue  is  dying,  and  called  by  Sanderson  "  the 
dhninutional  effect."  The  continuous  negative  change  is  possibly  attributable 
to  the  presence  of  a  continuous  contraction  process,  perhaps  the  contracture 
which  we  observed  in  studying  the  tetanus  curve  (see  Fig.  52).  This  "  diminu- 


Fi<;.  66— Record  of  changes  in  electric  potential  in  a  tetanized  injured  muscle  of  a  frog.  The  leading- 
off  non-polarizable  electrodes  connected  with  the  capillary  electrometer  touched  the  normal  longitud- 
inal and  injured  cut  surface  of  the  muscle.  The  muscle  was  tetanized  by  an  induction  current  applied 
to  its  nerve,  the  rate  of  interruptions  being  210  per  second.  A  rise  of  the  curve  indicates  an  electrical 
change  of  opposite  direction  to  that  caused  by  the  injury.  The  diminution  of  the  current  of  injury, 
which  was  less  than  in  some  other  experiments,  was  0.008  volt.  The  time  record  at  the  bottom  of  the 
curve  was  obtained  from  a  tuning  fork  making  500  double  vibrations  per  second  (after  Burdon  San- 
derson). 

tional  effect"  is  only  to  be  observed  upon  an  injured  muscle,  since  it  repre- 
sents a  difference  in  potential  between  the  normally  contracting  and  the  injured, 
imperfectly  contracting  muscle-substance.  When  all  parts  of  the  muscle  are 
normal  and  contracting  to  an  equal  amount,  the  electrical  forces  would  be 
everywhere  of  the  same  nature,  balance  one  another,  and  give  no  external 
evidence.  Although  the  dimiuutional  effect  is  only  to  be  observed  upon  the 
injured  muscle,  the  temporary  negative  changes  which  follow  each  excitation 
1  Journal  of  Physiology,  1895,  vol.  xviii.  p.  717. 


152  AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 

are  to  be  observed  on  the  normal  muscle.  To  understand  this  we  must  con- 
sider the  diphasic  current  of  action. 

Diphasic  Current  of  Action. — If  a  normal  muscle  be  locally  stimulated  by 
a  single  irritation,  either  directly  or  indirectly  through  its  nerve,  the  part 
excited  will  be  the  first  to  become  active  and  electrically  negative,  and  this 
condition  will  be  taken  on  later  by  other  parts.  Our  methods  only  permit  us 
to  observe  the  relative  condition  of  the  parts  of  the  muscle  to  which  the  elec- 
trodes are  applied,  the  changes  in  the  intermediate  tissue  failing  to  show  them- 
selves. If  an  electrode  be  applied  near  the  place  where  the  uninjured 
muscle  is  stimulated,  A,  and  another  at  some  distant  point,  13,  and  these 
electrodes  be  connected  with  a  capillary  electrometer,  a  diphasic  electrical 
change  will  be  observed  to  follow  each  stimulation.  At  the  instant  the  irritant 
is  applied  the  muscle-substance  at  A  will  become  suddenly  negative  with 
respect  to  that  at  B ;  when  the  spreading  irritation  wave  has  reached  B,  that 
part  too  will  tend  to  be  negative,  and  an  electrical  equality  will  be  temporarily 
established ;  finally,  B  continuing  to  be  active  after  A  has  ceased  to  act,  B 
will  be  negative  in  respect  to  A.  Since  the  wave  of  excitation  spreads  along 
the  fibres  in  both  directions  from  the  point  irritated,  each  excitation  will  cause 
diphasic  electrical  changes  to  either  side  of  the  place  to  which  the  irritant  is 
applied. 

If  the  muscle  has  been  injured  at  B,  the  dying  fibres  there  will  react  but 
poorly  to  the  stimulus,  and  therefore  the  antagonistic  influence  of  the  negative 
change  at  B  will  incompletely  compensate  for  the  negativity  at  A,  and  hence 
only  a  single  phase  due  to  the  condition  of  negativity  at  A  will  be  seen. 

The  normally  beating  heart  ventricle  shows  diphasic  currents  of  action  : 
in  the  first  phase  the  base,  where  the  contraction  process  starts,  is  negative  to 
the  apex,  and  in  the  second  phase  the  apex  is  negative  to  the  base.  In  case 
the  heart  be  injured,  the  negative  change  corresponding  to  action  fails  at  the 
injured  part,  and  therefore  a  single  and  because  not  antagonized  more  pro- 
longed negative  change  is  observed.  Under  certain  conditions  a  tri phasic 
change  is  observed,  which  need  not  be  discussed  here.  Waller l  has  succeeded 
in  recording  the  electrical  changes  which  accompany  the  beat  of  the  human 
heart. 

These  diphasic  changes  of  the  electric  condition  are  sufficiently  strong  and 
rapid  in  the  mammalian  heart  to  excite  the  nerve  of  a  nerve-muscle  prepara- 
tion, and  the  muscle  will  be  seen  to  give  one,  or,  if  the  heart  is  uninjured, 
sometimes  two,  contractions  every  time  the  heart  beats. 

Bernstein2  found  the  time  between  the  two  portions  of  diphasic  change  to 
be  proportional  to  the  distance  between  the  leading-off  electrodes,  and  to  cor- 
respond to  a  rate  of  transmission  the  same  as  that  of  the  wave  of  excitation, 
as  revealed  by  the  spread  of  the  contraction  process  (in  the  muscle  of  the  frog 
3  meters  per  second).  Hermann,3  by  using  cord  electrodes  on  the  human  fore- 

1  -Archiv  fur  Anatomie  und  Physiologic,  1890;  physiol.  Abtheil.,  S.  187. 

2  Untersuchungen  uber  den  Erregungsvorgang  im  Nerven-  und  Muskel-system,  1871. 

3  Handbuch  der  Physiologic,  1879,  i.  1,  8.  224. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVi:.      ir,:>, 

arm,  found  the  rate  of  spread  of  the  active  process  by  the  voluntary  contraction 
of  human  muscle  to  be  from  10  to  13  meters  per  second.  Du  Bois-Reymond 
dipped  a  finger  of  each  hand  into  fluid  contained  in  cups  connected  with  a 
galvanometer.  If  the  muscles  of  one  arm  were  vigorously  contracted,  a 
deflection  of  the  magnet  was  seen.  This  was  probably  due  to  electric  currents 
from  the  glands  of  the  skin  and  not  from  the  contracting  muscles. 

Relation  of  the  Negative  Variation  Current  to  the  Contraction. — Bernstein 
observed  the  negative  variation  of  the  demarcation  current  of  the  muscle 
almost  at  the  instant  that  the  muscle  was  excited  and  before  the  contraction 
began  to  be  recorded — i.  e.,  during  the  mechanical  latent  period  of  the  muscle 
—and  concluded  that  it  is  associated  with  the  excitation  rather  than  the  con- 
traction process.  This  view  is  supported  by  others/  who  find  that  the 
highest  point  of  the  negative  change  is  generally  passed  before  the  contrac- 
tion shows  itself. 

On  the  other  hand,  it  is  reported  that  the  negative  state  may  continue 
throughout  the  contraction.  Sanderson  and  Page 2  saw  the  diphasic  change 
which  accompanies  the  beat  of  the  heart  to  last  throughout  the  systole ;  and 
Lee3  found  the  diphasic  change  which  occurs  when  a  skeletal  muscle  of 
a  frog  is  excited  by  a  single  stimulus,  to  continue  as  long  as  the  muscle  re- 
mains active,  including  the  period  of  relaxation ;  in  some  cases  it  lasted  from 
0.05  to  0.06  second.  Sanderson,  as  we  have  seen  (see  Fig.  66),  tetanized 
injured  skeletal  muscles  of  the  frog,  and  found  not  only  a  series  of  negative 
variations  corresponding  to  the  contraction  processes  which  resulted  from  the 
separate  excitations,  but  also  a  continuous  negative  variation,  the  diminutional 
effect,  which  developed  comparatively  slowly  and  lasted  after  the  irritant 
had  ceased  to  act. 

Still  other  observers,  who  claim  that  the  electrical  state  of  the  muscle  is 
closely  related  to  the  contraction  process,  find  sometimes  a  negative  and 
sometimes  a  positive  change,  according  as  the  contractions  are  isometric  or 
iso  tonic. 

A  further  consideration  of  this  subject  would  be  out  of  place  here.  Suf- 
fice it  to  say  that  there  can  be  no  doubt  that  the  change  which  occurs  in 
muscle  substance  when  it  is  excited  is  associated  with  a  change  in  its  elec- 
trical condition  ;  whether  the  subsequent  activity  of  the  muscle  protoplasm, 
manifested  in  the  change  of  form,  has  also  an  electrical  change  associated 
with  it,  must  be  left  an  open  question. 

4.  Currents  of  Action  in  Nerves. — In  general,  the  facts  which  have 
been  stated  with  regard  to  the  current  of  action  in  muscles  apply  to  nerves. 
When  a  normal  nerve  is  excited  a  negative  change  is  forthwith  developed  at 
the  stimulated  point  and  passes  thence  in  both  directions  along  the  nerve  at 
the  same  rate  as  the  nerve-impulse.  This  change  is  diphasic,  first  the  part 
excited  and  later  distant  parts  showing  the  negative  change.  If  the  nerve 

1  Jensen  :  PfliigeSs  Archiv,  1899,  Bd.  Ixxvii.  S.  107. 

1  Journal  of  Physiology,  1879,  vol.  ii.  p.  396. 

8  Archiv  fur  Anatomic  und  Physiologic,  1887,  S.  204. 


154  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

be  injured,  and  the  normal  surface  be  compared  with  the  dying  or  dead  cross 
section,  the  second  phase  is  absent.  If  the  nerve  be  frequently  excited,  each 
excitation  awakens  a  separate  current  of  action. 

Although  nerves  are  excited  most  readily  by  electric  currents,  negative 
variations  of  the  demarcation  current  may  be  called  out  by  various  chemicals 
— e.  g.,  salt  or  glycerin1 — and  by  mechanical  excitation,  such  as  a  sharp  cut 
with  the  shears.2  It  is  a  physiological  phenomenon,  for  a  negative  change 
may  be  observed  to  accompany  a  nerve  impulse  which  has  been  caused  by 
the  spread  of  excitation  from  central  neurones  along  their  peripheral  axones. 

Du  Bois-Reymond  observed  with  the  galvanometer  a  lessening  ("  negative 
variation  ")  of  the  demarcation  current  ("  current  of  rest ")  when  in  strychnia- 
poisoning  the  spinal  motor  neurones  were  sending  vigorous  impulses  along 
their  axones  and  causing  cramp-like  tetanic  muscular  contractions.  Gotch 
and  Horsley 3  applied  electrodes  connected  with  a  capillary  electrometer  to 
peripheral  nerves,  spinal  nerve-roots,  and  tracts  of  motor  fibres  within  the 
spinal  cord,  and  discovered  that  if  the  cortical  brain-cells  in  the  motor  zones 
were  excited,  the  nerves  showed  currents  of  action  corresponding  in  rate  to 
the  discharge  of  motor  impulses  from  these  brain-cells — e.  g.,  if  the  epilepti- 
form  convulsions  were  occurring  at  the  time,  the  capillary  electrometer 
revealed  changes  of  potential  of  like  rate  in  the  nerves. 

Macdonald  and  Reed 4  observed  currents  of  action  in  the  phrenic  nerve 
of  mammals  which  corresponded  in  time  with  the  respiratory  movements. 
These  were  due  to  the  normal  discharge  of  nerve  impulses  from  the  respira- 
tory centers.  When  a  condition  of  apnoea  was  established  and  the  respiratory 
movements  ceased,  the  electrical  change  failed  to  appear ;  when  the  respiratory 
movements  were  quickened  in  dyspnoea,  the  rhythmic  movements  of  the  gal- 
vanometer were  quickened  to  correspond  ;  when  during  asphyxia  the  respi- 
rations were  of  the  Cheyne-Stokes  type,  the  currents  of  action  showed  a  like 
rhythm. 

Even  physiological  sensory  nerve-impulses  have  been  found  to  produce 
negative  variation  currents.  Steinach5  observed  currents  of  action  to  be 
caused  by  mechanical  pressure  on  the  skin  of  the  frog.  If  the  pressure  \vas 
continued,  the  negative  change  gradually  decreased,  and  a  new  negative 
variation  was  seen  if  the  pressure  was  suddenly  removed. 

Light  falling  on  the  retina  of  the  eye  of  a  frog  has  been  seen  to  cause  a 
negative  variation  of  the  current  of  rest  of  the  optic  nerve. 

The  electrical  change  which  we  call  the  current  of  action  can  be  thought 
to  sweep  over  the  nerve  as  a  wave,  having  in  the  medullated  nerves  of  the 
frog  a  length  of  18  mm.,  and  travelling  at  the  rate  of  28  meters  per  second. 
The  duration  of  the  negative  variation  is  different  in  different  kinds  of  nerves 

1  Kiihne  and  Steiner :  Untersuchungen  aus  der  physiologisehen  Laboratoriitm  in  Heidelberg,  1880, 
Bd.  iii.  S.  149. 

2  Bering :  Lotos,  Neue  Folge,  1888,   Bd.  9,  S.  35. 

3  Physiological  Transactions,  1891,  vol.  182,  pp.  267-526. 

*  Macdonald  and  Reed  :  Journal  of  Physiology,  1898,  vol.  xxiii.  p.  100. 
5  Steinach :  Pfliiger's  Archiv,  1896,  Bd.  Ixiii.  S.  495. 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     155 

and  in  the  same  nerve  under  dilVnvnt  conditions.  In  the  medullated  nerves 
of  the  frog  it  lasts  0.008  to  0.024  second  ; l  in  the  non-medullated  m-rvcs  of 
the  pike  the  rise  of  negativity  requires  0.08-0.029  second  and  the  fall  0.40- 
1.2  second.2  The  rate  of  conduction  is  slowed  by  cold,  and  this  at  the  san it- 
time  lengthens  the  accompanying  electrical  change.  This  fact  has  been 
made  use  of  to  ascertain  that  in  the  uninjured  nerve,  as  in  the  muscle,  there 
is  a  diphasic  current  of  action  spreading  in  both  directions  from  the  point  of 
excitation. 

The  strength  of  the  electrical  change  which  takes  place  in  a  nerve  when 
it  is  excited  is  the  best  evidence  which  we  have  of  the  activity  of  the  nerve. 
"The  physiological  structures  which  the  nerve  normally  excites  obey  laws 
peculiar  to  themselves,  and  are  liable  to  give  results  which  are  open  to  mis- 
interpretation. For  example,  if  a  nerve  be  stimulated  in  the  middle,  the 
condition  of  activity  aroused  spreads  in  both  directions,  and  causes  at  the 
one  end  a  contraction  of  the  muscle,  and  at  the  other  a  negative  variation  of 
the  current  of  rest,  which  may  be  observed  with  a  galvanometer.  If  the 
nerve  be  excited  many  times  in  succession,  the  height  of  the  muscular  con- 
tractions is  seen  to  decrease  while  the  electrical  changes  show  no  sign  of 
fatigue.  The  decrease  in  the  height  of  the  contractions  is  really  due  to  the 
fatigue  of  the  nerve  ends  and  the  muscle,  and  the  constancy  of  the  electrical 
changes  is  the  truer  expression  of  the  state  of  the  nerve. 

A  difficulty  presents  itself  here,  however :  the  negative  variation  currents 
observed  in  such  an  experiment  may  be  so  very  regular  as  to  suggest  that 
they  are  physical  rather  than  physiological  phenomena.  That  they  are  not 
purely  physical  can  be  ascertained  by  subjecting  the  nerve  to  influences  of  a 
type  to  alter  the  physiological  activity  of  the  protoplasm,  without  perma- 
nently, or  indeed  markedly,  altering  its  chemical  and  physical  structure.  A 
study  of  the  effect  of  anaesthetics  on  the  nerve  is  especially  instructive.  In 
general,  anaesthetics  are  found  iirst  to  heighten,  later  to  lower,  and  finally  to 
destroy  the  irritability. 

If  the  anaesthesia  is  not  carried  too  far,  the  nerve  may  completely  recover 
its  function  on  the  removal  of  the  drug.  Waller3  describes  the  following 
experiment :  A  fresh  nerve  is  placed  on  two  pairs  of  non-polarizable  electrodes 
in  a  moist  chamber.  One  pair  is  connected  with  the  galvanometer  which  is 
to  record  the  current  of  action,  the  other  pair  is  connected  with  an  induction 
apparatus,  and  brings  the  exciting  current  to  the  nerve.  Induction  shocks 
of  equal  strength  are  sent  into  the  nerve  at  regular  intervals,  and  the  extent 
of  the  currents  of  action  awakened  is  noted.  After  the  electrical  response 
of  the  nerve  has  been  tested,  fumes  of  ether  (diethyl  oxide)  are  blown  through 
the  chamber.  At  first  the  electrical  response  is  found  to  be  increased,  later 
it  decreases,  and  at  the  end  of  three  or  four  minutes  it  is  altogether  lost.  If 
air  be  now  allowed  to  enter  the  chamber,  the  current  of  action  reappears, 

1  Biedermann :  Electrophysiology,  translation  by  F.  A.  Welby,  1898,  vol.  ii.  p.  260. 

2  Gartner:  PflugeSs  Archiv,  1899,  Bd.  Ixxvii.  S.  498. 

3  Waller:  Lectures  on  Animal  Electricity,  1897,  Lecture  II. 


156  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

and  at  the  end  of  from  five  to  ten  minutes  may  be  even  stronger  than  it  was 
at  the  start.  Chloroform,  likewise,  inhibits  the  activity  of  the  nerve ;  but  it 
is  even  more  vigorous  in  its  action  and  the  nerve  is  less  likely  to  recover. 
The  vapor  of  ethyl  alcohol,  after  a  preliminary  exhilarating  effect,  paralyzes 
the  nerve ;  under  favorable  conditions  the  nerve  recovers  in  a  few  hours. 
The  action  of  CO2  is  particularly  interesting,  since  this  is  one  of  the  normal 
waste  products  of  the  body.  In  small  doses  it  is  found  to  increase  the 
strength  of  the  current  of  action,  while  in  large  doses  it  has  an  anaesthetic 
effect.  The  nerve  is  so  sensitive  to  CO2  that  even  a  fiftieth  of  a  milligram 
suffices  to  change  its  irritability :  the  amount  that  is  contained  in  the  expired 
air,  4  per  cent.,  suffices  to  do  away  with  the  current  of  action  in  three 
minutes. 

In  general,  whatever  increases  the  irritability  of  the  nerve  increases  the 
negative  variation  currents  which  result  from  its  excitation.  For  instance, 
if  a  nerve  be  excited  at  regular  short  intervals  with  induction  shocks  of  equal 
strength,  there  is  a  staircase-like  increase  in  the  negative  variation  current ; 
moreover,  if  it  be  subjected  for  a  short  period  to  tetanic  excitation,  the  cur- 
rent of  action  called  out  by  a  single  shock  is  found  to  be  increased.1  Indeed 
if  one  photographically  records  the  movements  of  the  galvanometer  magnet 
in  such  experiments,  he  obtains  a  curve  closely  resembling  that  obtained  by 
records  of  muscular  contractions  when  the  muscle  is  so  excited  (see  Fig.  57). 
Waller  suggests  that  the  effect  of  excitation  to  increase  the  irritability  of  the 
nerve  may  be  due  to  the  production  of  CO2  by  the  nerve,  and  a  consequent 
internal  stimulation.  The  fact  that  the  nerve  does  not  fatigue  he  would 
explain  as  the  result  of  rapid  restoration  of  the  protoplasm  and  a  rapid 
neutralization  of  waste  products. 

Apparently  the  strength  of  the  current  of  action  can  be  taken  as  a  fair 
index  of  the  activity  of  the  nerve,  and  consequently  of  the  strength  of  the 
nerve  impulse.  This  view  corresponds  with  the  fact  that  there  is  a  close 
relation  between  the  strength  of  the  current  of  action  of  the  muscle  and  the 
height  of  the  contraction.  Waller 2  and  Green 3  found  experimentally  that  a 
current  of  action  can  be  detected  with  difficulty  with  subminimal  irritants ; 
but  as  the  strength  of  the  current  is  raised  above  the  threshold  intensity  the 
strength  of  the  electrical  change  increases  proportionally  to  the  strength  of 
the  excitation,  until  a  point  is  reached  which  is  far  beyond  what  is  needed 
to  excite  maximal  muscular  contractions.  After  this  the  increase  is  less  and 
less,  until  finally  a  maximal  current  of  action  is  reached.  This  occurs  with 
a  strength  of  excitation  much  stronger  than  is  required  to  call  out  a  maximal 
muscular  contraction,  and  probably  beyond  the  limit  of  functional  action. 
It  is  doubtful  whether  the  nerve-cell  could  excite  such  a  condition  in  a 
nerve. 

On  account  of  the  great  resistance  of  the  nerve,  and  the  short-circuiting 

1  Waller  :  Lectures  on  Animal  Electricity,  1897,  p.  59. 

8  Waller :  Brain,  xvii.  p.  200. 

3  Green  :  American  Journal  of  Physiology,  1898,  p.  104. 


GENERAL    PHYSIOLOGY    OF  MUSCLE   AND    NERVE.      157 

of  a  part  of  the  current  by  the  lymph,  etc.,  the  currents  which  may  be  obtained 
are  usually  small.  Under  especially  favorable  conditions  the  current  may 
be  twice  as  great  as  the  current  of  rest,  and  Gotch  and  Burch  state  that 
they  have  found  it  in  the  sciatic  nerve  of  the  frog  to  have  the  excitatory  effect 
of  0.033  volt,1  Hering 2  has  shown  that  it  may  suffice  to  excite  another  nerve 
in  close  contact  with  it,  the  experiment  being  of  the  same  type  as  that  de- 
scribed as  secondary  tetanus,  p.  150. 

The  fact  that  a  negative  variation  of  the  demarcation  current  of  a  bundle 
of  nerve-fibres  may  excite  other  fibres  lying  beside  them  in  the  same  nerve, 
is  the  explanation  of  the  so-called  "  paradoxical  contraction."  This  may  be 
seen  under  the  following  conditions :  Take  a  frog  that  has  been  kept  in  the 
cold  so  that  its  nerves  are  very  irritable ;'  dissect  out  the  branches  of  the 
sciatic  nerve  at  the  knee,  ligature,  divide  below  the  ligatures,  and  then  dissect 
up  the  nerve  as  far  as  the  branches  given  off  to  the  muscles  of  the  thigh ; 
expose  the  nervous  plexus  at  the  back  of  the  abdominal  cavity,  and  divide 
close  to  the  spinal  cord ;  remove  all  extra  fluid,  and  avoid  wetting  the  nerves 
with  salt  solution.  Kaise  up  the  knee  end  of  the  nerve,  and  place  on  elec- 
trodes connected  -with  the  secondary  coil  of  an  induction  apparatus :  excite 
with  weak  tetanizing  current  close  to  the  end  of  the  nerve,  and  see  the  thigh 
muscles  contract,  As  the  fibres  excited  at  the  knee  have  no  anatomical  or 
physiological  connection  with  the  fibres  of  the  branches  of  the  nerve  dis- 
tributed to  the  thigh  muscles,  the  nerve  impulse  which  passes  up  the  nerve 
could  not  reach  these  fibres,  and  the  contraction  could  not  occur  were  it  not 
for  the  spread  of  the  -current  of  action  under  these  abnormal  conditions.  To 
make  sure  that  the  contraction  is  not  due  to  a  spread  of  the  exciting  current 
or  to  unipolar  stimulation  effects,  one  can  ligature  the  nerve  above  the  excited 
point.  This  would  stop  the  progress  of  the  nerve  impulse  and  the  accom- 
panying current  of  action,  and  prevent  the  "paradoxical  contraction."  There 
is  still  another  possible  source  of  error  that  has  to  be  guarded  against :  the 
exciting  current,  through  electrolytic  effects,  may  cause  electrotonic  currents 
(see  p.  62)  to  be  developed  in  the  nerve,  and  these  may  spread  sufficiently 
to  excite  branches  of  the  thigh  muscles.  A  ligature  will  stop  the  spread  of 
these  currents,  but  their  presence  can  be  recognized  from  the  fact  that  the 
contractions  will  be  the  stronger  the  nearer  the  electrodes  are  brought  to  the 
part  of  the  nerve  from  which  the  branches  to  the  thigh  muscles  are  given 
off.  A  contraction  called  out  in  this  way  is  called  the  "  electrotonic  twitch." 
A  spread  of  the  current  of  action  to  neighboring  fibres  never  occurs  when 
the  nerve  is  intact,  the  rule  of  isolated  conduction  by  nerve-fibres  holding 
good. 

Relation  of  the  Electrical  Phenomena  of  the  Nerve  to  Physiological 
Processes. — Certain  writers  of  the  extreme  mechanical  school  would  explain 
all  the  forms  of  functional  activity  of  the  nerve  as  purely  physical  processes, 
which  result  from  chemical  change  occurring  within  it.  This  point  of  view 

1  Proceedings  of  the  Royal  Society,  1900,  vol.  Ixv.  p.  441. 

2 Hering:  Sitzungsberichte  der  Wiener  Akadamie,  1882,  Ixxxv.  3,  S.  237. 


158  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

is  largely  based  on  the  remarkable  results  which  have  been  obtained  by  the 
study  of  artificial  models  of  nerves,  called  core-conductors.  These  were  first 
carefully  studied  by  Hermann,1  and  since  then  by  Boruttau,2  Waller,3  and 
others.  Such  a  model  can  be  made*  by  placing  a  platinum  wire  in  a  glass 
tube,  and  surrounding  it  with  a  0.6  per  cent,  salt  solution.  The  wire  represents 
the  axis-cylinder  of  the  nerve,  and  the  salt  solution  the  medullary  sheath. 
In  other  models  both  core  and  sheath  have  been  made  of  fluid  electrolytes. 
With  such  a  core-conductor,  one  can  observe  electrical  phenomena  so  closely 
resembling  those  manifested  by  the  normal  nerve  that  the  statement  has 
been  made  that  all  the  electrical  phenomena  of  the  living  nerve  can  be 
explained  if  one  will  look  upon  it  as  a  core-conductor.4  According  to  this 
idea,  conduction  in  nerve  depends*  on  the  transmission  of  an  electrical  phase 
caused  by  local  differences  in  potential,  resulting  from  chemical  changes  and 
consequent  polarization  effects  produced  when  the  nerve  is  excited. 

This  extreme  view  is  accepted  by  few  physiologists.5  The  electrical  effects 
which  follow  excitation  are  exhibited  not  only  by  nerves,  but  also  by  a  great 
variety  of  protoplasmic  structures ;  they  are  stopped  by  anesthetics,  which 
do  not  alter  the  core-conductor-like  structure  of  the  nerve,  and  are  greatly 
modified  by  all  influences  which  are  capable  of  changing  the  irritability  of 
living  protoplasm,  even  though  they  are  too  feeble  to  produce  any  recogniz- 
able change  in  dead  matter ;  finally,  they  are  called  forth,  not  only  by  electric 
currents,  but  by  every  form  of  stimulus  capable  of  exciting  living  protoplasm 
to  action. 

It  is  very  easy  to  be  led  astray  by  the  similarity  of  processes  observable 
on  very  different  structures,  and  think  to  see  the  whole  truth  in  what  is  only 
a  partial  truth.  As  Engelmann  has  shown,  the  anisotropic  substance  in  a 
piece  of  catgut  suspended  in  water  will  cause  it  to  shorten  and  then  lengthen 
if  quickly  heated  and  then  cooled,  and  if  a  lever  be  connected  with  it,  to 
write  a  curve  strikingly  like  that  of  the  contracting  muscle.  In  muscle  there 
is  anisotropic  material  surrounded  by  fluid,  and  heat  is  produced  at  the 
instant  of  contraction ;  it  is  doubtful,  however,  whether  the  physiological 
contraction  process  is  of  the  same  type  as  that  of  the  piece  of  catgut.  Within 
the  body  we  have  oxidation  processes  going  on,  and  heat  is  liberated  as  it  is 
outside  of  the  body  in  combustion,  but  the  two  sets  of  changes  giving  this 
result  are  not  identical.  Similarly  we  may  say  that  the  heart  is  a  pump,  and 
the  eye  a  camera,  but  the  behavior  of  these  living  organs  is  very  different 
from  that  of  lifeless  machines. 

All  physiological  phenomena  are  to  be  regarded  as  of  chemico-physical 
nature,  but  many  of  them  differ  so  widely  from  the  chemical  and  physical 
processes  associated  with  dead  matter  that  a  sharp  distinction  should  be 

1  Pftuger*  8  Archiv,  1872,  Bd.  v.,  vi.,  vii.;  also  Handbuch  der  Physiologic,  1879,  Bd.  ii.  S.  17. 

2  Pfluger's  Archiv,  1894-1897,  Bd.  Iviii.,  lix.,  Ixiii.,  Ixv.,  Ixvi.,  Ixix. 

3  Lectures  on  Animal  Electricity,  London,  1897. 

4  Boruttau :  Pfliiger's  Archiv,  1894,  Bd.  Iviii.  S.  64. 

5  Biedermann:  Electrophysiology,  translated  by  F.  A.  Welby,  1898,  vol.  ii.  p.  303. 


GENERAL   PHYSIOLOGY   OF  MUSCLE   AND    NEltVl-:.      159 


made.  Purely  physiological  phenomena  are  such  as  can  he  exhibited  only 
by  a  mechanism  which  has  the  chemical  and  physical  structure  of  living 
protoplasm,  and  such  as  cease  with  the  life  of  the  protoplasm. 

The  electrical  phenomena  of  nerve  are  capable  of  being  divided  into  two 
classes,  the  one,  purely  chemico-physical,  resulting  from  the  core-conductor- 
like  structure,  and  the  other,  physiological,  intimately  dependent  on  the  reac- 
tions of  the  living  protoplasm.  The  medullated  nerve  is  not  merely  a  core- 
conductor. 

It  is  too  soon  to  try  to  separate  these  two  classes  of  phenomena  ;  we  must 
wait  not  only  for  more  work  to  be  done  on  nerves,  but  on  other  irritable  forms 
of  protoplasm,  for  many  of  these,  although  of  entirely  different  structure  from 
the  nerve,  exhibit  very  similar  electrical  reactions. 

F.  CHEMISTRY  OF  MUSCLE  AND  NERVE. 
I.  CHEMISTRY  OF  MUSCLE. 

Muscles  consist  of  muscle-fibres  bound  together  by  connective  tissue. 
Between  the  fibres  we  find  nerves,  blood-vessels,  and  lymphatics.  Fat-cells 
containing  considerable  fat  may  also  be  found  in  the  midst  of  the  connective- 
tissue  network.  Each  fibre  consists  of  a  sheath,  the  sarcolemma,  which 
resembles  elastin  in  its  constitution,  and  within  this  the  muscle-substance 
proper,  together  with  certain  substances  of  nutritive  value  and  waste  prod- 
ucts. 

Muscle  which  has  been  freed  as  far  as  possible  from  blood,  connective 
tissue,  and  fat,  has  a  mean  specific  gravity  of  1.060;  the  extreme  variations 
found  for  the  muscles  of  different  animals  being  1.  053-1.  074.1  When  it  is 
fresh  the  reaction  is  slightly  alkaline. 

It  contains  about  75  parts  of  water  and  25  parts  of  solids  ;  nearly  20 
parts  of  the  solids  are  proteids,  the  remaining  5  parts  consisting  of  fats,  ex- 
tractives, and  salts. 

Little  is  known  concerning  the  chemistry  of  living  muscle;  the  instability 
of  the  complex  molecules  which  makes  possible  the  rapid  development  of  energy 
peculiar  to  muscles  renders  exact  analysis  impossible.  The  manipulations 
essential  to  chemical  analysis  necessarily  alter  and  kill  the  muscle  protoplasm. 

Death  of  the  muscle  is  ordinarily  associated  with  a  peculiar  chemical  change 
known  as  rigor  mortis.  To  understand  the  chemical  composition  of  muscle  it 
is  necessary  that  we  should  consider  the  nature  of  this  change. 

1.  Rigor  Mortis.  —  Rigor  mortis,  the  rigidity  of  death,  is  the  result  of  a 
chemical  change  in  the  substance  of  a  muscle  by  which  it  is  permanently 
altered,  its  irritability  and  other  vital  properties  being  irretrievably  lost.  The 
change  is  manifested  by  a  loss  of  translucency,  the  muscle  becoming  opaque,  and 
by  a  gradual  contraction,  accompanied  by  a  development  of  heat  and  acidity, 
and  resulting  in  the  muscle  being  stiff  and  firm  to  the  touch,  less  elastic,  and 
less  extensible.  Whenever  muscle  dies  it  undergoes  this  change. 
1  Carvallo  and  Weiss  :  Journal  de  Physiologic,  1899,  i.  p.  204. 


160  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Conditions  which  Influence  the  Development  of  Rigor. — Ordinarily  on  the 
death  of  the  body  the  muscle  enters  into  rigor  slowly — the  muscle-fibres  are 
involved  one  after  the  other,  and  through  the  gradual  contraction  and  harden- 
ing of  the  antagonistic  muscles  the  joints  become  fixed  and  the  body  acquires 
the  rigidity  which  we  associate  with  death.  Rigor  usually  affects  the  different 
parts  of  the  body  in  a  regular  order,  from  above  downward,  the  jaw,  neck, 
trunk,  arms,  and  legs  being  influenced  one  after  the  other.  The  position  taken 
by  the  body  is  generally  determined  by  the  weight  of  the  parts  and  the  rela- 
tive strength  of  the  contractions  of  the  muscles. 

The  time  required  for  the  appearance  of  rigor  is  very  variable.  It  is  deter- 
mined in  part  by  the  nature  of  the  muscle,  its  condition  at  the  moment  of 
death,  and  the  temperature  to  which  it  is  subjected.  The  muscles  of  warm- 
blooded animals  enter  into  rigor  more  quickly  than  those  of  cold-blooded 
animals ;  of  the  warm-blooded  animals,  pale  muscles  more  quickly  than  red, 
and  the  flexors  before  the  extensors  ;  of  the  cold-blooded  animals,  frog's  muscles 
more  quickly  than  those  of  the  turtle.  In  general,  the  more  active  the  muscle 
protoplasm,  the  more  rapid  are  the  chemical  changes  which  it  undergoes,  and 
amongst  these  the  coagulation  of  rigor  mortis. 

The  condition  of  the  muscle  plays  a  very  important  part  in  determining  the 
onset  of  rigor.  If  the  muscles  are  strong  and  vigorous  and  death  of  the  body 
has  come  suddenly,  rigor  develops  slowly ;  if  the  muscles  have  been  enfeebled 
by  disease  or  fatigued  by  great  exertion  shortly  before  death,  it  comes  rapidly. 
In  the  case  of  wasting  diseases  rigor  comes  quickly,  is  poorly  developed,  and 
passes  off  quickly ;  when  the  muscles  are  fatigued  at  the  time  of  death,  as  in 
the  case  of  a  hunted  animal,  it  comes  quickly.  We  hear  of  soldiers  found  dead 
on  the  field  of  battle  grasping  the  sword,  as  if  the  muscular  contractions  of  life 
had  been  continued  by  the  contractions  of  death.  In  the  case  of  certain  dis- 
eases of  the  spinal  cord  and  brain,  too,  rigor  may  come  so  rapidly  that  the 
limbs  may  maintain  the  position  which  they  had  at  the  time  of  death,  "  cata- 
leptic rigor,"  as  i't  has  been  called.  The  coming  on  of  rigor  is  particularly 
striking  in  the  case  of  diseases  which,  like  cholera,  are  accompanied  by  violent 
muscular  cramps  and  lead  to  a  rapid  death.  It  is  not  uncommon,  in  such 
cases,  for  the  contractions  of  rigor  to  cause  movements  which  may  mislead  a 
watcher  into  supposing  the  dead  man  to  be  still  alive.  This  idea  is  favored  by 
the  fact  that  the  body  may  remain  warm,  owing  to  the  heat  which  is  produced 
in  the  muscles  as  a  result  of  the  chemical  changes  occurring  during  rigor. 
The  post-mortem  muscular  contractions  and  the  rise  of  temperature  observed 
in  such  cases  are  only  excessive  manifestations  of  what  always  occurs  on  the 
death  of  the  muscle.  The  movements  are  probably  due,  in  part,  to  the  rapidity 
with  which  the  muscles  contract  in  rigor,  and  in  part  to  the  fact  that  the 
antagonistic  muscles  are  not  affected  at  the  same  time  to  the  same  degree. 
Whether  the  contractions  are  partly  excited  by  changes  accompanying  the 
death  of  the  motor  nerve-cells  in  the  central  nervous  system  is  uncertain,  but 
not  impossible.  Muscles  are  still  able  to  respond  by  contractions  to  stimuli 
coming  to  them  through  the  nerve,  even  after  rigor  has  become  quite  pro- 


GENERAL    PHYSIOLOGY   OF  MUSCLE   AND    NERVE.     161 

nounced,  probably  because  the  coagulation  process  attacks  the  different  fibres 
at  different  rates,  and  certain  of  the  fibres  are  still  alive  and  irritable  after  the 
others  are  dead  and  coagulated. 

Many  observers  favor  the  view  that  the  central  nervous  system  influences 
muscles  after  the  death  of  the  body  as  a  whole,  and  by  weak  stimuli  resulting 
from  the  changes  in  the  nerve-cells  excites  chemical  changes  in  the  muscles 
which  favor  the  coming  on  of  rigor.1  In  proof  of  this  it  is  stated  that  cura- 
rized  muscles  enter  into  rigor  more  slowly  than  non-curarized.  Undoubtedly 
stimulation  of  the  nerve,  or,  indeed,  anything  which  would  excite  a  muscle  to 
action,  tends  to  put  it  in  a  condition  favorable  to  the  coming  on  of  rigor; 
whether  the  influence  exerted  by  the  central  nervous  system  is  more  than  this 
is  very  questionable. 

Temperature  has  a  marked  influence  on  the  development  of  rigor  mortis. 
Cold  delays  and  warmth  favors,  38°— 40°  C.  being  most  favorable.  Since  rigor 
is  the  result  of  a  chemical  change,  these  effects  of  temperature  are  what  one 
would  have  expected.  Other  forms  of  chemical  change  which  are  attributable 
to  ferment  action  are  found  to  be  the  most  vigorous  at  a  temperature  of  about 
40°  C. 

In  general,  it  may  be  said  -that  rigor  in  warm-blooded  animals  comes  on 
in  from  ten  minutes  to  seven  hours  after  death,  although  some  state  that  it 
may  come  as  late  as  eighteen  hours.  It  lasts  anywhere  from  one  to  six  days. 
The  sooner  it  comes  on,  the  sooner  it  goes  off.  The  stiffness  can  be  broken  up 
artificially  by  forced  movements  of  the  parts,  and  when  thus  destroyed  does 
not  return,  provided  the  rigor  was  complete  at  the  time. 

The  Cause  and  Nature  of  the  Contraction  of  Rigor  Mortis. — The  most  likely 
explanation  of  the  contraction  of  the  dying  muscle  is  that  it  is  the  result  of 
the  coagulation  of  a  part  of  the  semi-fluid  muscle-substance  within  the  sarco- 
lemma.  This  was  suggested  by  Bruecke,  and  Kuehne  proved  that  such  a 
coagulation  change  takes  place,  by  showing  that  the  semi-fluid  muscle-sub- 
stance, "  the  muscle-plasma/7  if  expressed  from  the  frozen  muscle,  coagulates  on 
being  warmed.  The  coagulation  is  the  result  of  a  chemical  change,  by  which 
two  proteids  of  the  plasma,  paramyosinogen  and  myosinogen,  are  converted 
into  the  coagulated  proteid  myosin,  this  change  being  produced  by  the  action 
of  a  ferment,  the  myosin  ferment,  which  is  thought  to  be  formed  at  the  death 
of  the  muscle. 

Another,  though  less  generally  accepted  view,  is  that  the  contraction  of  the 
muscle  seen  in  rigor  is  of  the  same  nature  as  ordinary  muscular  contractions.2 
Prolonged  muscle  contractions  are  observed  under  a  great  variety  of  condi- 
tions (see  p.  127),  and  there  are  many  points  of  resemblance  between  the 
contraction  of  normal  and  dying  muscle — viz.,  the  change  of  form,  the  pro- 
duction of  heat,  the  formation  of  sarcolactic  acid,  the  using  up  of  oxygen  and 
the  production  of  carbon  dioxide,  and  the  fact  that  the  dying  and  presumably 
coagulating  muscle  is,  like  normal  contracting  muscle,  electrically  negative 

1  Brown-Se'quard  :  Archives  de  Physiologic,  1889,  p.  675. 

2  Hermann  :  Handbuch  der  Physiologic,  1879,  Bd.  i.  S.  146. 
VOL.  II.— 11 


162 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


as  compared  with  normal  resting  muscle.  To  this  may  be  added  that,  as  has 
been  said,  the  muscle  continues  to  be  irritable  even  when  rigor  is  quite 
advanced,  and  that  it  enters  into  rigor  more  quickly  if  left  in  connection  with 
the  central  nervous  system. 

On  the  other  hand,  one  cannot  fail  to  be  impressed  with  the  differences 
between  the  two  forms  of  contraction. 


Normal  Contracting  Muscle. 
Contains  uncoagulated  proteids. 
Is  translucent. 
Is  soft  and  flexible. 
Is  no  less  elastic  than  in  repose. 
Is  more  extensible  than  in  repose. 
Contracts  rapidly. 
Fatigues  rapidly  and  relaxes. 


Muscle  contracting  by  Rigor  Mortis. 
Contains  coagulated  myosin. 
Is  opaque. 
Is  firm  and  stiff. 
Is  less  elastic  than  before. 
Is  less  extensible  than  before. 
Contracts  very  slowly,  as  a  rule. 
Remains  contracted  a  long  time. 


Furthermore,  it  may  be  added  that  normal  contractions  only  occur  when 
the  irritable  muscle  is  stimulated,  while  a  muscle  can  enter  into  rigor  when  its 
irritability  has  been  taken  away  by  subjecting  it  to  oxalate  solutions,1  also, 
when  it  has  been  curarized  and  so  shut  out  from  all  nervous  influences.2 

Rigor  is  not  confined  to  the  voluntary  muscles,  though  it  is  less  easily 
observed  in  the  case  of  most  involuntary  muscles.  The  heart  enters  rapidly 
into  rigor,  with  the  formation  of  sarcolactic  acid.  The  non-striated  muscle 
of  the  stomach  and  ureters,  too,  has  been  seen  to  undergo  this  change. 

The  passing  off  of  rigor  mortis  is  usually  accompanied  by  beginning 
decomposition,  and,  indeed,  it  has  been  thought  that  the  decomposition  is 
the  cause  of  softening  of  the  muscle.  This  is  denied  by  certain  observers, 
and  it  is  stated  that  rigor  may  pass  off  when  the  presence  of  putrefactive 
organisms  is  excluded  by  special  aseptic  precautions.  Another  explanation 
is  that  a  process  of  intramuscular  digestion  goes  on.  Pepsin,  a  proteolytic 
ferment,  has  been  found  in  small  amounts  in  the  muscle ;  and  acid,  which  is 
necessary  to  the  action  of  this  digestive  ferment,  is  formed  in  the  muscle 
when  it  coagulates.  The  presence  of  these  substances  would  make  the  diges- 
tion of  proteid  material  possible,  and  the  fact  that  proteoses  and  peptone, 
products  of  such  digestion,  are  to  be  found  in  the  muscle  after  death,  though 
not  present  during  life,  favors  the  view.  It  cannot  be  considered,  however, 
to  be  definitely  established. 

The  rigidity  of  muscles  in  rigor  can  be  readily  broken  up  by  stretching 
and  massaging  the  muscles,  and  when  this  has  been  done  it  does  not  return. 

The  Chemical  Changes  which  accompany  the  Development  of  Rigor. — Rigor 
mortis  is  characterized  by  the  coagulation  of  a  part  of  the  muscle-substance ; 
this  can  be  prevented  by  a  temperature  a  little  below  0°  C.  Cold,  although 
temporarily  depriving  the  muscle  of  its  irritability,  does  not,  unless  extreme  and 
long-continued,  kill  the  muscle  protoplasm.  Frogs  can  be  frozen  stiff  and 
recover  their  activity  when  they  thaw  out.  Indeed,  this  probably  happens  not 

1  Howell :  Journal  of  Physiology,  1893,  vol.  xiv.  p.  476. 

2  Nagel :  Pftilger^s  Archiv,  Bd.  Iviii.  S.  279. 


GENERAL    PHYSIOLOGY    OF  MUSCLE    AND    NERVE.     163 

infrequently  to  the  frogs  hibernating  in  holes  in  the  banks  of  ponds.  Since  cold 
prevents  coagulation  without  destroying  the  life  of  the  muscle  protoplasm,  we 
can  by  its  aid  isolate  the  living  muscle-substance  from  the  nerves,  blood- 
vessels, connective  tissue,  and  sarcolemna  of  the  muscle,  but  as  soon  as  we 
begin  to  analyze  it  it  loses  its  living  structure.  This  method  of  obtaining 
muscle-plasma  was  introduced  by  Kuehne1  in  the  study  of  the  muscles  of  frogs, 
and  was  later  employed  with  slight  modifications  by  Halliburton2  for  the  mus- 
cles of  warm-blooded  animals.  The  blood  was  washed  out  of  the  vessels  with 
a  stream  of  0.6  per  cent,  sodium-chloride  solution  at  5°  C. ;  the  irritable  mus- 
cles were  then  quickly  cut  out  and  frozen  in  a  mixture  of  ice  and  salt.  The 
frozen  muscle  was  then  cut  up  fine  in  the  cold,  and  a  yellowish,  some- 
what viscid,  and  faintly  alkaline  muscle-plasma  was  squeezed  out.  This 
fluid  was  found  to  coagulate  in  twenty  to  thirty  minutes  at  a  temperature  of 
40°  C. ;  if  the  temperature  were  lower  the  coagulation  was  slower.  The  clot, 
which  was  jelly-like  and  translucent,  contracted  slowly  and  in  a  few  hours 
squeezed  out  a  few  drops  of  serum.  The  coagulated  material  formed  in  the 
clot  is  called  myosin.  It  dissolves  readily  in  dilute  neutral  saline  solutions, 
as  a  10  per  cent,  solution  of  sodium  chloride  or  a  5  per  cent,  solution  of  mag- 
nesium sulphate,  and  its  saline  solutions  are  precipitated  in  an  excess  of  water 
or  by  saturation  with  sodium  chloride,  magnesium  sulphate,  or  ammonium 
sulphate ;  it  has,  in  short,  the  characteristics  of  a  globulin.  Chittenden  and 
Cummins  state  that  it  has  the  following  composition:  C  52.82,  H  7.11,  N 
16.17,  S  1.27,  O  22.03. 

Halliburton,  in  studying  the  coagulation  of  muscle,  followed  for  the  sake  of 
comparison  the  methods  which  have  been  employed  in  the  study  of  coagulation 
of  blood.  He  found  that  muscle-plasma,  like  blood-plasma,  is  prevented  from 
coagulating  not  only  by  cold,  but  by  neutral  salts,  such  as  magnesium  sulphate, 
sodium  chloride,  and  sodium  sulphate;  and  further,  that  the  salted  plasma  if 
diluted  coagulates. 

The  points  of  resemblance  between  the  coagulation  of  myosin  in  the 
muscle  and  fibrin  in  the  blood  suggest  a  similar  cause,  and  Halliburton  suc- 
ceeded in  obtaining  from  muscles  coagulated  by  long  standing  in  alcohol  a 
watery  extract,  which  greatly  hastened  the  coagulation  of  muscle-plasma. 
He  called  the  substance  thus  obtained  myosin  ferment.  The  extract  obtained 
contained  an  albumose  which  was  either  the  ferment  or  held  it  in  close  com- 
bination. The  pure  ferment  has  not  been  isolated.  In  the  case  of  coagula- 
tion of  the  blood,  a  proteid  of  the  plasma,  fibrinogen,  is  changed  by  coagula- 
tion to  fibrin,  this  change  being  brought  about  by  the  action  of  the  fibrin 
ferment,  for  the  formation  of  which  calcium  is  necessary.  The  calcium 
does  not  enter  into  the  chemical  change  independently,  and  it  can  go  on  in 
the  absence  of  calcium  provided  the  ferment  has  been  already  formed.  In 
the  case  of  coagulation  of  muscle,  two  proteids  of  the  muscle  plasma, 
paramyosinogen  and  myosinogen,  are  changed  by  coagulation  to  myosin,  or, 

1  Untersuchungen  ijber  das  Protoplaxma,  Leipzig,  1864. 
1  Journal  of  Physiology,  1887,  vol.  viii.  pp.  133-202. 


164  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

as  Fiirth  says,  to  myogen-fibrin  and  myosin-fibrin l  by  the  action  of  myosin 
ferment.  The  change  can  go  on  in  the  absence  of  calcium,  but  whether  this 
is  essential  to  the  formation  of  the  ferment  is  not  yet  known.  The  myosin 
ferment  is  not  the  same  as  fibrin  ferment,  since  neither  can  do  the  work  of 
the  other.  Moreover,  fibrin  ferment  is  destroyed  at  75°-80°  C.  and  myosin 
ferment  is  not  destroyed  till  100°  C. 

The  chemical  change  which  results  in  the  formation  of  myosin  is  diiferent 
from  that  which  produces  fibrin.  The  clotting  of  muscle-plasma  and  the 
formation  of  myosins  are  accompanied  or  closely  followed  by  the  production 
of  an  acid,  while  no  such  change  occurs  during  the  coagulation  of  blood-plasma. 
In  the  earlier  stages  of  clotting  the  acidity  may  be  due  in  part  to  acid  potas- 
sium phosphate,  but  the  final  acidity  is  chiefly  due  to  lactic  acid.  The  source 
of  the  lactic  acid  has  not  been  definitely  made  out.  The  view  that  it  comes 
from  glycogen  is  made  questionable  by  Bohm's 2  observation  that  the  amount 
of  glycogen  is  not  lessened  in  rigor ;  besides,  the  muscles  of  starving  animals 
become  acid  when  entering  into  rigor,  although,  as  Bernard  found,  they  con- 
tain no  glycogen.  Bohm  concluded  that  the  sarcolactic  acid  may  be  formed 
from  the  proteids.  Probably  both  glycogen  and  proteids  can  yield  lactic 
acid. 

Some  writers  have  thought  that  the  coagulation  of  the  muscle  was  due  to 
the  formation  of  an  acid  by  the  dying  muscle.  This  is  unlikely,  although  the 
presence  of  acid,  like  that  of  many  other  substances,  quinine,  caffein,  digitalin, 
veratrin,  hydrocyanic  acid,  ether,  chloroform,  etc.,  which  lead  to  altera- 
tions in  the  conditions  of  the  normal  muscle-substance,  may  hasten  the  proc- 
ess. Apparently,  anything  which  causes  a  deterioration  of  the  muscle-sub- 
stance, chemical  reagents,  drugs,  or  the  products  of  fatiguing  work,  hastens 
the  coming  on  of  rigor.  On  the  other  hand,  anything  which  helps  maintain 
the  normal  constitution  of  the  muscle  appears  to  postpone  the  change.  Thus 
Latimer3  reports  that  the  circulation  of  dextrose  through  fatigued  muscle 
largely  does  away  with  the  effect  of  fatigue  to  hasten  rigor  mortis.  Nor  is 
this  because  fatigue  products  are  washed  out  of  the  muscle,  for  the  circula- 
tion of  other  fluids  through  the  muscle,  whether  neutral,  acid,  or  alkaline, 
fails  to  have  the  effect. 

Rigor  Caloris. — If  a  muscle  be  heated  beyond  its  normal  temperature,  its 
irritability  is  increased,  and  it  undergoes  rapid  katabolic  changes  which  lead 
to  its  death.  These  changes,  if  sufficiently  rapid,  may  bring  about  a  con- 
traction of  the  muscle,  and  this  contraction,  involving  the  diiferent  fibres  of 
the  muscle  to  different  degrees,  may  be  continued  without  break  by  the  con- 
traction that  is  peculiar  to  rigor  mortis  ;  in  addition  to  this,  if  the  temperature 
is  raised  sufficiently,  there  will  be  a  heat  precipitation  of  the  various  proteids 
of  the  muscle,  which  will  lead  to  a  still  further  shortening,  the  contraction 

1  Furth:  Archivfur  experimentelle  Pathologic  und  Pharmakologie,  1895,  xxvi.  231 ;  and  1896, 
xxxvii.  389. 

2  Pfliiger's  Archiv,  1880,  Bd.  xxiii.  S.  44. 

3  Latimer :  American  Journal  of  Physiology,  1899,  ii.  p.  29. 


GENERAL    PHYSIOLOGY   OF  MUSCLE  AND  NERVE.     165 

of  rigor  caloris.  The  heated  muscle  may,  therefore,  be  the  seat  of  three 
dill 'crent  kinds  of  processes,  each  of  which  leads  to  a  shortening.  If  frog's 
muscle  be  gradually  heated,  it  shows  three  separate  contraction  movements 
at  three  separate  temperatures,  at  about  34°,  47°,  58°  C.  The  last  two  con- 
tractions are  due  to  heat  coagulation  of  paramyosinogen  (myosin  of  v. 
Furth)  at  47°  C.,  and  myosinogen  (myogen  of  v.  Furth)  at  58°  C.  These 
are  undoubted  effects  of  heat  rigor.  There  is  a  difference  of  opinion  as  to  the 
nature  of  the  first  contraction.  It  has  been  generally  attributed  to  the 
coming  on  of  rigor  mortis — i.  e.,  to  a  post-mortem  coagulation  of  para- 
myosinogen and  myosinogen.  In  case  a  muscle  be  rubbed  between  the 
fingers,  so  that  its  anatomical  condition  is  altered,  although  the  chemical 
structure  remain  the  same,  this  form  of  shortening  does  not  occur,  and  it  is 
not  until  the  temperatures  at  which  paramyosinogen  and  myosinogen  are 
coagulated  by  heat  are  reached  that  the  muscle  begins  to  shorten.1  Probably 
the  rigor-mortis  change  occurs,  but  on  account  of  the  physical  change  in  the 
fibre  it  does  not  reveal  itself.  This  suggests  the  well-known  fact,  that  when 
the  rigidity  of  a  muscle  in  rigor  has  been  broken  up  by  mechanical  means  it 
does  not  return.  The  condition  of  the  muscle  has  an  important  influence  on 
the  temperature  at  which  it  will  enter  into  rigor  when  heated.  Latimer2 
reports  that  a  fatigued  muscle  will  go  into  rigor  at  a  temperature  10  degrees 
lower  than  a  fresh  muscle  will.  Probably  both  fatigue  and  high  tempera- 
ture are  favorable  to  the  formation  of  the  myosin  ferment,  and  heat  hastens 
the  fermentation  process,  resulting  in  coagulation.  Another  view  of  the 
nature  of  the  first  form  of  contraction  has  been  advanced  lately.  According 
to  Brodie  and  Richardson,3  this  contraction  in  the  case  of  frog's  muscle  may 
be  very  considerable,  and  is  due  to  heat  coagulation  of  soluble  myogen  fibrin, 
a  form  of  proteid  which  v.  Furth  found  to  be  formed  from  myosinogen  (what 
he  termed  myogen)  at  30°  C.  Mammalian  muscles  do  not  show  any  marked 
contraction  at  this  temperature,  and  have  not  been  found  to  contain  myogen 
fibrin.  This  form  of  shortening  is  seen  only  by  light  loads,  for  the  coagula- 
tion of  the  proteids  of  the  muscle  causes  increased  extensibility,  in  addition 
to  the  tendency  to  contract.4  The  change  of  form  in  rigor  caloris  is  more 
in  voluntary  than  in  involuntary  muscles,  as  much  as  60  per  cent,  in  the 
former  and  10  per  cent,  in  the  latter.  The  beginning  of  heat-rigor  comes 
at  very  different  temperatures  in  the  different  muscles  of  different  animals. 
Mammalian  muscle  can  stand  several  degrees  higher  than  the  muscles  of 
cold-blooded  animals,  heart-muscle  can  be  heated  two  or  three  degrees  higher 
than  the  skeletal  muscles,  and  skeletal  muscles  differ,  e.  g.,  the  semimem- 
branosus  of  the  frog  enters  into  rigor  sooner  than  the  gastrocnemius.5  These 
facts  are  brought  out  in  experiments  in  which  the  temperature  of  the  muscle 

1  Stewart  and  Sollmann  :  1899,  xxiv.  p.  428. 

2  Latimer :  American  Journal  of  Physiology,  1899,  ii.  p.  29. 

3  Philosophical  Transactions  of  the  Royal  Society  of  London,  1899,  Series  B.,  vol.  cxci.  p.  353. 

4  Gotschlich  :  Journal  of  Physiology,  1897,  vol.  xxi.  p.  353. 

5  Ward,  H.  C. :  Unpublished  experiments  at  the  University  of  Michigan. 


166  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

is  more  or  less  rapidly  raised,  at  the  same  time  that  the  changes  in  the  length 
of  the  muscle  are  recorded  on  a  slowly  moving  surface.  Halliburton1  gives 
the  following  precipitation  temperatures  for  muscle  proteids. 

Name.  Temperature  of  coagulation. 

Proteids  obtained  from  I  Paramyosinogen 47°  C 

the  dissolved  clot .    .    I  Myosinogen 56°  C. 

Proteids  obtained  from  f  Myoglobulin 63°  C. 

muscle-serum    ...  1  Myo-albumin 73°  C. 

v  Myo-albumose (not  coagulated  by  heat). 

Constituents  of  Muscle-serum. — Proteids. — The  fluid  which  can  be 
expressed  from  the  coagulated  fresh  muscle  is  called  muscle-plasma.  This 
undergoes  a  change  in  the  process  of  coagulation,  two  of  the  globulins 
present,  the  para  -myosinogen  and  the  myosinogen,  being  precipitated  in  the 
form  of  myosin,  which  makes  the  substance  of  the  clot.  The  fluid  which 
can  be  expressed  from  the  clotted  muscle,  the  muscle-serum,  therefore  lacks 
at  least  two  of  the  proteid  constituents  of  the  normal  muscle.  The  proteids 
of  the  muscle-serum  are  :  myoglobulin,  myo-albumin,  and  myo-albumose. 

The  myoglobulin  resembles  serum-globulin,  although  precipitated  at  63° 
C.  instead  of  73°  C.  The  myo-albumin  is  apparently  identical  with  serum- 
albumin. 

To  these  proteids  we  must  add  the  pigment  haemoglobin.  Another  pig- 
ment, myohsematin,  is  also  found.  It  is  not  unlikely  that  these  pigments  have 
here  as  elsewhere  a  respiratory  function. 

Although  the  proteids  form  the  larger  part  of  the  solids  of  the  muscle- 
substance,  but  little  is  known  as  to  the  form  in  which  they  exist  in  the  living 
muscle  or  the  part  that  they  play  in  its  activity.  They  seem  to  have  a  two- 
fold function,  they  are  at  once  the  machine  and  the  fuel.2  Under  normal 
conditions  they  probably  supply  but  a  small  part  of  the  energy  set  free  by 
the  muscle  during  ordinary  work.  In  excessive  muscular  work  they  undergo 
katabolic  change,  as  is  shown  by  the  increased  excretion  of  nitrogen  and  sul- 
phur in  the  urine.  In  the  case  of  an  individual  not  in  training  it  would 
appear  that  during  excessive  muscular  exercise,  as  in  starvation,  other  parts 
of  the  body  may  give  up  their  proteids  to  the  muscles,  for  under  such  cir- 
cumstances uric  acid  and  phosphorus-holding  extractives,  the  waste  products 
of  nuclein,  appear  in  the  urine,  and  the  muscle  contains  but  little  nuclein.3 
This  is  much  less  the  case  if  the  individual  is  in  training,  from  which  it 
would  appear  that  through  training  muscles  acquire  the  capacity  of  storing 
more  proteid  or  of  utilizing  their  stock  to  better  advantage.  In  any  case  if 
a  large  amount  of  muscular  work  is  to  be  done  the  amount  of  proteid  in  the 
food  should  be  increased. 

Nitrogenous  Extractives. — The  chief  nitrogenous  extractive  is  creatin ;  in 

1  Halliburton  :   Physiological  Chemistry,  p.  414. 

2Pfliiger:  Pfliige/s  Archiv,  1899,  Bd.  Ixxvii.  S.  425. 

3  Dunlop,  Pat  on,  Stockman,  Maccadam  :  Journal  of  Physiology,  1897,  xxii.  p.  67. 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND   NERVE.     167 

addition  to  this  we  find  small  amounts  of  crcatinin  and  <>f  various  xanthin 
bodies,  as  xanthin,  hypoxanthin,  carnin,  carnic  acid,  and  sometimes  traces  of 
urea,  uric  acid,  taurin,  and  glycocoll.  The  chemical  nature  of  these  bodies 
need  not  be  considered  here.  Physiologically  they  may  be  regarded  as  waste 
products  which  result  from  the  partial  oxidation  of  the  proteids  of  muscle 
during  the  katabolic  processes  which  are  continually  occurring  even  in  the 
resting  muscle  protoplasm.  Monari  has  shown  that  the  amount  of  creatin 
and  creatinin  is  increased  by  the  wear  and  tear  of  muscular  work,  although 
the  proteids  of  the  well-fed  muscle  probably  supply  but  little  of  the  energy 
which  is  set  free. 

The  non-nitrogenous  constituents  of  muscle  are  fats,  glycogen,  inosit,  dex- 
trose, and  lactic  acid. 

Fats  are  usually  found  in  intermuscular  connective  tissue,  and  there  is 
some  within  the  normal  fibre.  It  is  doubtful  whether  fat  plays  a  direct  part 
in  the  ordinary  metabolic  processes  involved  in  the  action  of  muscles,  although 
it  is  probable  that  if  more  available  sources  of  energy  are  lacking  it  may,  like 
the  proteids,  be  altered  and  employed.  Bogdanow l  states  that  the  fat  which 
is  within  the  muscle-fibre  is  of  different  constitution  from  that  between  the 
fibres ;  the  extracts  contain  more  free  fatty  acid.  He  further  found  that  the 
fat  within  the  fibre  is  used  up  during  the  work  of  the  muscle,  and  is  con- 
tinually renewed  from  the  blood.  If  a  muscle  of  a  frog  be  removed  from 
the  circulation  and  tetanized,  it  stains  much  less  with  osmic  acid  than  one 
with  its  circulation  unimpaired ;  while  a  muscle  which  is  curarized,  and  so 
does  no  work,  if  it  has  a  good  blood-supply,  stains  much  darker  than  ordinary 
muscle.  Under  pathological  conditions  large  amounts  of  fat  may  be  found 
inside  the  sarcolemma  ;  in  phosphorus-poisoning  the  degenerated  muscle-pro- 
toplasm may  be  replaced  by  fat  in  the  form  of  fine  globules. 

Glycogen  is  found  jn  very  variable  amounts  in  different  muscles.  The  work 
of  many  observers  has  shown  that  it  is  here,  as  in  the  liver,  a  store  of  carbo- 
hydrate material,  and  is  employed  by  the  muscle,  either  directly  or  after  con- 
version into  some  other  body,  as  a  source  of  energy.  The  quantity,  which  is 
rarely  more  than  ^  per  cent.,  lessens  rapidly  during  starvation  and  muscle 
work. 

When  it  is  required,  it  is  changed  to  dextrose,  and  is  finally  oxidized  to 
OO2  and  H2O.  If  the  action  of  the  muscle  is  prevented  by  the  cutting  of 
the  nerve  or  of  the  tendon,  the  glycogen  is  found  to  accumulate. 

Sugar  is  found  in  muscles  in  small  quantities  only ;  nevertheless  it  probably 
plays  an  important  part,  for  Chauveau  and  Kaufmann,  by  studying  the  levator 
labii  superioris  of  the  horse,  found  that  the  muscles  take  sugar  from  the  blood, 
and  that  they  take  more  during  action  than  rest.  The  sugar  which  the  mus- 
cle takes  during  rest  is  for  the  most  part  stored  as  glycogen.2  Although  sugar 
is  considered  a  source  of  muscle-energy,  the  exact  way  in  which  it  is  employed 
is  doubtful. 

1  Archiv  fur  Anatomic  und  Physiologic,  1897,  S.  149. 

2  Comptes  rendus  de  la  Societe  de  Biologic,  1886,  civ. 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Ergographic  experiments  on  the  human  subject  have  proved  that  muscles 
which  have  been  fatigued  by  long-continued  voluntary  work  recover  much 
more  rapidly  if  sugar  be  eaten.  Curiously  enough,  Waller  and  Miss  Sowton 
observed  that  the  endurance  of  an  isolated  frog's  nerve  was  increased,  or  at 
least  its  capacity  to  develop  strong  currents  of  action  was  enhanced,  if  it  was 
put  for  a  time  in  a  0.6  per  cent,  solution  of  sodium  chloride  containing  dex- 
trose. 

Lactic  Acid. — This  is  formed  in  the  muscle  during  work  and  during 
coagulation.  It  has  the  form  of  para-lactic  acid  or  sarco-lactic  acid,  though 
it  is  doubtful  whether  it  exists  in  a  free  state.  It  is  a  decomposition  product 
of  the  carbohydrates  and  perhaps  of  proteid  or  some  complex  muscle-sub- 
stance of  which  proteid  forms  a  part.  It  is  only  partly  responsible  for 
acidity  of  the  muscle  which  has  been  worked.  The  acidity  may  well  be  in 
part  caused  by  acid  potassium  phosphate  produced  from  alkaline  phosphates 
as  a  result  of  the  formation  of  phosphoric  acid  from  lecithin,  nuclein,  etc.1 
Rohmann 2  attributes  the  acidity  of  worked  muscles  to  monopotassium  phos- 
phate, and  the  alkaline  reaction  of  the  resting  muscle  to  dipotassium  phos- 
phate and  sodium  bicarbonate. 

Inorganic  Constituents  of  Muscle. — Among  the  bases,  potassium  has  the 
greatest  prominence,  and  sodium  next;  magnesium,  calcium,  and  small  amounts 
of  iron  are  also  found.  Of  the  acids,  phosphoric  is  present  in  the  largest  quan- 
tities. 

The  quantity  of  a  given  substance  present  in  a  tissue  is  not  an  evidence 
of  its  value,  and  the  salts  in  the  muscles,  although  present  in  comparatively 
small  quantities,  are  absolutely  essential,  not  only  to  their  functional  activity, 
but  to  their  life.  According  to  Loeb,3  salts  are  not  only  present,  as  such,  in 
the  muscle,  but  the  ions  Na,  Ca,  K,  and  Mg,  are  in  combination  with  the 
proteids,  and  these  ion-proteid  compounds  are  essential  to  its  irritability  (see 
p.  58). 

Gases  of  Muscle. — No  free  oxygen  can  be  extracted,  but  carbon  dioxide 
may  be  obtained,  in  part  free  and  in  part  in  combination.  A  little  nitrogen 
can  also  be  extracted,  but  apparently  it  has  no  physiological  significance. 
The  amount  of  carbonic  acid  developed  varies  greatly  with  the  condition  of 
the  muscle ;  for  instance,  it  is  much  increased  by  muscle  work.  Muscles 
take  up  oxygen  from  the  blood  freely,  especially  when  active,  and  when 
removed  from  the  body  may  absorb  small  amounts  from  the  air.  Moreover, 
a  certain  amount  of  oxygen  comes  to  the  muscle  from  the  food.  More  oxy- 
gen is  taken  up  by  the  muscle  during  rest  than  is  liberated  as  carbon  dioxide, 
but  during  action  the  reverse  is  the  case.4  Oxygen  is  not  retained  as  free 

1Weyl  und  Seither:  Zeitschrift  fur  physiologische  Chemie,  vi.  S.  557. 
2 Rohmann:    Pflugeijs  Archiv,  1892,  1.  S.  84,  and  1893,  Iv.  589. 

3  American  Journal  of  Physiology,  1900,  vol.  iii.  p.  327. 

4  Ludwig  und  Sczelkow :  Sitzungsberichte  der  'k.  Akad.  Wien,  1862,  Bd.  xlv.  Abthl.  1  ;  and 
Ludwig  und  Schmidt :  Sitzungsberichte  der  math.-phys.  Classe  d.  k,  Sachs.  Gesellschaft  der  Wissen- 
schaft,  1868,  Bd.  xx. ;  Eegnault  and  Reiset :  Annales  de  Chimie  et  de  Physique,  1849,  3me  s6r., 
xxvi. ;  Pfliiger :  Pfliiger's  Archiv,  1872,  vi.  ;  and  others. 


GENERAL   PHYSIOLOGY   OF  MUSCLE  AND   NERVE.     169 

oxygen,  but  is  stored  in  some  combination  more  stable  than  oxyhsemoglnbin. 
It  is  by  virtue  of  the  combined  oxygen  that  the  muscle  is  enabled  to  do  its 
work,  but  the  process  is  not  one  of  simple  oxidation.  That  muscles  hold  oxygen 
in  available  combinations  was  shown  by  Hermann,  who  ascertained  that  a 
muscle  can  contract  hundreds  of  times  in  an  atmosphere  free  from  oxygen, 
and  produce  water  and  carbon  dioxide.  If  a  muscle  be  thus  fatigued,  it  will 
recover  somewhat  in  case  it  be  supplied  with  oxygen,  but  not  otherwise 
(Joteyko  et  Richet). 

Zuntz  l  found  that  the  amount  of  oxygen  absorbed  by  the  body  during 
muscular  work  gives  a  proportional  measure  of  the  energy  expended.  He 
gives  the  following  figures  for  bicycle-riding : 

Rate  per  hour.  Oxygen  absorption  per  meter. 
9  kilometers  4.5  cu.  cm. 

15          "  4.8      « 

21.5       "  5.76     " 

A  comparison  of  bicycling  and  walking  showed  that  by  moderate  speeds 
(riding  15  kilometers  and  walking  6  kilometers  per  hour)  about  double  the 
amount  of  oxygen  was  used  for  like  distances  in  walking,  but  about  22  per 
cent,  more  was  required  during  like  periods  of  time  in  riding. 

II.  CHEMISTRY  OF  NERVES. 

Most  of  our  ideas  concerning  the  chemistry  of  nerves  are  based  on  analysis 
of  the  white  and  gray  matter  of  the  central  nervous  system.  The  white  matter 
is  largely  made  up  of  nerve-fibres  and  supporting  tissue,  and  the  gray  matter 
of  the  bodies  of  nerve-cells.  The  peripheral  nerve-fibres  are  a  continuation 
of  the  structures  in  the  central  nervous  system,  or  are  composed  of  similar 
elements ;  the  active  part  of  the  fibre,  the  axis-cylinder,  is  an  outgrowth  of 
the  cytoplasm  of  the  body  of  a  nerve-cell,  and  the  surrounding  medullary 
sheath  is  a  continuation  of  the  material  which  sheathes  the  axis-cylinder  while 
in  the  brain  and  cord.  It  is  probable,  therefore,  that  the  chemistry  of  the 
axis-cylinder  approaches  to  that  of  the  cytoplasm  of  the  body  of  the  nerve- 
cell  of  which  it  is  a  branch,  and  that  the  chemistry  of  the  medullary  substance 
is  the  same  outside  as  inside  the  central  nervous  system. 

The  white  matter  of  the  brain  of  the  ox,  which  is  largely  made  up  of  nerye- 
fibres,  is  composed  of  about  70  parts  water  and  30  parts  solids,  about  one-half 
the  latter  being  cholesterin,  about  a  quarter  proteids  and  connective-tissue  sub- 
stance, and  about  a  quarter  complex  fatty  bodies,  neuro-keratin,  salts,  chiefly 
potassium  salts  and  phosphates,  and  traces  of  xanthin,  hypoxanthin,  etc. 

Analysis  of  human  sciatic  nerve  gives  the  following  percentage  for  the 
principal  organic  constituents  :  Proteids,  36.8 ;  lecithin,  32.57 ;  cholesterin 
and  fat,  12.22;  cerebrins,  11.30;  neurokeratin,  3.07;  other  substances,  4.O.2 
The  nerve-fibre  has  a  delicate  sheath,  the  neurilemma,  the  exact  constitution 

1  Zuntz :  Pfliiyer's  Archiv,  1897,  Bd.  Ixx.  S.  346. 

2  J.  Chevalier  :  Zeitschrift  fur  physiologische  Chemie,  x.  8.  97. 


170  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

of  which  is  unknown,  but  which  is  supposed  to  resemble  the  sarcolemma  and 
to  be  composed  of  a  substance  similar  to  elastin.  The  fibres  are  bound  together 
by  connective  tissue  which  on  boiling  gives  gelatin.  Within  the  neurilemma  is 
the  medullary  sheath,  which  is  composed  of  two  elements — viz.  (1)  neuro-kera- 
tin,  a  material  similar  to  the  horny  substance  of  epithelial  structures,  which 
forms  a  sort  of  loose  trellis,  or  network,  and  probably  acts  as  a  supporting 
framework  to  the  fibre ;  (2)  a  white,  highly  refracting,  semi-fluid  material, 
which  fills  the  meshes  of  the  neuro-keratin  network,  and  which  is  composed 
largely  of  protagon  and  cholesterin  combined  with  fatty  bodies.  Protagon  is 
a  complex  phosphorized  nitrogenous  compound,  which  many  observers  believe 
to  contain  lecithin  and  cerebrin.  According  to  Noll,  it  makes  up  7.47  per 
cent,  of  the  dried  nerve.  Both  lecithin  and  cerebrin  are  fatty  bodies  possess- 
ing nitrogen,  and  the  former  phosphorus.  These  -and  some  other  complex 
fatty  bodies  probably  exist  in  addition  to  protagon  in  the  medullary  sub- 
tance.  The  formation  of  the  "myelin  forms77  seen  in  the  medulla  of  dead 
nerves  is  attributed  to  lecithin.  The  axis-cylinder  probably  contains  most  of 
the  proteids  of  the  fibre,  chiefly  globulins,  mixed  with  complex  fatty  bodies. 

The  reaction  of  the  normal  living  fibre  is  neutral  or  slightly  alkaline.  It 
is  said  to  become  acid  after  death,  but  this  change  is  not  known  to  accompany 
functional  activity.  Indeed,  nothing  is  known  of  the  physiological  import  of 
the  chemical  constituents  of  the  nerve-fibre  or  of  the  chemical  changes  which 
occur  in  the  axis-cylinder  when  it  develops  or  transmits  the  nerve  impulse. 
The  peculiar  chemical  composition  of  the  medullary  substance  would  suggest 
that  it  has  a  more  important  function  than  simply  to  protect  the  axis-cylinder. 
Some  have  attributed  to  it  nutritive  powers,  and  others  have  supposed  it  helped 
to  insulate :  it  is  certain  that  the  axis-cylinder  can  develop  and  transmit  the 
nerve  impulse  without  the  aid  of  the  medullary  sheath,  for  there  is  a  large 
class  of  important  nerves — the  non-medullated  nerves — in  which  it  is  lacking. 


II.  CENTRAL  NERVOUS  SYSTEM. 


INTRODUCTION. 

The  Unity  of  the  Central  Nervous  System. — The  human  nervous 
system  is  formed  by  a  mass  of  separate  but  contiguous  nerve-cells.  Indeed, 
a  group  of  nerve-cells  disconnected  from  the  other  nerve-tissues  of  the  body, 
as  the  muscles  or  glands  are  disconnected  from  each  other,  would  be  without 
physiological  significance.  To  understand,  therefore,  the  physiology  of  the 
nervous  system,  it  is  important  to  keep  in  mind  the  fact  that  by  dissection  it 
is  found  to  be  continuous  throughout  its  entire  extent. 

When  the  nervous  system  is  described  as  formed  of  a  central  and  a 
peripheral  portion,  and  the  peripheral  portion  is  further  divided  into  its 
spinal  and  sympathetic  components,  the  parts  distinguished  are  found  to 
have  no  sharply  marked  boundaries  separating  them,  but  really  to  merge  one 
into  the  other. 

For  topographical  descriptions  the  convenience  of  such  subdivisions  is 
undoubted ;  but  the  physiological  processes  which  it  is  our  purpose  to  study 
overstep  in  so  large  a  measure  these  limits  that  the  picture  of  events  in  the 
central  nervous  system  would  be  very  incomplete,  should  they  be  separately 
traced  only  within  such  prescribed  anatomical  boundaries. 

By  virtue  of  its  continuity  the  nervous  system  puts  into  connection  all  the 
other  systems  of  the  body.  Conforming  as  it  does  in  shape  to  the  frame- 
work of  the  body,  its  branches  extend  to  all  parts.  These  branches  form 
pathways  over  which  nerve-impulses  travel  toward  the  central  system — the 
brain  and  spinal  cord — and,  in  consequence  of  the  impulses  that  come  in, 
there  pass  out  from  the  central  system  other  impulses  to  the  muscles  and 
glands.  * 

All  incoming  impulses  must  reach  the  central  system.  It  is  a  fact  of  the 
greatest  significance  that  until  they  have  entered  the  central  system  the 
incoming  impulses  do  not  give  rise  to  those  outgoing,  for  thus  all  incoming 
impulses  are  first  brought  to  the  spinal  cord  and  brain,  where  the  outgoing 
impulses  are  co-ordinated. 

By  means  of  the  central  system  reactions  are  established  in  parts  of  the 
body  not  directly  affected  by  the  variation  of  the  external  conditions.  Owing 
also  to  the  wide  connections  of  the  nervous  system  and  the  conduction  of  all 
incoming  impulses  to  its  central  parts,  a  measure  of  harmony  is  maintained 

171 


172  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

between  the  activities  of  the  several  systems  composing  the  body.  Thus,  not 
only  the  systems  forming  the  body  are  in  this  manner  controlled,  but  the 
body  as  a  whole,  in  relation  to  all  things  outside  of  it  and  forming  its 
environment,  is  even  more  plainly  under  the  guidance  of  these  administra- 
tive cells. 

Phenomena  Involving-  Consciousness. — It  is  with  the  brain  and  espe- 
cially the  cerebral  hemispheres  that  the  phenomena  of  consciousness  are  most 
closely  linked.  Strictly,  physiology  concerns  itself,  at  present,  with  the  reac- 
tions of  the  nervous  system  which  can  be  studied  without  an  appeal  to  con- 
sciousness. A  moment's  consideration  shows,  however,  that  in  the  physiology 
of  the  brafa  the  assistance  to  be  obtained  by  passing  beyond  the  limit  thus 
laid  down  is  of  more  value  than  any  boundary,  and  hence,  although  the  field 
of  consciousness  is  sacred  to  psychology,  physiology  should  not  be  deprived 
of  any  of  the  advantages  which  come  from  the  privilege  of  occasional 
trespass. 

Growth  and  Organization. — The  physiological  connections  existing  be- 
tween the  nerve-elements  in  infancy  are  very  incomplete  and  poorly  estab- 
lished, more  so  than  in  any  other  system  of  the  body  ;  in  the  history  of 'the 
growth  of  the  nervous  system,  the  increase  in  weight  and  change  in  shape 
run  parallel  with  an  increase  in  its  complexity.  This  increase  in  complexity 
is  accompanied  by  more  perfect  organization,  which  results  in  better  and 
more  numerous  physiological  pathways,  thus  permitting  the  system,  as  a 
whole,  not  only  to  do  more  perfectly  at  maturity  those  things  which  it  could 
do  in  some  degree  at  an  earlier  age,  but  also,  by  virtue  of  its  increased  com- 
plexity, to  do  at  maturity  those  things  which  previously  it  could  not  do  at  all. 

Growth /in  the  case  of  this  system  implies,  therefore,  an  increase  in  com- 
plexity such  as  nowhere  else  occurs,  and,  since  this  growth  can  be  modified 
by  the  experience  of  the  individual  during  the  growing  period,  the  impor- 
tance of  jinderstanding  it  and  its  relation  to  organization  is  evident. 

Plan  of  Presentation. — Our  subject  may  be  discussed  under  three  heads 
dealing  respectively  with  the  physiology  of  the  (1)  single  cells,  (2)  groups  of 
cells,  and  (3)  the  entire  central  system. 

Part  I.  The  physiology  of  the  nerve-cells,  considered  as  a  peculiar  kind 
of  tissue-element,  endowed  with  special  physiological  characters. 

Part  II.  The  activities  of  groups  of  these  elements.  The  physiological 
grouping  is,  of  course,  mainly  dependent  on  the  anatomical  arrangement.  At 
the  same  time,  the  activities  of  one  group  modify  those  of  others.  Stated  in 
general  terms,  the  problem  in  this  part  is  that  of  the  pathway  of  any  impulse 
through  the  central  system. 

Part  III.  The  reactions  of  the  system  taken  as  a  whole.  Here  its  capa- 
bilities as  a  unit  are  contrasted  with  those  of  the  other  systems,  and  its 
growth,  organization,  and  rhythms  of  rest  and  activity  are  most  readily  de- 
scribed. 


CENTRAL    NERVOUS  SYSTEM. 


173 


PART  I.— PHYSIOLOGY  OF  THE  NERVE;CELL. 

A.  ANATOMICAL  CHARACTERISTICS  OP  THE  NERVE-CELL. 

Form  of  Nerve-cells. — Morphologically  the  mature  nerve-cell  is  regarded 
as  composed  of  a  cell-body,  containing  a  nucleus,  together  with  other  modified 
inclusions,  and  possessed  of  one  or  more  outgrowths  or  branches.  Some  of 
these  branches  may  be  very  long,  such,  for  instance,  as  those  which  form 
nerve-fibres ;  other  branches  are  sjiort  and  differ  from  the  nerve-fibres  in 
their  structure. 

The  terms  employed  in  describing  the  nerve-elements  are  as  follows  :  To 
the  entire  mass  under  the  control  of  a  given  nucleus  and  forming  both  cell- 
body  and  branches,  the  term  neurone  is  applied.  The  inclusions  within  the 


FIG.  67.— A  group  of  human  nerve-cells,  all  drawn  to  the  same  scale,  from  preparations  according  to 
Nissl's  method,  made  by  Dr.  Adolph  Meyer,  and  kindly  furnished  for  this  purpose  ;  X  300:  a,  small  motor 
cell  from  ventral  horn  of  cervical  spinal  cord  ;  b,  cell  from  "  Clarke's  column,"  thoracic  cord ;  r,  small 
nerve-cell  from  tip  of  dorsal  horn,  thoracic  cord  ;  d,  sr?l  nal  ganglion-cell .  cervical  root;  e,  three  granules 
from  the  granular  layer  of  the  cerebellum;  /,  Purkinje's  cell  from  the  same  preparation  as  e;  g,  small 
pyramidal  cell  from  the  second  layer  of  the  cerebral  cortex  of  the  central  gyri ;  h,  giant  pyramidal  cell 
from  the  same  region.  * 

% 

cell-body  have  the  usual  designations.  Nerve-cells  differ  greatly  in  the 
number  of  the  branches  arising  from  them.  In  some  cells  there  appear  to  be 
two  nerve-fibres  arising  from  the  cell-body,  in  others,  only  one.  For  conve- 
nience the  descriptions  about  to  be  given  will'  apply  to  the  latter  group  only. 
From  most' cells  there  arises  one  principal  branch,  which  when  considered 
alone  is  described  as  a  nerve-fibre,  but  when  considered  as  the  outgrowth  of 
the  cell-body  from  which  it  originates  is  called  the  axone.  The  axotie,  in 
many  cases,  has  branches,  both  near  its  origin  from  the  cell-body  and  also 
along  its  course.  These  branches  are  designated  as  collaterals.  -  At  their 
distal  ends  the  main  stem  of  the  axone  and  also  the  collaterals  subdivide 


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AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


into  finer  twigs,  forming  the  terminal  arborizations.  Contrasted  with  this 
principal  outgrowth  are  the  other  branches  of  the  cell,  which  are  individu- 
ally much  less  extensive  and  which  divide  dichotomously  at  frequent  in- 
tervals. From  the  tree-like  form  which  they  thus  acquire  they  have  been 
designated  dendrites. 

The  accompanying  illustration  (Fig.  67)  shows  the  cell-bodies  of  several 
neurones,  together  with  the  beginning  branches,  and  also  gives  some  idea  of 
the  variations  in  the  size  of  the  cell-bodies  as  found  in  man.  The  nerve-cell 
body  is  at  first  ovoid  in  shape,  although  this  type  is  much  modified  in  many 
cases.  As  will  be  seen  from  Fig.  67,  the  diameters  of  nerve-cells  range 
from  4  fj.  to  65  n*  though  in  some  instances  in  the  spinal  cord  cell-bodies  of 
even  larger  diameter  are  found. 

Peculiarities  of  Nerve-cells.— As  compared  with  the  other  cells  of  the 
body,  the  best  developed  nerve-cells  are  of  large  size,  but  the  nucleus,  pro- 
portionately to  the  remainder  of  the 
neurone,  is  not  large.  Moreover,  its  pro- 
portional value  decreases  with  the  increase 
in  the  size  of  the  entire  cell.  The  most 
striking  feature  of  the  nerve-cell,  how- 
ever, is  the  great  length  to  which  its  chief 
branch,  the  axone,  may  attain,  for  in  no 
other  tissue  does  anything  like  so  great  a 
proportion  of  the  cell-substance  occur  as 
a  branch.  Since  the  axone  is  the  direct 
outgrowth  of  the  cell-body  and  can  have 
attained  its  length  only  gradually,  it  is 
not  surprising  to  find  that  all  varieties  of 
length  occur. 

The  form  of  cell  represented  in  Fig. 
68  is  one  in  which  the  axone  shows  a 
very  short  stem  between  the  cell-body  and 
its  terminal  twigs.  In  such  an  instance 
the  entire  extension  of  the  axone  may  be 
less  than  a  millimeter.  With  this  are  to 
be  contrasted  those  forms  in  which  the 
axone  is  very  long  and  its  mass  great. 
What  its  greatest  length  may  be  is  easily  determined.  Within  the  central 
system  there  are  cells  whose  axones  extend  from  the  cerebral  cortex  to  the 
lumbar  enlargement  (60  centimeters),  and  again  in  the  peripheral  system 
there  are  cell-bodies  in  the  lumbar  enlargement  of  the  spinal  cord,  the  axones 
of  which  extend  to  the  skin  and  muscles  of  the  foot,  a  distance  of  100  centi- 
meters or  more.  Among  the  neurones  found  in  the  spinal  ganglia  of  the 
lumbar  region,  some  cells  send  one  axonic  branch  to  the  level  of  the  nuclei 
of  the  dorsal  funiculi  in  the  bulb  and  the  other  branch  to  the  skin  of  the 

1  i  =  0.001  of  a  millimeter. 


Fro.  68.— A  cell,  with  a  short  axone, 
giving  off  many  branches.  In  such  a  cell 
the  axone  is  less  in  volume  than  the  cell- 
body.  This  is  the  extreme  form  of  the 
"central  cell"  of  Golgi  (Ram6n  y  Cajal). 
D.,  dendrites  ;  N.,  axone. 


CENTRAL   NERVOUS  SYSTEM.  175 

toes,  thus  spanning  nearly  the  entire  length  of  the  human  body.  These  are 
the  extreme  cases,  but  as  the  axoues  are  distributed  to  all  intermediate  points 
both  in  the  eentral  and  peripheral  system  every  intermediate  length  is  to  be 
found. 

Volume  Relations. — Calculation  shows  that  the  volume  of  the  axone  of 
a  large  motor  cell  of  the  spinal  cord  when  it  extends  from  the  lumbar 
enlargement  to  the  foot,  may  be  1500  times  that  of  the  cell-body.  This 
would  include  in  the  term  axone  both  the  axis-cylinder  and  the  medullary 
sheath.  If  either  of  these  is  taken  alone,  the  volume  is  reduced  by  one-half. 

It  is  extremely  difficult  to  estimate  the  volume  of  the  dendrites.  In 
some  instances,  as  in  the  cells  of  the  spinal  ganglia  (Fig.  71),  they  are 
rarely  present,  while  in  the  large  cells  of  the  cerebellum  (Purkinje's  cells) 
they  form  a  mass  which  must  be  several  times  greater  than  that  of  the  cell- 
body  proper.  In  most  neurones,  however,  the  dendrites  have,  at  best,  a 
volume  as  great  as  that  of  the  cell-body. 

Size  of  Nerve-cells  in  Different  Animals. — In  considering  the  size  and 
form  of  cells  in  man  it  becomes  of  interest  to  determine  how  far  the  obser- 
vations apply  to  the  lower  mammals.  The  facts  are  briefly  these  :  It  can 
be  said  that  the  smaller  mammals  usually  have  the  smaller  nerve-cells,  but 
the  decrease  in  the  volume  of  the  nerve-cells  is  not  proportional  to  the 
decrease  in  the  volume  of  the  entire  body.  For  example,  Kaiser l  has  shown 
that  the  cell-bodies  occupying  the  ventral  horn  in  the  cervical  enlargement 
of  the  spinal  cord  of  the  bat,  the  rabbit,  and  the  monkey  are  in  many  cases 
as  large  or  larger  than  those  found  in  man. 

Though  the  volume  of  the  cell-body  and  the  diameter  of  the  associated 
axone  are  approximately  similar  in  any  two  animals  of  different  size,  as,  for 
instance,  in  a  bat  and  in  man,  it  is  evident  that  the  axones  could  not  have 
the  length  in  the  bat  that  they  do  in  man,  and  that  in  this  last  dimension  at 
least  there  is  a  diminution  corresponding  to  the  size  of  the  animal. 

The  bearing  of  this  fact  on  the  comparative  physiology-  of  the  nervous 
system  is  evident,  for,  under  these  conditions,  as  the  volume  of  the  entire 
nervous  system  is  diminished,  the  number  of  cell-elements  constituting  it 
must  also  be  diminished,  and  thus  the  structure  of  this  system  in  the  smaller 
mammals  becomes  numerically  simplified. 

Size  ancUFunction. — A  nerve-cell  contains  a  mass  of  living  substances 
capable  of  being  broken  down  and  built  up  chemically,  and  there  is  nothing 
against  the  inference  that  the  larger  the  cell  the  greater  is  the  quantity  of 
these  living  substances,  and  hence  the  larger  the  amount  of  stored  energy 
represented  by  it.  The  larger  cells  are  therefore  those  capable  rfH^^ing 
free  the  greater  amount  of  energy.  The  energy-producing  change!^  |  be 
associated  with  the  cell-body  rather  than  with  any  of  the  branches^^Kore- 
over,  the  nerve-cells  with  large  cell-bodies,  sending  out,  as  they  do,  branches 
which  are  more  voluminous  than  those  the  cell-bodies  of  which  are  small, 
furnish  a  greater  amount  of  material  to  form  the  terminal  twigs  into  which 

1  Die  Funktionen  der  Ganglienzellen  des  Halsmarkes,  Haag,  1891. 


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AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


these  branches  finally  split.  From  this  it  follows  that,  in  general,  the  large 
nerve-cells  have  more  points  of  connection  with  the  structures  about  them,  as 
well  as  the  capacity  for  the  liberation  of  a  greater  amount  of  energy. 

Growth  of  Nerve-cells. — The  nerve-elements  are  derived  from  germinal 
cells  found  in  the  epiblast  of  the  embryo  (Fig.  69). 

These  divide  rapidly  and  in  such  a  way  that  one  daughter  cell  continues 
as  the  germinal  cell,  while  the  other  moves  away  from  the  primitive  surface 
of  the  body  and  becomes,  without  further  division,  a  young  neurone,  or 
neuroblast.  The  formation  of  neuroblasts  in  man  ceases,  or  becomes  very 
slow  and  unimportant,  by  the  end  of  the  third  month  of  fetal  life. 

Two  characters  of  the  neuroblast  are  worthy 
of  careful  consideration.  First,  there  is  good 
indirect  evidence  that,  in  early  life  at  least, 
and  before  their  branches  have  been  formed, 
they  are  migratory,  moving  in  an  amoeboid 
manner.  This  being  so,  the  perfection  with 
which  they  arrange  themselves  in  the  adult 
system  depends  on  the  accuracy  with  which 
they  respond  to  those  conditions  that  deter- 
mine their  migration  as  well  as  upon  the 
normal  character  of  these  directing  influ- 
ences (mechanical  strain  ;  *  chemotaxis  ;  nu- 
tritive attraction  or  electrical  influences).2 
But  with  so  much  liberty  of  movement  and 
with  directing  influences  that  are  so  compli- 
cated, the  chances  for  deviation  from  a  fixed 
arrangement  are  much  enhanced. 

Second,  very  early  in  the  history  of  the 
neuroblast  the  point  on  the  cell-body  from 
which  the  axone  will  grow  appears  in  many 
cases  to  be  determined,  and  the  cell  is  thus 
physiologically  polarized.3  This  polarity  being 
established,  the  direction  in  which  the  axone  first  grows  is  determined,  and 
where  the  cells  are  misplaced  this  polarization  can  lead  to  a  confusion  of 
arrangement. 

The  volume  of  either  the  germinal  cell  or  of  the  first  form  of  the  neuro- 
blast was  found  by  His4  to  be  697  cubic  //  in  a  human  fetus  (embryo  R- 
length  5.5  millimeters),  aged  3-3.5  weeks  ;  and  it  can  be  shown  that  the  mature 
neurone  must  often  attain  a  volume  more  than  50,000  times  that  of  the  orig- 
inal neuroblast. 

1  His  :    Unsere  Korperform,  1874. 

2  Davenport:  Bulletin  of  the  Museum  of  Comparative  Zoology,  Harvard  College,  Nov.,  1895  ; 
Herbst:  Biologisches  Centralblatt,  1 894,  Bd.,  xiv.;  H.  Strasser:    Ergebnisse  der  Anatomic  u.  Ent- 
wickelungsgeschichte,  Merkel  and  Bonnet,  1891,  Bd.  i.  S.  731. 

3  Mall :  Journal  of  Morphology,  1893,  vol.  viii. 
"*  Archiv  fiir  Anatomic  und  Physiologic,  1889. 


FIG.  69.— Portion  of  developing  medul- 
lary tube  (human)  seen  in  frontal  section. 
X 1100  diameters  (His) :  G,  germinal  cell ; 
N,  neuroblasts. 


CENTRAL    NERVOUS  SYSTEM. 


177 


Maturing  of  the  Nerve-cell. — The  maturing  of  the  nerve-cell  involves 
several  changes.  First,  the  outgrowth  of  the  axone  or  axones ;  next,  the 
formation  of  the  dendrites ;  and,  finally,  in  some  cases,  the  medullation  of 
the  axone,  while  simultaneously  and  with  greater  or  less  rapidity  the  absolute 
amount  of  substance  in  both  cell-body  and  axone  is  being  increased,  together 
with  a  chemical  differentiation  of  the  cytoplasm  and  the  nucleus.  The  time 
in  the  life-history  of  the  individual  at  which  these  several  events  occur  is 
variable,  and  may  be  delayed  beyond  puberty  at  least,  while  the  rate  at  which 
they  occur  is  different  in  different  cases.  Furthermore,  many  nerve-cells 
never  develop  beyond  the  first  stage  of  immaturity  (Fig.  70). 


showing  the  phylogenetic  development  of  mature  nerve-cells  in  a  series  of  vertebrates ; 
a-e,  the  ontogenetic  development  of  growing  cells  in  a  typical  mammal ;  in  both  cases  only  pyramidal 
cells  from  the  cerebrum  are  shown ;  A,  frog ;  B,  lizard  ;  C,  rat ;  D,  man ;  a,  neuroblast  without  dendrites  ; 
6,  commencing  dendrites  ;  c,  dendrites  further  developed  ;  d,  first  appearance  of  collateral  branches  ;  c, 
further  development  of  collaterals  and  dendrites  (from  S.  Ram6n  y  Cajal). 

Form  of  the  Axone  as  a  Means  of  Classification. — Of  the  various  de- 
vices used  tot classify  nerve-cells,  the  form  of  the  axone  is  the  most  useful. 

Physiologically,  the  nerve-cell  is  significant  as  the  source  and  pathway  for 
the  nerve-impulses.  The  current  conception  of  the  change  called  the  nerve- 
impulse  is  that  it  begins  at  one  point  of  the  cell  and  travels  from  there  to  the 
other  parts  ;  one  of  the  other  parts  is  the  axone,  and  along  this  the  impulse 
can  be  shown  to  pass.  Indeed,  the  nerve-cell  body  stimulated  at  any  point 
may  be  responsive  just  an  an  amoeba  is  responsive  at  any  portion  of  its  sur- 
face. When,  however,  the  branches  are  formed  they  become  the  channels 
through  which  the  impulses  pass,  and  hence  assume  a  special  significance 

VOL.  II.— 12 


178  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

without  indicating  any  fundamental  change  in  the  structure  of  the  cell. 
Where  the  cell  has  well-developed  branches  we  explain  the  arrangement  by 
assuming  that  the  impulse  enters  the  cell-body  by  one  branch  and  leaves  it 
by  another. 

On  examining  the  mature  nerve-cells  of  man  with  this  idea  in  mind,  two 
types  are  found.  The  first  type  may  be  exemplified  by  the  pyramidal  corti- 
cal cells  shown  in  Fig.  70.  Here,  from  a  pyramidal  body  (D)  there  arise 
a  number  of  dendrites,  while  from  the  lower  portion  of  the  cell  the  axone 


FIG.  71. — Spinal  ganglion  of  an  embryo  duck;  composed  of  diaxonic  nerve-cells  (van  Gehuchten). 

gro  vvs  out  and  branches.  In  the  other  type  the  axone  alone  grows  out.  Its 
branches  are  but  two  in  number  and  both  are  medullated.  They  pass  in  oppo- 
site directions,  and  in  this  type  there  are,  as  a  rule,  no  dendrites.  Such 
are  the  typical  spinal  ganglion-cells  of  the  mammal.  To  understand  the 
arrangement  in  these  cases,  recourse  must  be  had  to  the  facts  of  develop- 
ment. The  second  type  begins  its  development  as  a  diaxonic  cell,  an  axone 
growing  from  each  pole  (Fig.  71).  In  the  adult  spinal  ganglion  of  the 
higher  mammals,  however,  such  diaxonic  cells  are  rarely  found,  the  great 
majority  having  a  single  axone  which  soon  divides  into  two  branches.1 


FIG.  72.— Diaxonic  changing  into  monaxonic  cells :  from  the  Gasserian  ganglion  of  a  developing 

guinea-pig  (van  Gehuchten). 

Fig.  72  beautifully  illustrates  the  phases  of  this  change  as  seen  in  a  sin- 
gle section.  At  first  one  axone  arises  from  each  pole  of  the  ovoid  cell-body. 
Later  the  cell-body  occupies  a  position  at  the  side  of  the  two  axones,  which 
appear  to  run  into  one  other.  •  Finally  the  cell-body  is  separated  from  the 
two  axones  by  an  intervening  stem.  The  stem  has  the  characters  of  a  medul- 
lated nerve-fibre,  and  from  the  end  of  it  the  two  original  axones  pass  off  as 
branches. 

1  Dogiel:  Anat.  Anz.  Jena,  1896,  Bd.  xii.  S.  140-152,  describes  the  several  kinds  of  neu- 
rones which  take  part  in  the  formation  of  the  spinal  ganglion. 


CENTRAL   NERVOUS  SYSTEM.  179 

From  this  mode  of  development  it  is  plain  that  the  single  stem  must  be 
looked  upon  as  containing  a  double  pathway,  although  it  appears  to  be  in  all 
ways  a  single  fibre,  for  on  the  one  hand  it  contains  the  path  for  the  incoming 
and  on  the  other  for  the  outgoing  impulses.  Recent  investigations  have  shown 
in  a  striking  way  that  cells  modified  in  this  manner  are  by  no  means  limited 
to  the  spinal  ganglia,  but  occur  in  the  cortex  of  the  cerebellum  and  elsewhere. 
Classifying  the  nerve-cells,  therefore,  in  the  light  of  these  facts,  we  find  : 
(1)  The  pyramidal  type,  in  which  the  dendrites  and  axone  are  both  well 
developed,  and  in  which  the  greater  number  of  the  impulses  most  probably 
enter  the  cell  by  way  of  the  dendrites  and  leave  by  way  of  the  axone  ;  (2)  The 
spinal  ganglion  type,  in  which  originally  the  impulse  passes  in  at  one  pole  of 
the  cell  and  out  at  the  other,  but  in  the  course  of  development  the  two  axones 
become  attached  to  the  cell-body  by  a  single  stem,  and  by  inference  there 
must  be  in  this  stem  a  double  pathway.  In  this  latter  case  there  are  usually 
no  dendrites. 

Growth  of  Branches. — After  the  cells  have  taken  on  their  type-form  the 
branches  still  continue  to  grow,  not  only  in  length,  but  also  in  diameter.  In 
man,  for  example,  the  diameter  of  the  nerve-fibres  (axones)  taken  from  the 
peripheral  nerves  at  birth  is  1.2-2  p.  for  the  smallest,  up  to  7-8  p.  for  the 
largest,  with  an  average  of  3-4  //,  while  at  maturity  it  is  10-15  //  for  the 
larger  fibres.1 

Internal  Structure  of  the  Neurones. — The  status  of  this  problem  has 
been  admirably  summarized  by  Barker,2  to  whose  book  the  reader  is  referred. 
For  our  purpose  it  is  sufficient  to  state  that  the  cytoplasm  of  nerve-cells  is 
composed  of  fibrils  (the  character  of  which  is  much  discussed),  and  an  inter- 
mediate, non-fibrillar  material.  These  constituents  are  distributed  in  different 
proportions  in  the  several  parts  of  the  neurone.  The  axone  contains  the 
fibrils  most  closely  packed.  The  intermediate  substance  is  most  evident  in 
the  body  of  the  cell,  and  in  general  the  dendrites  more  closely  resemble  in 
their  structure  the  cell  body.  Part,  at  least,  of  the  intermediate  material 
forms  the  "stainable  substance"  of  Nissl,  also  called  "tigroid,"  which,  in  its 
susceptibility  to  change  under  disturbed  nutritive  conditions,  acts  like  a  stored 
food  material.  But  which  portion  of  the  cell  acts  to  conduct  the  nerve  im- 
pulse is  not  known,  and  the  contention  that  one  or  the  other  of  the  compo- 
nent structures^ is  the  conductor  of  the  nerve  impulses  rests  on  histological 
evidence  alone.  For  the  present  it  is  sufficient  to  know  that  the  neurone 
appears  to  be  conductive  in  all  its  gross  parts. 

While  the  axone  is  growing  as  a  naked  axis-cylinder,  it  is  usually  slightly 
enlarged  at  the  tip  (Cajal),  suggesting  that  it  is  specially  modified  at  that 
point.  The  nutritive  exchange  on  which  the  increase  of  the  entire  axone 
depends  appears  to  take  place  along  its  whole  extent,  and  not  to  be  entirely 
dependent  on  material  passed  from  the  cell-body  into  the  axone. 

Medullation. — After  the  production  of  its  several  branches,  the  next  step 

1  Westphal:  Neurologisckes  Centralblatt,  1894,  No.  2. 
J  Barker:  The  Nenous  System,  1899,  pp.  101-114. 


180  AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 

in  the  growth  of  the  cell  is  the  formation  of  the  medullary  sheath  about  the 
axone.  Not  all  axones  have  a  medullary  sheath,  nor  is  any  axone  com- 
pletely medullated.  In  the  sympathetic  system  there  is  a  very  large  pro- 
portion of  unmedullated  axones.  In  the  central  system  the  number  is 
also  very  large,  although  their  mass  is  small.  Of  the  significance  of  the 
medullary  sheath  we  know  nothing.  The  suggestion  that  it  acts  to  insulate 
the  nerve  impulse  within  a  given  axis-cylinder  has  little  or  no  evidence 
in  its  favor.  The  suggestion  that  it  is  nutritive  is  plausible,  but  important 
differences  in  the  physiological  reactions  of  the  two  classes  of  nerve-fibres 
have  not  yet  been  found,  if  we  except  the  observation  that  the  non -medullated 
nerves  rapidly  lose  their  irritability  at  the  point  of  stimulation  with  the 
faradic  current,  thus  exhibiting  a  "stimulation  fatigue"  not  found  in  nerves 
unquestionably  medullated. 

Growth  of  Medullary  Sheath  in  Peripheral  Nerves. — Whatever  may 
be  the  significance  of  the  medullary  sheath,  it  is  usually  formed  before  the 
neurone  has  attained  its  full  size.  In  the  peripheral  system  it  depends  on 
the  presence  of  mesodermal  cells  which  envelop  the  axis-cylinder,  forming  a 
tube  about  it.  Each  ensheathing  cell  is  physiologically  controlled  by  a  nu- 


FIG.  73. — Longitudinal  (I?)  and  transverse  (A)  sections  of  nerve-fibres.  The  heavy  border  represents 
the  medullary  sheath,  which  is  thicker  in  the  larger  fibres.  Human  sciatic  nerve.  X200  diameters 
(modified  from  van  Gehuchten). 

cleus  which  becomes  situated  about  midway  between  its  extremities.  Accord- 
ing to  Ranvier  and  his  school,  the  cell-substance  is  largely  transformed  into 
myelin,  and  the  line  of  junction  between  two  of  these  sheathing  cells  forms  a 
node  of  the  nerve-fibre.  In  the  sheath  of  a  growing  axone  at  least  two  changes 
can  be  readily  followed  :  As  the  axis-cylinder  increases  in  diameter  the  medul- 
lary sheath  becomes  thicker.  The  change  is  such  that  in  the  mammalian 
peripheral  system  the  areas  of  the  axis-cylinder  and  of  the  medullary  sheath, 
as  shown  in  cross-sections  of  osmic  acid  preparations,  remain  nearly  equal 
(Fig.  73).  On  the  other  hand,  the  length  of  the  internodal  segments  tends  to 
increase  with  an  increase  in  the  diameter  of  the  nerve-fibre,  and  for  nerves  of 
the  same  diameter  it  is  less  in  man  than  in  the  lower  animals.  In  a  given 
fibre  the  segments  are  shorter  at  the  extreme  peripheral  end  (Key  and  Retz- 
ius).  In  the  young  fibres,  also,  they  are  shorter  and  increase  in  length 
with  age. 

A  physiological  significance  attaches  to  these  segments,  because,  as  Ran- 
vier long  since  pointed  out,  it  is  at  the  nodes  that  various  staining  reagents 
easily  reach  the  axis-cylinder.  This  suggests  that  normal  nutritive  exchanges 
may  follow  the  same  path,  and  thus  short  internodal  segments  giving  rise  to 


CENTRAL    NERVOUS  SYSTEM.  181 

many  nodes  would  represent  the  condition  most  favorable  to  exchange  be- 
tween the  axis-cylinder  and  the  surrounding  plasma.  Thus  far,  histological 
observation  shows  the  more  numerous  nodes  where  the  physiological  processes 
are  presumptively  most  active,  and  hence  supports  the  hypothesis  suggested. 

In  the  peripheral  nervous  system  the  nerve-fibres  conduct  impulses  before 
they  acquire  their  medullary  sheaths :  witness  the  activities  of  new-born 
rats,  in  which  the  whole  nervous  system  is  entirely  unmedullated.  Moreover, 
Langley l  has  reported,  in  the  regenerating  cervical  sympathetic  nerve,  a 
return  of  function,  while  the  majority  of  the  fibres  are  still  without  their 
medullary  sheaths. 

Medullation  in  Central  System. — Concerning  the  relation  of  the  medul- 
lary sheath  to  the  axis-cylinder  in  the  central  system,  our  information  is  less 
complete.  The  elements  which  give  rise  to  the  medullary  substance  are  not 
known  and  the  myelin  is  not  enclosed  in  a  primitive  sheath.  There  are  no 
internodal  nuclei  regularly  placed,  yet  Porter 2  has  demonstrated  in  both  the 
frog  and  the  rabbit  the  existence  of  nodes  in  some  fibres  taken  from  the  spinal 
cord.  The  conditions  which  there  exist  must  be  further  studied  before  any 
general  statements  concerning  the  development  of  the  medullary  substance  in 
the  nerve-centres  can  be  ventured.  Yet,  it  is  an  important  observation  that 
whereas  medullation  in  the  peripheral  system  is  mainly  completed  during  the 
first  five  years  of  life,  the  process  continues  in  the  central  system,  and  espe- 
cially in  the  cerebral  cortex,  to  beyond  the  fiftieth  year.3 

Whatever  views  may  be  held  concerning  the  capacities  of  a  medullated 
fibre,  it  is  to  be  remembered  that  the  medullary  sheath  does  not  cover  the 
first  part  of  the  axone  on  its  emergence  from  the  cell-body,  nor  are  ultimate 
branches  of  the  axone  medullated  in  the  region  of  their  final  distribution. 

What  has  just  been  said  applies  to  the  main  stem  of  the  axone.  As  shown 
in  Fig.  70,  the  axone  often  has  branches  near  its  origin,  the  collaterals,  and 
according  to  the  observations  of  Flechsig4  these  also  become  medullated. 
Concerning  the  time  of  the  medullation  of  these  branches  there  are  no  direct 
observations ;  but  if  it  is  controlled  by  the  same  conditions  which  appear  to 
control  the  process  in  the  main  stem,  then,  as  the  branches  form  their  physio- 
logical connections  later  than  the  main  stem,  it  would  follow  that  their 
medullation  should  also  occur  later,  and  the  studies  on  the  progressive  medul- 
lation of  the  cerebral  cortex  favor  such  a  view. 

The  acquisition  of  this  sheath  occurs  in  response  to  a  physiological  change 
that  appears  at  the  same  time  along  the  entire  length  of  the  fibre.  The  proc- 
ess, therefore,  is  not  a  progressive  one,  but  is  practically  simultaneous. 

From  the  observations  of  Ambronn  and  Held 5  on  rabbits  a  day  or  two  old, 
it  appears  that  the  efferent  (motor)  spinal  and  cranial  nerves  acquire  their 

1  Langley  :  Journal  of  Physiology,  1897,  xxii.  p.  223. 

2  Quarterly  Journ.al  of  Microscopical  Science,  1 890. 

3  Vulpius  :   Archivfiir  Psychiatric  und  Nervcnkrank.,  1892,  Bd.  xxiii. 
*  Archiv  fur  Anatomic  und  Physiologic,  1889. 

5  Ambronn  and  Held :  Archivfiir  Anatomic  und  Physiologic,  Anatom.  Abthl.,  1896,  S.  208. 


182 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


sheaths  before  the  corresponding  afferent  (sensory)  nerves  are  medullated 
(except  in  the  case  of  the  vestibular  branch  of  the  auditory  nerve,  which  is 
medullated  at  the  same  time  as  the  motor  nerves).  In  the  central  system 
the  continuations  of  the  afferent  pathways  become  medullated  before  the 
pyramidal  tracts,  while  in  the  cerebral  hemispheres  medullation  of  the  com- 
missural  and  association  fibres  follows  immediately  that  of  the  afferent  tracts. 

Ambronn  and  Held l  have  also  shown  that  when  the  eyelids  are  prema- 
turely opened  in  animals  born  blind,  such  as  the  rabbit,  dog  or  cat,  and  the 
animal  is  then  exposed  to  the  light,  the  medullary  sheaths  are  more  rapidly 
formed  in  the  optic  nerves  exposed  to  stimulation  than  in  those  developing 
normally. 

Changes  in  the  Cytoplasm. — While  the  nerve-cell  is  passing  from  the 
immature  to  the  mature  form,  increasing  in  mass  and  in  the  number  of  its 
branches,  as  well  as  acquiring  its  medullary  sheath,  it  also  undergoes  various 
chemical  changes.  The  stainable  substance  in  the  cytoplasm  becomes  more 
abundant  at  maturity  and  the  pigment-granules  increase  in  quantity.2 


FIG.  74.— To  show  the  changes  in  nerve-cells  due  to  age:  A,  spinal  ganglion-cells  of  a  still-born  male 
child ;  B,  spinal  ganglion-cells  of  a  man  dying  at  ninety-two  years ;  n,  nuclei.  In  the  old  man  the  cells 
are  not  large,  the  cytoplasm  is  pigmented,  the  nucleus  is  small,  and  the  nucleolus  much  shrunken  or 
absent.  Both  sections  taken  from  the  first  cervical  ganglion,  X  250  diameters  (Hodge). 

Old  Age  of  the  Nerve-cells. — But  the  nerve-cell,  though  possessing  in 
most  cases  a  life-history  coextensive  with  that  of  the  entire  body,  eventually 
exhibits  retrogressive  changes.  These  changes  of  old  age  consist,  in  some 
measure,  in  a  reversal  of  those  processes  most  evident  during  active  growth. 
The  cell-body,  together  with  the  nucleus  and  its  subdivisions,  becomes  smaller, 
the  stainable  substance  diminishes  and  becomes  diffused  instead  of  appearing 
in  compact  masses,3  the  pigment  increases,  the  cytoplasm  exhibits  vacuoles, 
the  dendrites  atrophy,  and  the  axones  also  probably  diminish  in  mass.  In 
some  instances  the  entire  cell  is  absorbed.  Some  of  these  facts  are  illustrated 
by  the  observations  of  Hodge  4  on  the  spinal  ganglion-cells  of  an  old  man  of 
ninety-two  years  as  compared  with  those  of  a  new-born  child  (see  Fig.  74). 

Since  the  chemical  and  morphological  variations  which  occur  during  the 
entire  growth-cycle  are  accompanied  by  variations  in  the  physiological  powers, 
1  LOG.  cit,  S.  222.  J  Vas  :  Archiv  fur  mikroskopische  Anatomic,  1892. 

3  Marinesco :  Revue  neurologique,  October,  1899,  No.  20. 

4  Journal  of  Physiology,  1894,  vol.  xvii. 


CENTRAL    NERVOUS  SYSTEM.  183 

we  are  led  to  anticipate  in  old  age  a  correlation,  on  the  one  hand,  between 
the  decrease  in  the  quantity  of  functional  substance  in  the  cytoplasm,  and  a 
decrease  in  the  energy-producing  power  of  the  cells,  and,  on  the  other, 
between  the  absorption  of  the  cell-branches  and  a  limitation  in  the  extent  t<> 
which  the  neurones  may  influence  one  another.  Both  of  these  condition.-  are 
characteristic  of  the  nervous  system  during  old  age. 

B.  THE  NERVE-IMPULSE  WITHIN  A  SINGLE  NEURONE. 

The  Nerve-impulse. — Neurones  form  the  pathways  along  which  nerve- 
impulses  travel.  As  introductory,  therefore,  to  the  study  of  the  composite 
pathways  in  the  central  system,  comprising,  as  they  do,  several  elements  ar- 
ranged in  series,  it  becomes  important  to  study  the  behavior  of  the  nerve- 
impulse  within  the  limits  of  a  single  cell-element. 

Experimentally,  the  passage  of  the  nerve-impulse  is  revealed  by  a  wave  of 
change  in  the  form  of  an  electrical  variation  which  passes  along  the  nerve- 
fibre  in  both  directions  from  the  point  of  stimulation.  Under  normal  condi- 
tions, the  intensity  of  the  electrical  change  does  not  vary  in  transit,  though 
for  moderate  electrical  stimuli  the  strength  of  the  electrical  change  ("action 
current ")  is  proportional  to  the  strength  of  the  stimulus.1  It  moves  in  the 
peripheral  nerves  of  the  frog  in  the  form  of  a  wave  some  1 8  millimeters  in 
length,  at  the  mean  rate  of  30  meters  per  second,  and  this  rate  can  be  some- 
what retarded  by  cooling  the  nerves  and  accelerated  by  warming  them.  In 
mammals  the  rate  in  the  peripheral  nerves  has  been  found  by  Helmholtz  and 
Baxt  to  be  34  meters  per  second,  The  nerve-impulse  can  be  aroused  at  any 
point  on  a  nerve-fibre  provided  a  sufficient  length  of  fibre  be  subjected  to 
stimulation.  Mechanical,  thermal,  chemical,  and  electrical  stimuli  may  be 
used  to  arouse  it,  but  just  how  the  impulse  thus  started  differs  from  that 
normally  passing  along  the  fibres,  as  a  consequence  of  changes  in  the  cell- 
bodies  of  which  these  fibres  are  outgrowths,  is  not  known.  It  appears,  how- 
ever, that  the  impulses  aroused  by  artificial  stimuli  are  usually  accompanied 
by  a  much  stronger  electrical  variation  than  accompanies  the  normal  impulses. 

In  the  peripheral  system  the  nerve-impulse,  when  once  started  within  a 
fibre  or  axone,  is  confined  to  that  track  and  does  not  diffuse  to  other  fibres 
running  parallel  with  it,  although  it  does,  of  course,  extend  to  all  the  branches 
of  the  axone,  whatever  their  distribution. 

The  above-mentioned  relations  have  been  deduced  from  the  study  of  the 
peripheral  nerves,  and  these  morphologically  are  but  parts  of  the  axones,  the 
cell-bodies  of  which  are  located  either  in  the  central  system  proper  or  in  the 
spinal  or  sympathetic  ganglia. 

The  observations  apply  therefore  to  but  one  portion  of  the  nerve-cell,  and 
our  present  purpose  is  to  determine  how  far  it  is  possible  to  apply  them  to 
the  entire  nerve-cell,  noting  at  the  same  time  the  modifications  thus  intro- 
duced. 

Owing  to  the  small  size  of  nerve-cell  bodies  there  are,  of  course,  very  few 

1  Greene:  American  Journal  of  Physiology,  1898,  vol.  i.  p.  115. 


184  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

instances  in  which  a  single  nerve-cell,  or  part  of  such  a  cell,  has  been  the 
object  of  direct  physiological  experiment.  We  shall  therefore  approach  the 
question  indirectly  by  showing  what  the  histological  relations  have  to  suggest. 

Direction  of  the  Nerve-impulse. — In  the  case  of  a  given  nerve-cell  the 
impulses  which  we  usually  consider,  pass  in  one  direction  only.  For  instance, 
along  the  ventral  nerve-roots  of  the  spinal  cord,  the  impulses  pass  from  the 
cord  to  the  periphery,  while  in  the  dorsal  roots,  so  far  as  the  fibres  take 
origin  from  the  cells  of  the  spinal  ganglia,  these  impulses  travel  in  the  oppo- 
site direction.  At  the  same  time  experiment  has  shown  that  if  a  nerve- 
trunk  be  stimulated  at  a  given  point,  then  the  nerve-impulse  can  be  demon- 
strated as  passing  away  from  the  point  of  stimulation  in  both  directions. 

We  are  therefore  led  to  inquire  what  limits  are  set  to  the  passage  of 
impulses  in  a  direction  opposite  to  the  usual  one.  The  narrowest  limits,  it 
appears,  are  those  of  the  single  cell  in  which  the  impulse  has  originated. 
The*  experimental  observations  are  as  follows  :  When  the  fibres  forming  the 
ventral  root  of  a  spinal  nerve  are  stimulated  electrically,  and  the  cross- 
section  of  the  spinal  cord,  somewhat  cephalad  to  the  level  at  which  the  root 
joins  it,  is  explored  with  an  electrometer,  there  is  not  found  any  evidence  of 
nerve-impulses  passing  cephalad  in  the  substance  of  the  cord.  The  arrange- 
ment of  the  cells  in  the  cord  is  such,  however,  that  the  cell-bodies  which  give 
origin  to  the  fibres  forming  the  ventral  root  are  physiologically  controlled  by 
fibres  running  toward  them  from  every  portion  of  the  cord,  and  under  normal 
conditions  these  fibres  convey  impulses  to  the  cell-bodies  in  question.  The 
experiment  shows  that  when  an  impulse  enters  the  cell-body  by  way  of  the 
ventral  root-fibre,  to  which  it  gives  origin,  the  impulse  does  not  stimulate  the 
other  elements  of  the  cord.1 

With  the  elements  forming  the  dorsal  spinal  root  the  case  is  at  first 
glance  apparently  diiferent,  though  in  reality  it  is  the  same.  These  elements 
have  the  cell-body  located  in  the  spinal  ganglion.  The  cells  are  essentially 
diaxonic  (Fig.  72);  one  axone  extends  from  the  point  of  division  toward 
the  periphery  and  the  other  enters  the  spinal  cord,  where  it  forms  two 
branches,  both  of  which  course  longitudinally  for  some  distance  within  it 
(see  Fig.  75).  In  this  case,  therefore,  the  normal  direction  of  the  effective 
impulses  is  from  the  periphery  toward  the  cord,  and  within  the  cord  they  are 
delivered  to  other  elements,  which  carry  them  in  all  directions.  It  is  there- 
fore to  be  expected  that  the  stimulation  of  the  dorsal  root-fibres  would  give 
rise  to  impulses  passing  in  both  directions  in  the  dorsal  columns  of  the  cord. 
When,  however,  the  dorsal  columns  of  the  cord  are  electrically  stimulated 
in  a  cross-section  made  just  above  the  level  of  the  entrance  of  a  dorsal  root, 
then  it  is  'found  that  the  electrical  variation  is  to  be  detected  in  the  nerve- 
fibers  on  the  distal  side  of  the  spinal  ganglion.  These  impulses  have  there- 
fore passed  in  a  direction  the  reverse  of  that  usually  taken.  The  fibres 
which  in  this  instance  are  stimulated  in  the  cross-section  of  the  cord  are, 
however,  outgrowths  of  the  spinal  ganglion-cells,  and  thus,  although  the 
1  Gotch  and  Horsley :  Proceedings  of  the  Royal  Society,  1888. 


CENTRAL    NERVOUS  SYSTEM. 


185 


stimulation  of  the  cord  does  give  rise  to  an  impulse  in  the  afferent  spinal 
nerve,  nevertheless  the  impulse  is  continually  within  the  limits  of  one  cell- 
element.  This  shows  that  the  reversed  impulse  can  pass  the  spinal  ganglion, 
and  in  doing  this  it  probably  traverses  the  cell-bodies  there  located.  There 
is,  however,  no  evidence  that  the  stimulation  of 
the  dorsal  columns  of  the  cord  produces  out- 
going impulses  in  the  dorsal  nerve-roots  except 
when  the  stimulus  is  applied  to  the  axones, 
which  are  outgrowths  of  the  cells  of  the  spinal 
ganglia. 

In  the  case  of  the  interpolation  of  the  cell- 
body  in  the  course  of  the  axones  there  is  every 
reason  to  think  that  the  nerve-impulse  traverses 
the  body  of  the  cell  itself.  This  is  suggested 
by  the  changes  caused  in  the  cell-body  of  the 
spinal  ganglion-cells  as  the  result  of  stimulating 
the  peripheral  axone.  Moreover,  some  observers 
report  an  appreciable  delay  (0.036  second)  in 
the  passage  of  the  nerve-impulse  through  the 
cell-body  in  the  case  of  those  cells  which  form 
the  spinal  ganglion.1  This  delay  has  recently 
been  denied.2 

The  observations  of  Steinach,3  on  the  capacity 
of  the  afferent  nerves  of  the  frog  to  conduct  the 
centripetal  impulses  through  the  region  of  the 
spinal  ganglion,  indicate  that  impulses  may  pass 
this  region  when  the  cell-bodies  are  very  prob- 
ably excluded  from  forming  a  part  of  the  possi- 
ble pathways,  thus  showing  that  the  two  branches 
of  the  T-process  are  physiologically  continuous. 
These  results  do  not  show,  however,  that  the 
centripetal  impulses  fail,  under  normal  condi- 
tions, to  pass  to  the  cell-bodies  also.  It  may  be 
pointed  out  that  this  is  another  piece  of  evi- 
dence in  favor  of  the  view  that  within  the 
limits  of  a  single  neurone  or  fraction  of  a  neurone  there  is  no  limitation  to  the 
passage  of  a  nerve-impulse  in  all  directions,  wherever  it  is  started. 

Double  Pathways. — If  the  view  is  correct,  that  in  passing  through  the 
spinal  ganglion  the  normal  impulse  traverses  the  cell-body,  then  the  nerve- 

1  Gad  and  Joseph  :  Archivf.  Andtomie  u.  Physiologic,  1889. 

2  Moore  and  Reynolds  :  Proceedings  of  the  Fourth   International   Physiological  Congress, 
held  at  Cambridge,  1898.     Supplement,  vol.  xxiii,  Journal  of  Physiology.     These  authors  deny 
the  delay. 

3 Steinach:  "  Ueber  die  centripetale  Erregungsleitung  im  Bereiche  des  Spinalganglions," 
PflugeSs  Archiv,  1899,  Bd.  78. 


Fio.  75.— A  longitudinal  section 
of  the  cord  to  show  the  branching 
of  incoming  root-fibres  in  dorsal 
columns.  At  the  left  are  three  (D  R) 
root-fibres,  each  of  which  forms  two 
principal  branches.  These  give  off 
at  right  angles  other  branches,  col- 
laterals, Col,  which  terminate  in 
brushes.  C  C,  central  cells,  whose 
axones  give  off  similar  collaterals 
(Ramon  y  Cajal). 


186 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


impulse  passes  to  and  fro  along  the  common  stem  which  joins  the  cell-body 
with  the  two  branches  (vide  Fig.  72),  the  stem  itself  having  all  the  characters 
of  a  medullated  fibre. 

The  study  of  this  modification  brings  with  it  the  following  suggestion  : 
If  the  single  stem  in  the  modified  spinal  ganglion-cells  must  by  virtue  of  its 
development  contain  a  double  pathway,  it  is  fair  to  inquire  whether  the  same 
may  not  be  true  of  the  other  forms  of  the  nerve-cell  in  which  the  axone  also 
appears  to  be  single.  Among  the  cortical  cells  the  arrangement  of  the 
branches  is  such  that,  for  aught  that  is  known,  the  stem  of  the  axone  may 
functionate  in  the  manner  suggested,  and  contain  more  than  one  pathway. 

The  same  arrangement  must  exist  in  the  case  of  cells  like  those  repre- 
sented in  Fig.  76,  in  which  the  axone  arises  from  the  base  of  a  dendrite 


FIG.  76.— Showing  the  relations  between  the  terminal  branches  of  the  dendrites  (D)  and  of  the- 
axones  (Nf)  of  the  optic  fibres  where  they  come  together  in  the  superficial  layer  of  the  optic  lobe  of  the 
chick ;  also  showing  the  origin  of  the  axone  (N)  from  a  dendrite  (van  Gehuchten). 

at  some  distance  from  the  cell-body,  and  in  which  nerve-impulses  arriving 
over  the  dendrites  and  leaving  by  the  axone  must  normally  follow  the  por- 
tion of  the  cell-branch  which  is  common  to  both,  passing  along  it  first  in  one 
direction  and  then  in  the  other.  This  last  result  has  been  extended  by 
Sherrington,1  who  found  that  he  could  produce  movements  of  the  hind  limb- 
in  both  monkeys  and  cats  when  the  cord  had  been  sectioned  just  below  the 
bulb,  and  the  stimulus  was  applied  to  the  fibres  in  the  fasciculi  graciles  at 
that  level.  The  reaction  is  explained  by  the  passage  of  impulses  down  the 
dorsal  columns  (in  a  direction  reverse  to  the  normal),  and  their  distribution 
by  way  of  the  collaterals  to  the  efferent  elements  located  in  the  ventral  horns. 
Significance  of  Cell-Branches. — Since  the  outgoing  nerve-impulses  are 
isolated  in  the  axone  until  they  reach  the  terminal  twigs,  it  follows  that  the 
impulses  destined  to  produce  an  effect  beyond  the  cell  limits  will  do  so  at  the 
extremities  of  the  branches.  This  leads  to  the  question  how  far  the  posses- 
sion of  branches  is  necessary  to  the  functional  activity  of  a  nerve-cell  either 
fur  the  reception  or  transmission  of  an  impulse.  Since  it  has  been  pointed 
out  that  the  spinal  cord  of  the  newt  and  fish  is  capable  of  conducting  impulses 
even  before  the  dendrites  are  developed,  it  follows  that  the  transmission  of 
impulses  is  in  some  way  dependent  on  the  condition  of  the  cell-wall,  inde- 
pendent of  cell-branches.  This  modification  may  exist  at  points  where  there 
are  no  branches,  or  during  this  early  period  be  a  general  property  of  the 
1  Proceedings  of  the  Royal  Society,  1897,  Ixi.  243-246. 


CENTRAL    NERVOUS  SYSTEM. 


187 


wall,  and  only  later  become  the  peculiar  property  of  those  portions  which  are 
drawn  out  to  form  the  tips  of  the  branches.  But  not  only  the  capacity  to 
receive,  but  also  the  capacity  to  deliver  impulses  is  a  function  of  the  ends 
of  the  branches,  and  the  cell-wall  at  these  points  must  therefore  be  peculiarly 
modified  with  a  still  further  differentiation,  determining  the  direction  in  which 
the  impulses  may  pass.  Each  dendrite  may  represent  at  least  one  pathway 
by  which  impulses  reach  the  cell-body.  If,  then,  there  are  many  dendrites, 
the  cell-body  is  subject  to  a  more  complicated  series  of  stimuli  than  if  the 
branches  are  few.  It  will  be  remembered  that  the  young  nerve-cell  has  no 
dendrites,  that  the  first  branch  to  be  formed  is  the  axone,  and  that  the  com- 
pletion of  the  full  number  of  dendrites  is  a  slow  process.  The  pathways 
formed  by  the  dendrites  are  therefore  continually  increasing  up  to  maturity 


FIG.  77.— Climbing  fibre  from  human  brain :  a,  nerve-fibre ;  6,  Purkinje's  cell  (Cajal). 

(Fig.  77).  The  relation  between  the  "  climbing  fibre  "  and  the  dendrites  of 
the  Purkinje  cell  illustrates  this  arrangement. 

Generation  of  Nerve-impulses. — The  impulses  which  arrive  at  the  cell- 
body  produce  there  chemical  changes.  These  changes,  when  they  reach  a 
given  volume,  cause  a  nerve-impulse  which  leaves  the  cell-body  by  way  of 
the  axone.  If  the  nerve-impulse  is,  as  we  assume,  dependent  on  the  chem- 
ical changes  occurring  in  the  cytoplasm,  then  it  must  vary  according  to  these 
changes,  which  in  turn  can  hardly  be  similar  when  the  incoming  impulses 
that  arouse  them  arrive  along  different  dendrites.  We  know  that  a  stimulus 
applied  directly  to  the  axone  will  give  rise  to  a  nerve-impulse ;  but,  as  we 
shall  see  later,  the  chemical  changes  accompanying  the  passage  of  this 
impulse  are  too  slight  to  be  detected.  Whether  in  the  cell-body  equally 
slight  changes  would  give  rise  to  an  impulse  cannot  be  determined. 

Birge l  found  upon  stabbing  the  spinal  cord  of  a  frog  with  a  needle  in 
1  Birge:  Arch.f.  Anat.  u.  PhysioL,  Physiol.  Abthl.,  1882,  S.  471. 


188 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


the  region  where  t*he  efferent  cell-bodies  are 
clustered  together,  that  not  only  were  impulses 
sent  out  of  the  cord,  causing  the  muscles  to 
contract,  but  they  continued  to  be  sent  out  for 
some  seconds  after  the  injury. 

When  an  electrical  stimulus  is  applied  to  the 
cerebral  cortex  so  as  to  stimulate  the  cells  there 
present,  the  discharging  cells  may  also  continue 
to  send  out  impulses  for  some  time  after  the 
cessation  of  the  stimulus. 

Experiments  showing  the  multiple  character 
of  the  impulses  aroused  within  the  central 
system  have  been  made  by  Gotch  and  Horsley.1 
When  the  motor  cortex  of  a  monkey  was 
stimulated  (Fig.  78)  by  means  of  the  faradic 
current,  the  muscles  which  by  this  means  were 
made  to  respond  showed  a  long  tonic  contraction 
followed  by  a  series  of  shorter  clonic  ones  (Fig. 
79,  D).  When  the  spinal  cord  had  been  cut 
across,  the  cortex  was  again  stimulated,  and  the 
electrical  changes  in  the  fibres  of  the  cord  which 
convey  the  impulses  from  the  cortex  to  the 
spinal  centres  were  investigated  by  means  of  the 
capillary  electrometer.  By  this  means  a  curve 
(Fig.  79,  D)  was  obtained  as  a  record  of  the 
negative  variations  passing  along  these  fibres. 
This  latter  curve  corresponds  with  the  record 
for  the  muscular  contraction,  and  hence  the 
relation  between  the  two  series  of  events  is 
evident.  It  appears,  therefore,  that  the  cortical 

cells  after  the  cessa- 
tion of  the  stimulus 
still  continue  to  dis- 
charge in  a  rhyth- 
mical manner.  When 
the  cortex  had  been 
removed,  and  the 
electrodes  were  ap- 


— Mercury. 


,-  Sulphuric  acid  10%. 


Microscope. 


--Mercury. 


FIG.  78.— Schema  illustrating  the  experiment  for  determining  the  num- 
ber of  separate  nerve-impulses  passing  down  the  spinal  cord  upon  stimula- 
tion of  the  cortex  (from  experiments  on  the  monkey ;  Horsley) :  E,  E,  elec- 
trodes, intended  to  be  on  the  "  leg  area."    Where  the  cord  is  interrupted,     plied    directly  to    the 
one  non-polarizable  electrode  is  placed  over  the  cut  end  of  the  pyramidal  i      ,    .        fl,  ,, 

fibres  going  to  the  lumbar  enlargement ;  the  other,  on  the  side  of  the  cord.  'rV  1D5  n      eb?  tr 

These  lead  to  the  capillary  electrometer,  in  which  the  column  of  mercury     discharge   of  the  im- 
moves  each  time  an  impulse  passes.  ^          -, 

pulses  was  tound  to 

cease  with  the  stoppage  of  the  stimulus.     The  presence  of  the  cortex  was 
therefore  necessary  for  the  continued  discharge  (Fig.  79,  C).     The  attempt 

1  Proceedings  of  the  Royal  Society,  London,  1888. 


CENTRAL    NERVOUS  SYSTEM.  189 

was  also  made  to  determine  the  rhythmic  character  of  the  negative  varia- 
tions in  the  motor  nerve-trunk  between  the  cord  and  the  contracting  muscle, 
but  the  changes  there  present,  though  sufficient  to  cause  contractions  of  the 
muscle,  were  not  strong  enough  to  be  recorded  by  a  delicate  capillary  electrom- 
eter. This  result  suggests  that  the  impulses  sent  out  from  the  spinal  cord 
by  the  normal  discharge  of  the  motor  nerve-cells  may  differ  from  the 
impulses  artificially  aroused  in  the  lesser  intensity  of  the  electrical  changes 
that  accompany  them. 

Rate  of  Discharge. — The  rate  at  which  the  nerve-cells  discharge,  as 
shown  by  the  number  of  impulses  which  produce  tetanus  of  a  muscle  indi- 
rectly excited,  either  by  artificial  stimulation  of  the  nerve-elements  in  animals 
or  by  voluntary  impulses  in  man,  is  about  ten  impulses  per  second.  It 
appears  that  at  least  the  cortical  cells  and  those  of  the  spinal  cord  have  the 
same  rate  of  discharge,  and  that  this  rate  is  the  same  in  some  mammals 
(dogs,  cats,  rabbits,  and  monkeys)  as  in  man.  Hence  a  tendency  to  discharge 
about  ten  times  a  second  may  be  assumed  as  characteristic  of  the  mammalian 
nerve-cell.1 

Points  at  which  the  Nerve-impulse  can  be  Aroused. — It  is  probable 
that  the  excitation  of  any  part  of  a  nerve-cell  is  capable  of  producing  a  nerve- 


UXCll 

ation.      | 

<            1            1            1            1            1            1            1 

1    1  Sec.    | 

fl                                   ^               \ 

1 

I  I  I  I  I  I  I  I  \     ISec.  \ 

FIG.  79. — From  a  photographic  record  of  the  movements  of  the  column  of  mercury  in  a  capillary 
electrometer  (Gotch  and  Horsley).  The  arrow  shows  the  direction  in  which  the  record  is  to  be  read. 
The  upper  curve  (D)  shows  the  period  of  excitation  by  the  interrupted  current ;  this  is  followed  by  a 
series  of  waves  in  the  record  showing  a  number  of  separate  impulses  sent  down  from  the  cortex  after 
electrical  stimulation  has  ceased.  In  the  lower  curve  (C'),the  exciting  electrodes  were  applied  to  the 
white  matter  directly,  the  cortex  having  been  removed.  The  record  shows  that  in  this  case  no  impulses 
pass  after  the  stimulation  has  ceased. 

impulse,  whether  the  stimulus  be  applied  at  the  tips  of  the  dendrites  or  to 
the  axone  in  its  course. 

Irritability  and  Conductivity. — In  general,  parts  of  the  system  which 
are  irritable  are  also  conductive,  but  there  are  special  cases  in  which  the 
irritability  of  the  nerve-ftbre  can  be  distinctly  separated  from  its  conductivity, 
the  latter  being  present  while  the  former  is  absent. 

It  is  an  old  observation  that  on  stripping  down  the  phrenic  nerve  by 
compressing  it  between  the  thumb  and  forefinger  and  sliding  these  along  the 
nerve,  a  contraction  of  the  diaphragm  is  caused.  The  part  of  the  nerve  thus 
stimulated  is  soon  exhausted.  If,  now,  the  same  operation  is  repeated  on  a 
portion  of  the  nerve  lying  nearer  the  spinal  cord,  contraction  of  the  diaphragm 
again  follows.  This  result  was  originally  used  to  support  the  theory  of  a 
^chafer  and  Horsley:  Journal  of  Physiology,  1885,  vol.  vii.;  Schiifer,  Ibid. 


190  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

nerve-fluid,  and  was  held  to  demonstrate  that  after  the  nerve-tubes  in  the 
portion  of  the  trunk  compressed  had  been  emptied  so  that  no  reaction 
followed  further  pressure,  then  if  the  pressure  were  applied  still  nearer  the 
cord,  the  fluid  from  that  part  of  the  nerve  could  be  driven  forward  and  a 
contraction  of  the  diaphragm  would  result.  The  notion  of  a  nerve-fluid  in 
the  sense  in  which  that  term  was  used  by  the  earlier  physiologists  has  long 
since  been  abandoned ;  but  for  our  purpose,  the  experiment  is  important  as 
showing  that  under  such  treatment  irritability  and  conductivity  do  not  dis- 
appear at  the  same  time,  but  that  the  fibres  remain  conductive  after  they 
cease  to  be  irritable. 

It  has  been  shown  also  that1  in  young  regenerating  motor-fibres  it  often 
happens  that  while  no  response  is  to  be  obtained  by  the  direct  stimulation 
of  the  regenerated  peripheral  portion,  yet  the  stimulation  of  the  central  and 
fully  grown  portion  does  cause  a  contraction  of  the  muscles  controlled  by 
these  fibres.  In  this  case  the  newly  formed  fibres  can  conduct  an  impulse 
which  gives  rise  to  a  contraction,  although  such  an  impulse  cannot  be  aroused 
by  directly  stimulating  them. 

Summation  of  Stimuli  in  Nerve-cells. — In  an  axone  a  single  stimulus 
if  followed  by  a  single  nerve-impulse ;  on  the  other  hand,  the  studies  which 
have  been  made  to  determine  the  number  of  weak  stimuli  necessary  to  dis- 
charge afferent  cell-elements,  when  stimulated  by  way  of  the  afferent  nerves, 
indicate  that  there  may  be  a  summation  of  stimuli — i.  e.,  the  discharge  does 
not  follow  until  a  series  of  stimuli  has  been  given.2 

Whether,  however,  the  delay  in  the  response  is  due  to  the  failure  of  the 
cytoplasm  of  the  receiving  cell  to  discharge  until  repeated  impulses  have 
reached  it,  or  whether  the  modification  of  the  cell  which  causes  the  delay  is 
a  process  taking  place  at  the  point  where  the  impulse  passes  over  from  the 
branches  of  one  cell  to  those  of  another,  is  not  directly  determined  by  the 
experiments.  The  indirect  .evidence  is,  however,  entirely  in  favor  of  the 
view  that  the  delay  which  is  notable  in  the  arousal  of  a  reflex  response 
occurs  at  the  point  where  the  impulse  passes  from  one  cell  to  another. 

0.  THE  NUTRITION  OF  THE  NERVE-CELL. 

The  metabolic  processes  within  the  nerve-cell  are  continuous,  and  the 
chemical  changes  there  taking  place  involve  not  only  those  prerequisite  to 
the  enlargement  of  the  cell  during  growth,  but  also  those  leading  to  the 
formation  of  such  substances  as  by  their  katabolism  cause  the  nerve-impulse. 
The  passage  of  the  nerve-impulses  probably  alters  the  osmotic  powers  of  the 
cell-wall  toward  the  surrounding  plasma,  and  this  of  course  is  fundamental 
to  the  nutritive  exchange.  It  follows,  therefore,  that  the  passage  of  nerve- 
impulses  is  one  factor  determining  the  nutrition  of  these  cells. 

Histologically  we  look  upon  the  cell-bodies  as  the  part  in  which  the  most 

1  Howell  and  Huber :  Journal  of  Physiology,  1892,  vol.  xiii. 

1  Ward :  Archiv  f.  Anatomic  u.  Physiologic,  1880 ;  Stirling :  Arbeiten  aus  den  physiologischen 
Anstalt  in  Leipzig,  1874. 


CENTRAL    NERVOUS  SYSTEM.  191 

active  changes  occur,  since  the  network  of  blood-vessels  is  most  dense  about 
them,  indicating  that  the  metabolic  processes  are  here  most  active1  (Fig.  80). 

Chemical  Changes. — For  the  direct  microchemical  determination  of 
special  substances  within  the  nerve-cells  there  are  but  few  methods,  though 
some  phosphorus-bearing  substances  (nucleins)  can  be  demonstrated,2  and  the 
occurrence  of  chemical  changes  due  to  activity  and  to  age  are  very  evident. 
Macallum  3  has  demonstrated  the  presence  of  iron  in  the  stainable  substance 
of  Nissl.  There  is  general  consensus  that  the  alkalinity  of  the  nerve-tissues 
is  decreased  during  activity,  and  this  decrease  in  alkalinity  may  amount  at 
times  to  a  positively  acid  reaction.  This  change,  too,  is  better  supported  by 
the  observations  made  where  the  cell-bodies  are  numerous  than  by  those 
made  where  the  fibres  are  alone  present. 

Fatigue. — Not  only  is  the  food-supply  to  the  nerve-cells,  as  represented 
by  the  quality  and  quantity  of  the  plasma,  variable,  but  the  cells  themselves 


FIG.  80.— Frontal  section  through  the  human  mid-brain  at  the  level  of  the  anterior  quadrigeminum 
(Shimamura).  On  the  left  side  the  blood-vessels  have  been  injected;  on  the  right  the  gray  matter  is 
indicated  by  the  heavy  lines.  It  appears  by  this  that  the  blood-vessels  are  most  abundant  in  the  gray 
matter. 

are  subject  to  wide  variations  in  their  power  to  utilize  these  food  materials, 
and  deviations  from  the  normal  in  either  of  these  respects  means  a  diminu- 
tion in  the  physiological  powers  of  the  cell,  which  we  may  call  fatigue.  In 
the  nervous  system  the  signs  of  fatigue  are  both  physiological  and  histological, 
but  it  is  to  the  latter  changes  only  that  attention  will  be  here  directed. 

If  a  faradic  current  is  applied  intermittently  to  the  mixed  nerve-trunk 
going  to  a  limb,  changes  are  to  be  observed  in  the  cell-bodies  belonging  to 
the  spinal  ganglia  of  the  several  roots  forming  the  nerve  (Hodge). 

When  this  experiment  is  made  on  a  cat,  and,  after  death,  the  sections 
from  the  stimulated  are  compared  with  those  of  the  corresponding,  but  un- 
stimulated,  spinal  ganglion,  a  picture  like  that  represented  by  Fig.  81,  is 
obtained.4 

1  Shimamura  :  Neurologisches  Centralblatt,  1894,  Bd.  xiii. 

2  Lilienfeld  and  Monti  :  Zeitschrift  fur  physiologische  Chemie,  1892,  Bd.  xvii. 

3  Macallum  :  British  Medical  Journal,  London,  1898,  vol.  ii.  p.  778. 

4  Hodge  :  Journal  of  Morphology,  1892. 


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The  sections  indicate  that  the  cytoplasm,  together  with  the  enclosed  nucleus 
and  nucleolus,  as  well  as  the  nuclei  of  the  enclosing  capsule  of  the  cell,  have 
all  suffered  change  by  this  treatment.  The  stimulus  was  applied  for  only 
fifteen  seconds  of  each  minute,  the  remaining  forty-five  seconds  being  given 
to  rest.  In  this  way  the  cells  here  figured  had  been  stimulated  over  a  period 
of  five  hours.  The  nuclei  of  the  sheath  are  flattened,  the  cytoplasm  of  the 


FIG.  81.— Two  sections,  A  and  B,  from  the  first  thoracic  spinal  ganglion  of  a  cat.  B  is  from  the  gan- 
glion which  had  been  electrically  stimulated  through  its  nerve  for  five  hours.  A,  from  the  correspond- 
ing resting  ganglion,  The  shrinkage  of  the  structures  connected  with  the  stimulated  cells  is  the  most 
marked  general  change.  «,  nucleus ;  n,  s,  nuclei  of  the  capsule ;  v,  vacuole ;  X  500  diameters  (Hodge). 

nerve-cells  somewhat  shrunken  and  vacuolated.  With  osmic  acid  the  nuclei 
of  the  stimulated  cells  stain  more  darkly  and  the  cytoplasm  less  darkly  than 
in  a  resting  cell.  In  the  nerve-cells  the  nucleus  is  shrunken  and  crenated, 
and  the  nucleolus  is  also  diminished  in  size. 

In  the  first  experiments  the  attempt  was  made  to  demonstrate  a  measur- 
able change  within  the  nerve  cell-bodies  as  the  result  of  stimulation.  Assum- 
ing the  nuclei  of  these  cells  to  be  approximately  spherical,  and  calculating 
their  volume  as  spheres,  the  shrinkage  amounted  to  that  .shown  in  the  follow- 
ing table  : 


CENTRAL    NERVOUS  SYSTEM. 


193 


Table  showing  the  Decrease  in  the  Volume  of  the  Nucleus  •/  Stimulated  Spinal 
Ganglion-cells  of  Cats.  Stimulation  for  fifteen  seconds  alternating  with 
rest  /or  forty-jive  seconds  (Hodge). 


Stimulation  continued  for 
1  hour. 
2.5  hours. 
5 
10 


Shrinkage  in  the  volume  of  the 
nuclei  of  the  stimulated  cells. 

22  per  cent. 

21       " 

24       " 

44       " 


This  table  further  shows  that  the  shrinkage  is  greater,  the  greater  the 
time  during  which  the  stimulus  was  applied.  There  is  thus  established  not 
only  the  fact  of  a  change  in  the  cell,  but  also  a  relation  between  the  amount 
of  this  change  and  the  length  of  time  during  which  the  stimulus  was  allowed 
to  act.  The  results  when  expressed  by  a  curve  yield  the  following : 


Per 
cent. 
100 


90 


80 


70 


50 


17 


23 


29 


Hours     1      2i         5  10     Hi 

FIG.  82.— The  broken  line  indicates  the  volume  of  the  nuclei  of  the  spinal  ganglion-cells  of  a  cat 
after  stimulation  for  the  times  indicated.  The  solid  line  indicates  the  volume  of  the  nuclei,  first  after 
severe  stimulation  for  five  hours,  and  then  in  other  cats,  also  stimulated  for  five  hours,  but  subsequently 
allowed  to  rest  for  different  periods  of  time.  The  period  of  rest  is  found  by  subtracting  five  hours  from 
the  time  at  which  the  record  is  made.  After  twenty-four  hours  of  rest  the  nucleus  is  seen  to  have 
regained  its  normal  volume  (Hodge). 

Whether  these  changes  could  be  considered  similar  to  the  normal  physio- 
logical variations  depended  on  whether  it  was  possible  to  demonstrate  recovery 
from  them.  This  wras  accomplished  in  the  following  manner : 

Under  fixed  conditions  a  cat  was  stimulated  in  the  usual  way  and  the 
amount  of  shrinkage  in  the  nuclei  of  the  spinal  ganglion-cells  was  determined. 
This  was  found  to  be  almost  50  per  cent.  Four  other  cats  were  similarly 
treated  and  then  allowed  various  periods  (six  and  a  half,  twelve,  seventeen, 
and  twenty-four  hours)  in  which  to  recover.  The  results  appear  in  Fig.  82. 

Having  thus  shown  that  the  change  was  physiological  in  the  sense  that  it 
was  one  from  which  the  cells  could  recover,  it  remained  to  be  shown  that  the 
features  of  the  change  were  discernible  in  the  living  cell,  and  were  not  caused 
secondarily  by  the  actions  of  the  reagents  employed  in  preparing  the  sections. 

For  the  study  of  the  living  cell,  frogs  were  chosen,  and  the  cells  of  the 
sympathetic  ganglia  examined.  In  these  experiments,  cells  from  different 
frogs  were  prepared  under  two  different  microscopes  and  kept  alive  in  the 

VOL.  II.— 13 


194 


AN  AMERICAN   TEXT-BOOK   OF   PHYSIOLOGY. 


rs.O-OO 


same  way  by  irrigation  with  a  nutrient  fluid.  In  one  case,  however,  the  cell 

was  stimulated  by  electricity,  while  in  the  other  no 
stimulation  was  applied.  During  the  time  of  the 
experiment  the  cell  which  was  not  stimulated  re- 
mained unchanged,  while  the  stimulated  cell  went 
through  the  series  of  changes  exhibited  in  Fig.  83.1 

It  followed  that  if  these  changes  were  really 
significant  of  normal  processes  they  should  be  found 
in  the  nerve- cells  of  those  animals  which  show 
well-marked  periods  of  activity,  alternating  with 
periods  of  rest.  To  determine  this,  birds  and  bees 
were  examined,  one  set  of  preparations  being  made 
from  animals  which  were  killed  at  the  beginning  of 
the  day,  after  a  night  of  rest,  and  the  other  from 
those  killed  at  the  end  of  the  day,  after  a  period  of 
activity.  The  cells  from  the  latter  animals  were 
found  altered  in  a  way  similar  to  that  following 
direct  stimulation  of  the  axone.  The  changes  were 
demonstrated  in  the  cells  of  the  spinal  ganglia  of 
English  sparrows,  of  the  cerebrum  of  pigeons  and 
cerebellum  of  swallows,  and  of  the  an  ten  nary  lobes 
of  bees.  These  observations  therefore  support  the 
conclusions  drawn  from  the  appearances  following 
direct  stimulation. 

Other  observers 2  have  obtained  similar  results. 

The  motor  cells  of  the  spinal  cord  and  cells  of 
the  retina  (dogs,  Mann)  have  been  added  to  the  list 
of  those  showing  fatigue  changes.  In  the  sympa- 
thetic cells  of  the  rabbit,  both  Yas  and  Mann 
found,  after  a  short  period  of  stimulation,  a  pre- 
liminary swelling  of  the  cell-body,  and  the  same 
has  been  noted  by  Mann  in  the  case  of  retinal  cells 
in  the  dog. 

The  application  of  these  observations  to  changes 
in  the  human  nervous  system  has  thus  far  been 
made  only  in  a  casual  way,  but  enough  has  been 
already  observed  to  make  certain  that  the  results 
are  applicable. 

It  will  be  noted  that  the  changes  above  described 
follow  variations  in  the  amount  of  stimulation,  the 
nutrient  conditions  represented  by  the  surrounding 
plasma  remaining  nearly  constant.  This  latter,  however,  may  undergo 


FIG.  83. — Showing  the  changes 
in  the  form  of  the  nucleus  re- 
sulting from  the  direct  electrical 
Stimulation  of  the  living  sym- 
pathetic nerve-cell  of  a  frog. 
The  hour  of  observation  is  given 
within  each  outline.  The  ex- 
periment lasted  six  hours  and 
forty-nine  minutes.  A  control 
cell  treated  during  this  time  in 
the  same  manner,  except  that 
it  was  not  stimulated,  showed 
no  changes  (Hodge). 


1  Hodge:  Journal  of  Morphology,  1892,  vol.  vii. 

2  Vas:  Archiv  fur  mikroskopische  Anatomic,  1892  ;  Mann  :  Journal  of  Anatomy  and  Physiology, 
1894. 


CENTRAL    NERVOUS  SYSTEM.  195 

alteration,  and  recent  observations  show  that  in  various  forms  of  poisoning 
by  inorganic  substances  or  in  zymotic  diseases  the  nervous  system  and  espe- 
cially the  cell-bodies  are  affected  early  and  in  a  profound  manner.1 

Fatigue  in  Nerve-fibres. — There  is  no  evidence  for  fatigue  changes  in 
nerve-fibres.  For  the  full  discussion  of  this  .question  the  reader  is  referred 
to  page  96. 

Atrophic  Influences. — When  a  nerve-cell  is  not  kept  active  by  the 
impulses  passing  within  it,  it  usually  atrophies  and  may  degenerate.  The 
reason  for  this  appears  to  be  that  the  loss  of  those  changes  which  accompany 
the  nerve-impulses  decrease  the  vigor  of  the  nutritive  processes. 

For  the  detailed  study  of  metabolic  changes  within  the  cell-body  the 
method  of  Nissl2  has  been  of  prime  importance.  This  method  consists  in 
fixing  and  hardening  the  nerve-tissue  in  96  per  cent,  alcohol  and  staining 
with  hot  methylene  blue.  As  a  result,  the  cell-bodies  especially,  retain  the 
stain,  and  in  the  cells  there  is  a  "stainable  substance"  characteristically 
arranged  in  small  masses. 

For  a  given  animal  the  arrangement  of  the  "  stainable  substance "  is  char- 
acteristic of  the  cells  from  different  divisions  of  the  nervous  system.  In 
a  general  way,  too,  cells  occupying  homologous  positions  in  the  central 
system  of  mammals  tend  to  have  the  substance  arranged  in  a  similar  manner. 
But  the  characteristic  picture  is  modified  in  any  given  case  by  the  age  of  the 
animal  and  by  the  pathological  conditions  which  may  have  surrounded  the 
cell  chosen  for  study.  The  changes  in  the  picture  may  be  described  as 
variations  in  (1)  the  stainable  substance;  (2)  in  the  non-stainable  fibrillar 
framework  which  appears  to  enclose  the  former. 

In  both  of  these,  variations  may  be  accompanied  by  gross  physical 
changes,  i.  e.,  alterations  in  the  size  of  the  cell-body,  the  nucleus  and  its  parts, 
and  alterations  in  the  position  of  the  nucleus,  which  may  appear  pushed  to 
the  periphery  of  a  swollen  cell,  or  even  extruded  from  it.  These  physical 
changes  are,  of  course,  the  effects  of  the  action  of  the  alcohol  and  other 
reagents  employed  on  the  cells  altered  from  the  normal,  and  while  these 
physical  changes  serve  most  admirably  to  distinguish  the  normal  from  the 
abnormal  cells,  they  do  not  necessarily  represent  the  condition  of  the  abnormal 
cells  during  life,  a  cell  with  an  extruded  nucleus,  for  example,  being  a  case 
in  point.  These  changes  may  ultimately  cause  the  death  of  the  element. 

The  stainable  substance  is  found  to  be  extremely  sensitive  to  variations 
in  the  physiological  conditions  surrounding  the  cell,  and  therefore  to  be  most 
important  for  the  revealing  of  the  effect  of  all  sorts  of  changed  conditions, 
such  as  starvation,  activity,  fatigue,  injury  to  the  axone,  or  injury  to  the 
afferent  neurones  bringing  impulses  to  this  particular  cell,  and,  finally,  the 
effects  of  toxins  circulating  in  the  blood. 

^chaffer:  Ungarisches  Archivfiir  Medicin,  1893;  Pandi :  Ibid.,  1894;  Popoff:  Virchow's 
Archiv,  1894;  Tschistowitsch  :  Petersburger  medicinische  Wochenschrift,  1895. 

2  The  publications  of  Nissl  have  not  yet  been  printed  in  a  compact  form.  The  voluminous 
bibliography  of  the  author  is  given  by  Barker  :  The  Nervous  System,  1899,  pp.  105,  106. 


196 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


Amputation  in  Man. — When  the  nerves  to  a  limb  have  been  severed, 
the  consequent  changes  in  the  spinal  cord  depend  on  the  age  of  the  patient  at 
the  date  of  operation,  the  length  of  time  elapsing  between  the  operation  and 
death,  and  the  level  on  the  limb  at  which  the  amputation  was  made.  When 
the  amputation  occurs  early  in  life,  and  the  time  before  death  is  long,  and 
the  level  of  the  amputation  high,  the  alterations  are  maximum,  and  consist 
in  an  atrophy  in  the  peripheral  efferent  nerve-fibres,  slight  atrophy  (or  some- 
times complete  disappearance)  of  the  spinal  ganglion-cell  bodies,  atrophy  of 
dorsal  root-fibres  and  their  continuations  within  the  cord,  and,  on  the  ventral 
side,  disappearance  or  atrophy  of  the  motor  (efferent)  cell-bodies  in  the 
ventral  horn  of  the  cord,  together  with  their  axonic  outgrowths,  the  ventral 
root-fibres,  the  effect  extending  outward  through  the  peripheral  nerve  to  the 
point  of  section  (see  Fig.  84).  The  final  appearances  are  brought  about  by 
slow  changes,  often  requiring  years  for  their  completion,  and  hence  most  of 
the  cases  examined  tend  to  show  less  change  than  is  here  described.1 


FIG.  84.— Cross-section  of  the  spinal  cord  of  the  chick,  X  100  diameters  (van  Gehuchten) ;  D,  dorsal  sur- 
face ;  V,  ventral  surface ;  d.  r,  dorsal  root ;  v.  r,  ventral  root ;  g,  spinal  ganglion.  On  the  left  the  arrows 
indicate  the  direction  of  the  larger  number  of  impulses  in  the  dorsal  and  ventral  roots  respectively. 
The  small  arrow  on  the  right  dorsal  root  calls  attention  to  the  fact  that  so^me  axones  arising  in  the  ven- 
tral lamina  emerge  through  the  dorsal  root  and  convey  impulses  in  the  direction  indicated. 

The  disturbance  caused  in  the  two  sets  of  cells  is,  however,  not  the  same. 
In  the  case  of  the  cells  of  the  spinal  ganglion,  the  chief  pathway  by  which 
they  are  stimulated  under  normal  conditions  is  so  far  mutilated  that  probably 
only  a  small  number  of  impulses  passes  over  them.  That  some  do  pass  is 
indicated  by  the  sensations  apparently  coming  from  the  lost  limbs — sensations 
which  are  often  very  vivid  and  minutely  localized.2 

Thus  the  cell-bodies  located  in  the  spinal  cord  are  to  a  great  degree 
deprived  by  such  an  operation  of  one  principal  group  of  incoming  impulses, 
namely — those  which  arrive  through  the  dorsal  root-fibres  that  are  most 
closely  associated  with  them ;  but  at  the  same  time  there  remain  many  other 
ways  in  which  these  same  cells  are  normally  stimulated.  The  efferent  path- 

1Marinesco:  Neurol.  Ceniralbl.,  1892  (reviews  the  literature);  Gregoriew :  Zeilschrift  /. 
Heilkunde,  1894,  Bd.  xv. 

2  Weir  Mitchell :  Injuries  of  Nerve*,  Philadelphia,  1872. 


CENTRAL    NERVOUS  SYSTEM.  197 

way  from  these  cells  is  incomplete,  and  the  impulses  which  must  pass  along 
the  stumps  are  inefficient.  That  impulses  do  pass  alon-r  the  stumps  of  the 
efferent  roots  is  beyond  question,  since,  when  the  distal  portion  of  an  effer- 
ent nerve  is  cut  off,  the  cell  can  be  shown  to  still  discharge  through  tin-  por- 
tion of  the  fibres  connected  with  the  cell-bodies,  and,  finally,  there  is  always 
a  tendency  for  the  cut  fibre  to  regenerate,  which  indicates  activity  tli rough 
its  entire  length. 

Wherever  in  the  central  system  a  group  of  fibres  forms  the  chief  pathway 
for  the  impulses  arriving  at  a  given  group  of  cells  then  the  destruction  of 
these  afferent  fibres  brings  about  the  more  or  less  complete  atrophy  of  the 
cells  about  which  they  terminate,  and  this  effect  is  the  more  marked  the 
younger  the  animal  at  the  -time  of  injury.  Examples  of  this  relation  are 
found  in  the  behavior  of  the  nuclei  of  the  sensory  cranial  nerves. 

Thus  the  activity  of  a  given  cell  contributes  to  the  strength  of  its  own 
nutritive  processes,  and  different  cell-elements,  so  far  as  they  are  physiologi- 
cally associated,  stand  in  a  nutritive  or  trophic  relation  to  one  another  such 
that  the  receiving  cell  is  in  some  measure  dependent  for  its  nutrition  on  the 
cell  which  stimulates  it. 

Degeneration  of  Nerve-elements. — All  parts  of  a  nerve-cell  are  under 
the  control  of  that  portion  of  the  cell-body  which  contains  the  nucleus ;  in 
this  respect  the  nerve-elements  are  similar  to  other  cells  which  have  been 
studied,  and  in  which  the  nucleated  portion  of  the  cell  is  found  to  be  alone 
capable  of  further  growth.  It  was  shown  by  Waller1  that  when  sepa- 
rated from  the  cell-body  of  which  it  was  an  outgrowth,  a  nerve-fibre  belong- 
ing to  the  peripheral  nerve  soon  degenerate  from  the  point  of  section  to  its 
final  distribution.  The  process  is  designated  as  secondary  or  "  Wallerian 
degeneration."  According  to  recent  studies  on  this  subject,2  this  degener- 
ative change  occurs  practically  simultaneously  along  the  entire  length  of  the 
portion  cut  off.  The  changes  following  the  section  of  medullated  nerve- 
fibres  consist  in  a  fragmentation  of  the  axis-cylinder  followed  by  its  dis- 
appearance ;  enlargement  and  multiplication  of  the  nuclei  of  the  medullary 
sheath,  and  absorption  of  the  medullary  substance,  so  that  in  the  course  of 
the  fibres  there  remains  at  the  completion  of  the  process  the  primitive  sheaths 
together  with  the  sheath-nuclei.  In  the  early  stages  of  this  process  the 
medullary  sheath,  moreover,  undergoes  some  changes,  the  result  of  which  is 
that  it  stains  more  deeply  with  osmic  acid,  and  hence  appears  very  black  in 
comparison  with  the  normal  fibres  about  it  (Marchi).  These  changes,  as 
shown  by  the  method  of  Marchi,  may  follow  even  slight  injuries  to  the  nerve- 
fibres — such  as  compression  for  a  shor,t  time. 

Concerning  the  progress  of  degenerative  changes  in  the  non-medullated 
fibres  information  is  scanty.  Bo wd itch  and  Warren3  observed  that  when 
the  sciatic  nerve  of  the  cat  was  sectioned,  degeneration  of  the  motor  and 

1  Nouvelle  methode  anatomigue  pour  I' investigation  du  Systime  nerveux,  Bonn,  1851. 
1  Howell  and  Huber  :  Journal  of  Physiology,  1892,  vol.  xii. 
3  Journal  of  Physiology,  1885,  vol.  vii. 


198  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

vase-constrictor  fibres  in  the  peripheral  portion  went  on  at  about  the  same 
rate.  Stimulation  of  the  peripheral  part  of  the  nerve  gave  a  vaso-dilator 
reaction  after  the  vaso-constrictor  reaction  had  entirely  disappeared,  suggest- 
ing that  the  constrictor  fibres  degenerate  more  rapidly  than  do  the  dilators, 
although  it  is  not  improbable  that  the  dilator  fibres  in  this  location  really 
belong  to  the  medullated  class  (Howell).  After  five  days  no  vaso-motor 
reaction  at  all  could  be  obtained.  In  a  recent  study  by  Tuckett1  of  the 
degeneration  of  the  non-medullated  fibres  contained  in  the  branches  springing 
from  the  superior  cervical  ganglion,  it  is  stated  that  the  degeneration,  as 
traced  by  histological  and  physiological  methods  is  complete  within  thirty 
to  forty  hours  after  section  of  the  fibres,  and  that  the  degenerative  changes 
involve  only  the  core  of  the  fibres,  the  outside  sheath  and  nuclei  being  un- 
affected. 

In  the  central  system,  the  distal  portion  of  the.  fibres  separated  from  the 
cell-body  degenerate,  as  at  the  periphery,  and  this  reaction  has  therefore 
formed  a  means  by  which  to  study  the  architecture  of  the  central  system. 
The  details  of  the  process  are,  however,  not  clear. 

Nutritive  Control. — So  far,  then,  as  the  principal  outgrowth  of  the  nerve- 
cell  is  concerned,  it  is  found  to  be  always  under  the  nutritive  control  of  the 
cell-body  from  which  it  springs.  The  changes  which  take  place  when  the 
spinal  roots  are  cut  will  serve  to  illustrate  this  control  (see  Fig.  85).  Sec- 


Mr. 


FIG.  85.— Schema  of  a  cross-section  of  the  spinal  cord,  showing  the  dorsal  and  ventral  roots  and  the 
points  at  which  they  may  be  interrupted :  D.  r.,  dorsal  root ;  V.  r.,  ventral  root;  G,  ganglion ;  M,  muscle ; 
S,  skin ;  1,  lesion  between  ganglion  and  cord  ;  2,  lesion  between  muscles  and  cord ;  3,  lesion  between  skin 
and  ganglion ;  4,  combination  of  2  and  3. 

tion  of  the  dorsal  root  at  the  distal  side  of  the  spinal  ganglion  at  3,  causes  a 
degeneration  of  all  the  fibres  which  form  the  dorsal  nerve-root  distal  to  the 
ganglion.  Section  of  the  dorsal  root  at  1,  causes  degeneration,  central  to  the 
section,  of  those  nerves  which  are  outgrowths  from  the  cell-bodies  of  the 
spinal  ganglion.  Section  of  the  ventral  root  at  2,  causes  a  degeneration  distal 
to  the  point  of  section  in  those  fibres  which  form  the  ventral  root  and  which 
arise  from  the  cells  within  the  spina-1  cord.  In  each  case,  therefore,  the 
necessary  degeneration  occurs  on  the  side  of  the  section  away  from  the  cell- 
body.  The  fraction  of  the  neurone  on  the  other  side  of  the  section  may  also 
degenerate  under  certain  conditions,  but  the  degeneration  is  not  inevitable.2 

1  Tuckett:  Journal  of  Physiology,  1896,  vol.  xix. 

2Bregmann  :  Arbeiten  am  dem  Insiitutfur  Anatomie  und  Physiologie  des  Centralnervensystems 
an  der  Wiener  Unwersildt,  1892-93. 


CENTRAL    NERVOUS  SYSTEM.  199 

It  is  sometimes  stated  that  degeneration  takes  place  in  the  direction  of 
the  nerve-impulse.  In  a  general  way  this  is  true,  since  the  impulses  usually 
travel  from  the  cell-body  along  the  axone.  In  the  case  of  the  fibres  arising 
from  the  cells  of  the  spinal  ganglion  it  is  not  true,  since  the  section  at  the 
distal  side  of  the  ganglion  causes  degeneration  away  from  the  spinal  cord, 
while  that  on  the  proximal  side  of  the  ganglion  causes  degeneration  toward 
the  spinal  cord;  yet  in  both  axones  the  impulse  is  in  the  same  direction — 
namely,  toward  the  cord  (see  Fig.  85). 

Degeneration  of  the  Cell-body. — It  was  discovered  by  .von  Gudden1 
that  when  the  nerves  of  young  animals  are  pulled  away  from  their  attach- 
ment with  the  central  system,  they  most  frequently  break  just  at  the  point 
where  they  emerge  from  the  cord  or  brain  axisc  When  an  efferent  nerve  is 
thus  broken,  in  animals  just  born  or  very  young,  the  remaining  portion — i.  e., 
the  cell-bodies  with  so  much  of  their  axones  as  lie  within  the  central  system — 
may  atrophy  to  complete  disappearance. 

The  bearing  of  such  a  fact  is  very  direct.  If  in  man  there  is  reason  to 
think  that  an  injury  was  suffered  during  fetal  life,  there  is  a  possibility  that 
the  injury  may  not  only  have  prevented  the  further  development  of  the  cells 
involved,  but  may  also  have  caused  the  complete  destruction  of  some  of  them, 
in  which  case,  of  course,  the  architecture  of  the  damaged  region  is  necessarily 
abnormal. 

Such  complete  disappearance  as  the  result  of  early  injury  has  not  been 
shown  for  cells  which  lie  entirely  within  the  central  system.  Those  forming 
the  spinal  ganglia  may  die,  however,  after  interruption  of  the-  axones,  even 
when  the  animal  is  mature  (van  Gehuchten).  In  the  case  of  those  central 
cells  which  form  the  sensory  nuclei,  like  the  sensory  nucleus  of  the  fifth 
nerve,  or  of  the  vagus,  pulling  out  the  nerve-trunk  formed  by  the  axones  of 
the  afferent  ganglion  cells,  causes  only  an  atrophy  of  the  central  cells,  and 
not  their  complete  disappearance.2 

Regeneration. — When  the  two  ends  of  the  sectioned  nerve  are  brought 
together  under  favorable  conditions,  the  peripheral  portion  may  be  regener- 
ated. This  regeneration  occurs  only  in  axones  possessing  a  nucleated  (medul- 
lated  or  unmedullated)  sheath,  or  in  the  anatomical  prolongations  of  these, 
such  as  the  dorsal  root-fibres  which  penetrate  the  spinal  cord.3  In  the 
typical  medullated  peripheral  nerve  this  process  occurs  in  the  following 
steps  as  described  by  Howell  and  Huber : 4 

"While  the  fragmentation  and  absorption  of  the  myelin  in  the  distal  portion 
of  the  cut  nerves  is  going  on,  the  protoplasm  in  the  neighborhood  of  the 
sheath-nuclei  tends  to  increase.  These  enlarged  masses  of  protoplasm  then 
appear  as  a  thread  of  substance  within  the  old  nerve-sheath.  A  new  sheath 
is,  however,  soon  formed  on  the  protoplasmic  thread,  and  the  whole  consti- 

1  Archivfiir  Psychiatrie,  1870,  Bd.  ii. 

2  Fore  1  :  Festschrift  zur  von  Nagdi  und  von  Kolliker,  Zurich,  1891. 

3  Baer,  Dawson,  and  Marshall  :  Journal  of  Experimental  Medicine,  1899,  vol.  iv.  No.  1. 

4  Journal  of  Physiology,  1892,  vol.  xiii. 


200  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

tutes  an  "  embryonic  fibre."  The  embryonic  fibres  lying  on  one  side  of  the 
cut,  unite  with  those  on  the  other,  union  taking  place  in  the  intervening 
cicatricial  tissue.  Next  the  myelin  appears  in  isolated  drops,  usually  near 
the  nuclei,  and  these  subsequently  unite  to  form  a  continuous  tube,  the  for- 
mation of  the  myelin  proceeding  centrifugally  from  the  wound.  Then  follows 
the  outgrowth  of  the  new  axis-cylinder  slightly  behind  the  organization  of 
the  myelin  into  the  tubular  form. 

It  must  not  be  forgotten  that  the  last  act,  the  formation  of  the  axis- 
cylinder,  is  the  important  event ;  and  while  the  whole  process  of  repair  may 
require  many  months,  the  rate  at  which  the  axis-cylinder,  when  started, 
grows  out  from  the  central  end  may  be  comparatively  rapid.  As  a  rule, 
regeneration  does  not  occur  in  the  central  system,1  and  thus  the  method  of 
experimentally  causing  degeneration  has  been  one  used  for  the  study  of  the 
architecture  of  both  the  brain  and  cord. 

That  the  regeneration  is  due  to  an  outgrowth  from  the  central  stump  has 
been  clearly  shown  by  Huber,2  who  inserted  a  bone  tube  between  the  cut 
ends  of  the  sciatic  nerve  of  the  dog,  and  obtained  regeneration  of  the  nerve 
with  a  return  of  function,  although  the  initial  interval  bet  ween  the  two  parts 
of  the  nerve  was  more  than  three  centimeters.  The  rate  of  growth  from  the 
central  end  has  been  specially  studied  by  Yanlair.3  In  the  facial  nerve  of 
the  rabbit,  function  was  restored  in  eight  months  after  section,  and  in  the 
pneumogastric  and  ischiadic  nerves  of  the  dog  in  about  eleven  months.  In 
the  latter  case,  this  gives  an  average  rate  of  growth  of  about  1  millimeter  a 
day.  In  the  scar-tissue  between  the  two  parts  of  the  nerve  the  rate  is  not 
more  than  0.25  millimeter  a  day,  and  hence  the  return  of  function  tends  to 
be  delayed  by  any  increase  in  the  distance  between  the  cut  ends  of  the  nerve.  It 
appears  also  that  the  return  of  the  cutaneous  sensibility  is  more  rapid  than 
the  return  of  motion  (Howell  and  Huber),  from  which  we  infer  that  the 
aiferent  fibres  (from  the  skin)  regenerate  more  rapidly  than  the  efferent  fibres 
to  the  muscles. 

Vanlair  found  that  when  the  regenerated  sciatic  nerve  of  a  dog  was  cut 
a  second  time,  it  not  only  again  regenerated,  but  did  so  more  rapidly  than  in 
the  first  case. 

Much  interest  has  always  attached  to  the  exact  course  taken  by  the  re- 
generating fibres.  They  appear  in  a  general  way  to  be  guided  by  the  old 
sheaths  of  the  peripheral  portion.  But  the  peripheral  nerves  contain  both 
afferent  and  efferent  fibres,  and  it  would  appear  most  probable  that  in  the 
process  of  reformation  the  new  fibres  should  undergo  much  rearrangement. 
Since  the  peripheral  portion  of  the  nerve  acts  as  a  guide  to  the  growing  fibres, 
the  experiment  has  been  tried  of  cross-suturing  two  mixed  nerves.  This  has 
been  done  with  the  median  and  ulnar  nerves  in  dogs.  Reunion  of  the  crossed 

1  Worcester :  Journal  of  Experimental  Medicine,  1898,  vol.  iii.  p.  597,  describes  a  case  of 
apparent  regeneration  of  a  fibre-bundle  in  the  mid-brain,  and  cites  the  literature. 
J  Journal  of  Morphology,  1895,  vol.  xi. 
3  Archives  de  Physiologic  normale  et  pathologique,  1894. 


CEXTH.  1  L     M-:ii  \ '()  I 'S    HYSTl-IM.  201 

nerves  occurred  and  sensation  and  motion  returned  to  the  affected  parts  of 
the  limbs.1  It  is  plain  that  by  this  arrangement  the  skin  and  muscles  at  the 
periphery  must  have  acquired  central  connections  with  the  spinal  cord  very 
different  from  those  normal  to  them. 

From  the  experiments  of  Cunningham,2  it  appears  that  the  results  of  the 
cross-suturing  of  nerve-trunks  are  about  what  other  facts  would  lead  us  to 
expect.  If  the  trunks  concerned  control  muscles  acting  in  a  similar  manner, 
then  cross-suturing  produces  but  slight  incoordination  as  a  result;  where, 
however,  the  central  trunks  normally  innervate  antagonistic  muscles,  then 
incoordination  follows  and  persists.  The  stimulation  of  the  cerebral  cortex  at 
the  centre  for  a  given  muscle  group  always  causes  impulses  to  pass  along  the 
efferent  fibres  which  normally  innervate  that  group,  no  matter  to  what  muscles 
these  fibres  may  have  been  secondarily  attached  by  cross-suturing.  More- 
over, striped  muscles  which  normally  exhibit  rhythmic  contractions  lose  this 
function  when  their  innervation  is  changed  by  cross-suturing  to  a  nerve-trunk 
which  normally  innervates  an  arhythmic  muscle.  Thus  the  central  nervous 
system,  in  dogs,  at  least,  does  not  adapt  itself  to  the  changed  conditions 
introduced  by  cross-suturing. 

In  a  series  of  investigations,  Langley 3  has  been  able  to  show  that  when 
the  preganglionic  fibres  of  the  thoracic  nerves,  which  send  branches  to  differ- 
ent groups  of  cells  in  the  superior  cervical  ganglion,  are  allowed  to  regenerate 
after  section,  the  several  bundles  of  fibres  appear  to  find  and  become  attached 
to  the  cell-group  which  they  normally  controlled,  since  stimulation  of  the 
several  roots  after  regeneration  gave  the  reactions  which  were  characteristic 
for  them.  However,  there  is  reason  to  think  that  the  arrangement  after 
regeneration  is  not  exactly  the  same  as  that  before,  and  that  some  fibres  have 
strayed  from  their  original  connections.  Further,  Langley 4  has  been  able  by 
cross-suturing  to  establish  a  connection  of  the  lingual  and  the  vagus  nerves 
respectively  with  the  cervical  sympathetic  nerve,  and  so  with  the  superior 
cervical  sympathetic  ganglion.  Thus  we  have  evidence  that  fibres  other  than 
those  normally  associated  with  the  ganglion  cells  can  at  times  form  functional 
connections  with  them  and  carry  impulses  which  excite  them  to  their  normal 
functions.  This  result  has  an  important  bearing  on  the  theory  of  the  stimu- 
lation of  one  element  by  another.  The  reaction  following  the  indirect  excita- 
tion of  these  cells  depends,  therefore,  on  the  connections  made  by  their  axones, 
and  not  on  the  source  of  the  fibres  which  excite  them.  The  regeneration 
thus  far  described  has  been  that  of  the  axone  by  the  cell-body  or  perikaryon. 
Concerning  the  regeneration  of  the  dendrites,  we  have  no  information. 

The  possibility  of  the  formation  in  mammals  of  new  nerve-cells  by  the 
division  of  nerve-elements  which  are  already  mature  and  have  been  func- 
tional, has  been  claimed. 

1  Journal  of  Physiology,  1895,  vol.  xviii. 

2  Cunningham  :  American  Journal  of  Physiology,  1898,  vol.  i. 

3  Langley  :  Journal  of  Physiology,  1897,  vol.  xxii.  p.  215. 

4  Langley  :  Ibid.,  1898-9,  vol.  xxiii.  p.  240. 


202  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Karyokinetic  figures  in  mature  nerve-cells  after  injury  have  been  demon- 
strated, but  we  have  yet  to  learn  exactly  what  cells  can  exhibit  this  reaction, 
and  what  becomes  of  them  at  the  end  of  the  nuclear  changes.  As  there  is  no 
reason  to  think  that  in  mammals  such  a  neoformation  of  neurones  in  the 
nervous  system  has  any  significance  for  the  general  physiology  of  the  animal,, 
we  shall  pass  the  point  with  a  mere  reference  to  the  literature.1 


PART  II.— THE  PHYSIOLOGY  OF  GROUPS  OF  NERVE-CELLS. 

A.    ARCHITECTURE   AND  ORGANIZATION  OF   THE   CENTRAL  NERVOUS 

SYSTEM. 

Since  the  nerves  form  the  pathways  by  which  the  sensory  surfaces  of  the 
body  are  put  into  connection  with  the  central  system,  and  also  the  pathways 
by  which  this  system  in  turn  is  rendered  capable  of  controlling  the  tissues  of 


D.p 

FIG.  86.— Schema  of  the  arrangement  of  the  human  spinal  cord  as  seen  in  cross-section ;  for  clearness 
the  afferent  fibres  are  shown  on  the  left  side  only,  efferent  and  central  cells  on  the  right  side  only  (von 
Lenhossek) ;  D.  R.,  dorsal  root ;  V.  R.,  ventral  root ;  D.  P.,  direct  pyramidal  fibres  ;  C.  P.,  crossed  pyramidal 
fibres  ;  C.,  direct  cerebellar  tract ;  A.  L.,  antero-lateral  tract ;  D.  C.,  dorsal  columns.  The  various  classes 
of  cell-bodies  are  indicated  by  the  manner  of  drawing. 

expression,  it  becomes  at  once  important  to  determine  over  what  nerves  the 
impulses  arrive,  how  they  travel  through  that  system,  and  by  what  other 
nerves  they  are  again  delivered  at  the  periphery.  The  arrangement  of  these 
paths  as  found  in  the  adult  human  nervous  system  is  our  principal  object;  at 
the  same  time  it  should  not  be  forgotten  that  the  reactions  of  simpler  mam- 
malian systems  have  furnished  the  greater  number  of  facts,  and  to  them  we 
must  constantly  refer. 

General  Arrangement  of  the  Central  Nervous  System.  —  As  the 
typical  arrangement  of  the  neurones  is  found  in  the  spinal  cord,  the  schematic 
representation  (Figs.  86,  87)  of  a  cross-section  through  this  part  will  most 
readily  illustrate  it. 

In  accordance  with  this  arrangement  of  the  nervous  system,  as  shown  in 

1  Tedeschi,  A. :  Anatomisch-experimentellen  Beitrag  zum  Studien  der  Regeneration  des  Gewebe  des 
Centralnervensystems.  Beitrdge  zur  pathologischen  Anatomie  und  zur  allegemeinen  Pathologic,  Jena, 
1897,  xxi.  43-72,  3  pi.  * 


CENTRAL    NERVOUS  SYSTEM. 


203 


Figs.  86,  87,  the  elements  which  compose  it  fall  into  three  groups:  (1) 
Ttie  afferent  neurones;  those  whose  function  it  is  to  convey  impulses  due  to 
external  stimuli  from  the  periphery,  including  the  muscles  and  joints,  to  the 
central  system.  The  expression 
"  external  stimuli "  is  in  this  case 
intended  to  include  beside  those 
outside  of  the  body,  also  such 
stimuli  as  act  within  the  tissues 
of  the  body  but  outside  of  the 
central  nervous  system  ;  for  ex- 
ample, those  acting  on  tendons 
and  muscles,  and  affecting  the 
afferent  nerves  which  terminate 
in  them.  The  dorsal  roots  of 
the  spinal  cord  arise  from  the 
cell  bodies  in  the  spinal  ganglion. 
Sir  Charles  Bell  (1811)  showed 
that  these  roots  are  sensory,  since 
in  animals  stimulation  of  the 
central  end  of  the  severed  root 
causes  reflex  movements  and  ex- 
pressions of  pain,  while  in  man 
stimulation  of  these  fibres  in  the 
stump  of  an  amputated  limb 
may  give  rise  to  all  the  sensa- 
tions which  would  be  derived 
from  their  stimulation  in  the 
normal  limb. 

In  some  vertebrates  a  few 
efferent  axones  leave  the  spinal 
cord  by  the  dorsal  roots.  These 
fibres  can  be  seen  in  the  chick 
(Fig.  87).  In  the  frog  stimula- 
tion of  the  peripheral  end  of  the 

i     i          i  FIG.  87.— Schema  of  the  distribution  of  the  efferent 

Severed    dorsal     root    may    cause  fibres  of  the  spinal  roots :  A  afferent  fibres  in  the  dorsal 

Contraction    of  the   skeletal  mils-  root  onlv  •  E>  E>  efferent  fibres  in  both  dorsal  and  ventral 

,       j        ™.             .                         ,          .  roots.    In  the  ventral  root  one  group  of  efferent  fibres 

Cles.          lliere     IS    no    good    evi-  g0es  to  M,  the  striped  muscles ;  another  group  to  ganglion- 

dence,  however,  that  these  fibres     cells- s- formin*  a  sinj?le  symPathetic  ganglion,  or  to  s< 

cells  located  in  more  than  one  sympathetic  ganglion,  but 

are  present  in  mammals. 

(2)  The     central     neurones; 
those  the  axones  of  which  never 

leave  the  central  system,  and  the  function  of  which  is  to  distribute  within 
this  system  the  impulses  which  have  there  been  received. 

(3)  The  efferent  neurones;  or  those  the  axones  of  which  pass  outside  of 

1  R.  J.    Horton-Smith  :  Journ.  Physiol.,  vol.  xxi.  p.  101. 


all  connected  with  one  efferent  fibre  by  means  of  its  col- 
laterals; P,  peripheral  plexuses  into  which  the  axones 
of  some  sympathetic  cells  run. 


204 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


the  central  system,  and  which  carry  impulses  to  the  periphery.  In  this  last 
group,  again,  two  minor  divisions  may  be  made,  namely  :  (a)  the  efferent  ele- 
ments, the  cell-bodies  of  which  lie  within  the  central  system,  as  is  the  case  with 
those  giving  rise  to  the  ventral  roots  ;  (b)  those  forming  the  peripheral  ganglia 
entirely  outside  of  the  central  system — the  sympathetic  ganglia  and  the  more 
or  less  solitary  cells  which  take  part  in  the  formation  of  the  peripheral  plexuses. 
It  was  Sir  Charles  Bell  who  also  showed  the  motor  character  of  the 
ventral  roots.  Nevertheless  the  observation  was  soon  made,  that  while 
stimulation  of  the  central  end  of  the  severed  ventral  root  was  always 
without  apparent  effect,  the  stimulation  of  the  peripheral  end  in  addition 
to  the  typical  motor  responses  might  sometimes  cause  expressions  of  pain. 
This  latter  result  was  obtained  even  when  the  mixed  nerve  trunk,  beyond 

the  union  of  the  two  roots,  had  been  severed,  so 
that  the  only  possible  pathway  for  the  impulses 
was  through  the  junction  of  the  two  roots  to 
the  spinal  ganglion,  and  so  by  the  dorsal  root 
to  the  cord.  It  appears  probable  from  studies 
on  the  degeneration  of  the  root  fibres  that  the 
peripheral  axones  of  some  afferent  neurones  on 
their  way  to  the  meninges  do  run  back  toward 
the  cord  within  the  ventral  root,  and  that  it  is 
the  stimulation  of  these  fibres  which  gives  rise 
to  the  phenomenon  of  "  recurrent  sensibility  " 
as  it  is  called. 

The  "afferent  neurones"  (1)  have  their 
cell-bodies  collected  to  form  the  spinal  gan- 
glia.1 The  distal  branches  of  these  cells  form 
the  peripheral  sensory  nerves,  and  the  prox- 
imal branches  combine  to  form  the  dorsal 
nerve  roots.  On  entering  the  walls  of  the 

spinal  cord  these  latter  fibres  divide  into  two  principal  longitudinal  branches 
which  lie  about  the  dorsal  horns  and  form  the  major  part  of  the  dorsal 
columns.  From  time  to  time  the  ends  of  these  branches,  or  their  collaterals, 
enter  the  gray  matter  of  the  cord. 

Thus,  in  a  cross-section  of  the  cord,  the  dorsal  column  and  the  gray  matter 
represent  the  localities  where  the  axones  of  the  afferent  elements  are  found. 
The  afferent  cranial  nerves  which  can  be  homologized  with  the  afferent 
spinal  nerves  have  a  corresponding  distribution  in  the  bulb. 

The  "efferent  neurones"  (3)  have  their  cell-bodies  only  in  the  ventral 
horns  of  the  spinal  cord,  or  the  homologous  localities  in  the  bulb  and  brain 
stem.  See  Figs.  88,  89,  which  show  the  part  of  the  medullary  tube  divided 

1  Recent  work  on  the  spinal  ganglia  shows  that  in  addition  to  the  elements  usually  described, 
they  probably  contain  cells,  the  axones  of  which  are  distributed  entirely  within  the  spinal  gan- 
glion, and  also  cells  which  send  their  branches  to  the  distal  side  only  of  the  ganglion.  See 
Dogiel:  Anat.  Am.  Jena,  1896,  Bd.  xii. 


FIG.  88.— Cross-section  in  the  cer- 
vical region  of  a  fetal  human  spinal 
cord  at  the  sixth  week  ;  X  50  diameters 
(v.  Kolliker) :  c.  central  canal ;  a.  a., 
groove  separating  the  two  laminae :  d.p., 
dorsal  lamina  ;  v,  p.,  ventral  lamina,  in 
which  alone  are  located  nerve-cells 
the  axones  of  which  leave  the  central 
system;  d.  r.,  dorsal  root;  v.  r.,  ventral 
root. 


CENTRAL    NEin'Ol'S   HYXTEM.  205 

by  His  into  the  ventral  and  dorsal  lamina*  during  development.  The  ventral 
horns  of  the  gray  substance  form  part  of  the  ventral  laminae. 

The. cells  of  the  sympathetic  system  \\hieh  are  interpolated  in  one  portion 
of  the  pathway  formed  by  the  efferent  elements  lie,  of  course,  entirely  out- 
side of  the  central  system.  (See  Fig.  87.)  The  central  neurones  (2)  occupy 
all  parts  of  the  central  system,  and  hence  where  the  bodies  or  branches  of  the 
first  two  groups  are  absent,  the  system  is  composed  of  central  neurones  only. 

Arrangement  of  the  Cells  Forming-  the  Several  Groups. — All  three 
groups  of  elements  are  grossly  arranged  so  as  to  be  bilaterally  symmetrical 
with  reference  to  the  dorso-ventral  median  plane  of  the  body.  There  are 
some  minor  exceptions  to  this  general  statement,  but  these  are  not  known  to 
have  any  physiological  significance. 


FIG.  89.— Schema  showing  the  encephalon  and  cord  ;  the  unshaded  portion  is  that  derived  from  the 
dorsal  lamina ;  the  shaded  that  from  the  ventral  (from  Minot) :  C,  cerebrum  ;  Cb,  cerebellum  ;  F,  foramen 
of  Monro;  /,  infundibulum ;  M,  bulb;  0,  olfactory  lobe;  P,  pons;  Q,  quadrigemina ;  Sp.c,  spinal  cord; 
III,  third  ventricle ;  IV,  fourth  ventricle. 

The  main  axones  of  the  afferent  elements  are  distributed  almost  entirely 
to  the  dorsal  columns,  and  to  the  gray  matter  of  that  side  of  the  cord  on 
which  they  enter,  though  some  crossing  occurs  in  the  dorsal  commissure.  In 
the  cord,  the  efferent  elements  have  their  cell-bodies  mainly  on  the  side  of 
the  cord  from  which  the  efferent  fibres  emerge.  In  the  case  of  some  cranial 
efferent  nerves  the  arrangement  is  different.  There  is  found,  for  instance,  a 
partial  decussation  of  the  fibres  of  the  oculo-motorius ;  complete  decussation 
in  the  case  of  the  patheticus,  and  no  decussation  in  the  case  of  the  abducens. 
It  is  the  central  cells  which  furnish  almost  all  the  axones  forming  the  com- 
missures, the  decussating  bundles  and  the  projection  systems,  while  the 
association  tracts  arise  entirely  from  them. 

Segmentation. — The  grouping  of  the  cell-bodies  of  the  afferent  fibres  is 
originally  segmental,  one  spinal  ganglion  corresponding  to  each  segment  of 
the  trunk.  In  the  brain,  the  original  segmental  arrangement  has  been 
greatly  modified.  In  the  trunk,  too,  the  distribution  of  the  distal  portion  of 


206  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  afferent  nerves  is  segmental,  the  area  of  skin  involved  forming  a  band 
about  the  body.  On  the  other  hand,  the  distribution  of  the  proximal  branches 
forming  the  dorsal  roots  is  such  that  while  part  of  the  axones  and  their 
collaterals  establish  connection  with  the  cord  and  bulb  near  the  level  at  which 
the  axone  joins  it,  the  principal  divisions  of  the  axone  often  pass  along  the 
cord  a  greater  or  less  distance  in  both  directions,  and  thus  a  long  stretch  of 
the  cord  may  receive  impulses  by  way  of  a  single  afferent  element. 

In  some  of  the  lower  vertebrates  the  arrangement  of  the  "  efferent "  cells 
is  plainly  segmental,  but  in  man  and  the  higher  mammals  this  is  hardly  to  be 
demonstrated.  In  the  least  modified  parts  of  the  cord,  the  efferent  fibres  do 
arise  from  cell-bodies  mainly  within  the  segment  from  which  the  ventral  root 
emerges.  But  this  massing  of  the  efferent  cell-bodies  is  largely  obscured  by 
the  presence  of  central  cells  through  the  entire  length  of  the  ventral  horns, 
while  in  the  portions  of  the  cord  controlling  the  limbs  the  columns  of  cells 
furnishing  fibres  to  a  given  ventral  root  may  extend  through  as  many  as  three 
segments  of  the  cord.  The  distribution  of  the  efferent  fibres  is  evidently 
segmental  in  plan,  though  highly  modified  everywhere  except  in  the  thoracic 
cord  supplying  the  portion  of  the  trunk  between  the  limbs.  The  principal 
peculiarity  in  the  group  of  central  cells  is  the  great  increase  in  the  mass  of 
them  as  we  pass  from  the  cord  cephalad,  the  cerebrum,  for  example,  being 
composed  entirely  of  central  cells. 

Relative  Development  of  Different  Parts. — The  bulk  of  the  three 
subdivisions  which  have  been  named  is  very  unequal.  The  central  system 
is  far  more  massive  than  the  afferent  and  efferent  taken  together,  but  the 
relation  cannot  be  stated  with  any  exactness,  since  the  mass  of  the  peripheral 
system  is  not  definitely  known. 

Connections  between  Cells. — In  determining  the  connection  between 
cells  which  permits  a  nerve  impulse  in  one  cell  to  stimulate  another,  the  fact 
that  the  axone  is  the  outgrowth  of  a  cell-body,  and  that  each  cell  is  an  inde- 
pendent morphological  unit,  forms  the  point  of  departure.  Under  these 
circumstances  the  question  of  the  connection  between  cells  takes  the  more 
explicit  form  of  the  question  whether  cell-branches  may  become  continuous 
by  secondary  union.  In  several  vertebrates  there  is  good  histological  evidence 
that  such  secondary  union  occurs  in  a  few  cases  in  the  central  system. 

In  one  type  the  axone  of  one  element  spreads  out  and  encloses  the  cell- 
body  of  a  second  after  the  manner  of  a  cup  holding  a  ball.  In  other  cases 
it  appears  that  the  terminals  of  a  given  axone  may  even  penetrate  the  cell 
substance  of  the  receiving  neurone. 

These  are  examples  of  concrescence.  In  the  majority  of  cases,  however, 
a  close  approximation  of  the  parts  of  two  nerve-cells  is  alone  to  be  seen 
(Fig.  90).  The  termination  of  the  discharging  axone  may  be  by  fibrils  or 
expanded  disks,  and  occur  either  close  to  or  upon  the  body,  dendrites,  or 
even  collaterals l  of  the  receiving  neurone.  If,  as  seems  probable,  the 
dendrites  form  an  important  pathway  by  which  the  receiving  neurone  is 
1  Held:  Archivf.  Anal,  u.  PhysioL,  Anat.  Abthl.,  Leipzig,  1897. 


(i:\TRAL    NERVOUS  SYSTEM. 


207 


excited,  then  a  cell  with  many  dendrites  should  oiler  more  receiving  points 
than  one  with  few.  It  is  perfectly  evident,  however,  that  in  inanv  cases  the 
dendrites  are  not  the  only  pathway  hy  which  impulses  may  travel  toward 
the  cell -body. 

Theories  of  the  Passage  of  the  Nerve-Impulses. — Accepting  the  view 
that,  with  the  exceptions  just  noted,  the  nervous  system  is  composed  of  dis- 
continuous but  closely  approximated 
cell-elements,  it  remains  to  explain 
how  impulses  arising  within  the  limits 
of  one  element  are  able  to  influence 
others.  The  relation  between  two 
neurones  is  quite  comparable  to  that 
between  a  muscle  and  the  nerve-fibres 
controlling  it,  but  the  recognition  of 
that  .fact  does  not  afford  us  much 
assistance. 

As  an  hypothesis  the  passage  of 
the  stimulus  may  be  assumed  to 
depend  on  chemical  changes  set  up 
at  the  tips  of  the  terminals  and  affect- 
ing the  surrounding  substance,  which, 
thus  affected,  acts  to  stimulate  some 
point  on  the  wall  of  the  neighboring 
cell,  either  along  a  dendrite  or  on  the 
cell-body  itself. 

The  suggestion  has  been  made  that 
in  some  cases  the  space  between  two  neurones  may  be  varied  by  amoeboid 
changes  in  the  dendrites  and  terminals  of  the  elements  concerned.  Although 
much  may  be  said  a  priori  in  favor  of  this  hypothesis,  good  histological  evi- 
dence is  still  wanting. 

The  structural  changes  which  permit  the  stimulation  of  one  element  to 
affect  another  are  completed  slowly,  and,  as  we  shall  later  see,  these  changes 
continue  in  some  parts  of  the  human  nervous  system  up  to  middle  life. 

From  what  has  just  been  stated,  it  follows  that  the  nervous  system  of  the 
immature  person  is  quite  a  different  thing  from  that  of  one  mature,  since  in 
the  former  it  is  more  schematic,  more  simple,  the  details  of  the  pathways  not 
having  been  as  yet  filled  out.  Moreover,  considering  the  slow  and  minute 
manner  in  which  the  central  system  is  organized  by  the  growth  of  the  cell- 
branches,  it  is  the  list  place  where  there  should  be  expected  structural  uni- 
formity in  the  details  of  arrangement. 

B.    REFLEX  ACTION. 

Conditions  of  Stimulation. — The  conditions  necessary  for  the  generation 
of  a  nerve  impulse  are  an  external  stimulus  acting  on  an  irritable  neurone. 
While  life  exists,  stimulation  of  varying  intensity  is  always  going  on,  and 


FIG.  90.— Showing  at  the  lower  edge  of  the 
figure  a  series  of  basket-like  terminations  of  axones 
which  surround  the  bodies  of  the  great  cells  of 
Purkinje  in  the  cortex  of  the  cerebellum  (Ramon 
y  Cajal) :  C,  cell-body  ;  N,  axones ;  B,  basket-like 
terminations  arising  from  cell  C,  and  enclosing  the 
cells  of  Purkinje. 


208  AN  AMERICAN    TEXT-BOOK    ^>F  PHYSIOLOGY. 

hence  there  is  no  moment  at  which  the  nerve  is  system  is  not  stimulated,  and 
no  moment  at  which  the  effectiveness  of  this  stimulus  is  not  varied.  The 
response  to  this  continuous  and  ever-varying  stimulation  is  not  necessarily 
always  evident,  but  occasionally  intensification  of  the  stimuli  renders  them  so- 
strong  that  an  evident  reaction  follows. 

Though  the  foregoing  statements  suggest  that  the  chief  variable  is 
that  represented  by  the  stimulus,  the  strength  of  which  changes,  yet  as 
a  matter  of  fact  the  variations  in  the  pi  ysiological  (chemical)  condition 
of  the  nerve-cells  are  equally  important ;  but  neither  factor  can  be  studied 
independently. 

The  term  "central  stimulation  "  has  been  sometimes  employed.  For  ex- 
ample, the  spasmodic  movements  of  the  young  child,  when  there  is  no  change 
noticeable  in  the  external  stimuli  acting  upon  it,  are  sometimes  attributed  ta 
this  cause ;  but  these,  although  doubtless  due  to  central  changes,  altering  the 
irritability  of  the  cells,  are  most  properly  classed  with  the  reactions  which 
follow  the  external  stimulus.  The  misconceptions  here  to  be  avoided  are 
those  of  supposing  that  the  nervous  system  is  at  any  time  unstimulated,  and 
that  the  evident  responses  follow  a  change  of  the  external  stimulus  only. 

When  the  impulse  in  one  cell-element  is  used  to  arouse  an  impulse  in 
another,  as  in  all  experiments  where  the  nerve-cells  are  examined  in  a  physi- 
ological series,  the  strength  of  the  impulse  from  the  second,  is  not  easily  pre- 
dicted. This  is  explained  as  due  to  variations  in  the  ease  with  which  the 
impulse  in  one  element  stimulates  the  next,  and  also  to  the  variations  in  the 
second  cell  of  those  conditions  which  determine  the  intensity  with  which  it 
shall  discharge. 

When  an  impulse  has  once  entered  the  central  system  by  way  of  a  dorsal 
nerve  root,  it  is  found  to  follow  the  course  of  the  afferent  axones  within  the 
central  system,  and  thus  must  be  distributed  almost  simultaneously  to  a 
length  of  cord  coextensive  with  that  of  the  branches  of  the  afferent  axones. 

The  arrangement  makes  possible  the  stimulation  of  a  large  number  of 
central  cells,  and  thus  greatly  increases  the  distribution  of  the  initial  disturb- 
ance. In  the  case  of  some  of  the  cells  about  which  the  branches  of  the  axone 
end,  the  impulse  will  not  be  adequate  to  cause  in  them  a  discharge,  although 
it  may  still  produce  a  certain  amount  of  chemical  change  in  them.  The  im- 
pulse thus  tends  to  disappear  within  the  system  by  producing,  in  part,  chemi- 
cal changes  strong  enough  to  cause  a  discharge  of  the  next  element  in  the 
series  and,  in  an  increasing  number,  similar  changes  of  a  less  intensity. 

Diffusion  of  Central  Impulses. — If  the  previous  description  has  been 
correct,  two  very  important  events  occur :  in  the  first  place,  the  impulse 
reaches  a  far  greater  number  of  cells  than  evidently  discharge,  and  in  the 
second,  the  pathway  followed  by  the  impulses  which  do  produce  the  discharge 
is  by  no  means  the  only  pathway  over  which  the  impulses  can  or  do  travel. 

Simple  Reflex  Actions. — We  turn  next  to  an  examination  of  these  groups 
of  neurones  in  action. 

The  simplest  and  most  constant  of  the  co-ordinated  reactions  of  the  nerv- 


CEX'IT .:        NERVOUS  8Y8TK.M.  209 

ous  system  are  termed  reflex.''  The  term  involves  tin-  idea  that  the  response 
is  not  accompanied  by  consciousness,  and  is  dependent  on  anatomical  Condi- 
tions in  the  central  system  which  are  only  in  a  slight  decree  subject  to  phvsi- 
ological  modifications.  This  view  of  reflex  activities  is  in  a  large  mea-un 
justified  by  the  facts,  but  at  the  same  time  it  must  be  held  subject  to  many 
modifications,  and  it  is  not  possible  to  make  a  hard  and  fast  line  between 
reflex  and  voluntary  reactions. 

The  principal  features  of  a  ivflex  act  may  be  illustrated  by  following  a 
typical  experiment : 

If  the  central  nervous  system  of  a  frog  be  severed  at  the  bulb,  so  as  to 
separate  from  the  spinal  cord  all  the  portions  of  the  central  system  above  it, 
and  the  brain  be  destroyed,  the  animal  is  fora  time  in  a  condition  of  collapse. 
If.  after  recovering  from  the  immediate  shock,  such  a  frog  be  suspended  by 
the  lip,  it  will  remain  motionless,  the  fore  legs  extended  and  the  hind  ICL-S 
pendent,  though  very  slightly  flexed.  If  such  a  frog  were  dissected  down  to 
the  nervous  system,  there  would  be  found  the  following  arrangement  : 
Afferent  fibres  running  from  the  skin,  muscles,  and  tendons,  and  entering  the 
cord  by  way  of  the  dorsal  nerve-roots.  The  central  mass  of  the  spinal  cord 
itself  in  which  these  roots  end,  each  root  marking  the  middle  of  a  segment. 
Within  the  cord  and  stretching  its  entire  length  are  to  be  found  the  central 
<r//x,  interpolated  more  or  less  numerously  between  the  terminals  of  the 
afferent  neurones  and  the  cell-bodies  of  the  efferent  neurones.  From  each 
segment  of  the  cord  go  the  ventral  root-fibres  passing  in  part  to  the  muscles 
and  in  part  to  the  ganglia  of  the  sympathetic  system.  The  mechanism 
demanded  for  a  reflex  response  is  an  afferent  path  leading  to  the  cord ;  cells 
in  the  cord  by  which  the  incoming  impulses  shall  be  there  distributed ;  and 
a  third  set  of  efferent  elements  to  carry  the  outgoing  impulses  to  the  terminal 
organ  which  gives  the  response.  It  is  important  to  consider  in  detail  what 
occurs  in  each  portion  of  this  reflex  arc. 

In  a  frog  thus  prepared,  stimulation  of  the  skin  in  any  part  supplied  by 
the  sensory  nerves  originating  from  the  spinal  cord  causes  a  contraction  of 
some  muscles. 

Influence  of  Location  of  Stimulus. — The  muscles  which  thus  contract 
tend  to  be  those  innervated  from  the  same  segments  of  the  cord  which  receive 
the  sensory  nerves  that  have  been  stimulated.  Thus  stimulation  of  the  skin 
of  the  breast  causes  movements  of  the  fore  limbs,  and  stimulation  of  the  rump 
or  legs  corresponding  movements  of  the  hind  limbs.  It  is  noticeable,  how- 
ever, that  wherever  the  stimulus  is  applied  the  hind  limbs  have  a  tendency 
to  move  at  the  same  time  that  the  muscles  most  directly  concerned  contract. 

If  the  attempt  is  made  to  correlate  these  variations  in  reaction  with  varia- 
tions in  the  structure  of  the  cord,  we  have  to  picture  the  simplest  reactions 
(from  the  same  level)  as  dependent  on  the  formation  of  terminals  on  the 
afferent  fibre  just  after  its  entrance  into  the  cord  and  in  the  immediate  neigh- 
borhood of  an  efferent  neurone.  In  the  second  case  either  the  afferent  axone 
is  extended  some  distance  through  the  cord  forming  several  terminations  by 
VOL.  II.— H 


210  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

its  collaterals,  or  a  central  cell  is  excited  and  serves  to  carry  the  impulse  to  a 
distance. 

Segmental  Reactions. — In  attempting  to  explain  this  associated  con- 
traction of  the  leg  muscles,  it  must  be  remembered  that  the  hind  legs  are, 
par  excellence,  the  motile  extremities  of  the  frog,  and  therefore  all  general 
movements  involve  their  use.  We  infer  from  this,  moreover,  that  the  arrange- 
ment in  the  spinal  cord  of  the  frog  is  not  such  that  the  sensory  impulse- 
ing  into  any  segment  tend  to  rouse  exclusively  the  muscles  innervated  by 
that  segment,  but  that  these  incoming  impulses  are  diffused  in  the  cord 
unevenly  and  in  such  a  way  as  to  easily  involve  the  segments  controlling  the 
legs.  As  reflex  co-ordinating  centres,  therefore,  the  several  segments  of  the 
^cord  have  not  an  equal  value. 

When  the  stimulus  is  applied  on  one  side  of  the  median  plane,  the  responses 
first  appear  in  the  muscles  of  the  same  side;  and  if  the  stimulus  is  slight  they 
may  appear  on  that  side  only.  The  incoming  impulses  are  therefore  first  and 
most  effectively  distributed  to  the  efferent  cells  located  on  the  same  side  of 
the  cord  as  that  on  which  these  impulses  enter.  Such  a  statement  is  most 
true,  however,  when  the  stimulus  enters  the  cord  at  the  level  where  the  nerves 
to  the  limbs  are  given  off.  At  other  levels  the  diffusion  to  the  limb  centres 
may  take  place  more,  readily  than  to  the  cells  in  the  opposite  half  of  the  same 
segment.  When  the  muscles  on  the  side  opposite  to  the  point  of  stimulation 
contract  it  is  found  that  they  correspond  to  the  group  of  muscles  giving  the 
initial  response  on  the  side  of  the  stimulus.  The  diffusion  then  tends  to  cross 
the  cord  and  to  involve  the  cells  located  at  the  same  level  as  that  at  vvhiVh 
the  incoming  impulses  enter  it. 

There  is  some  reason  to  think  that  when  the  impulses  enter  the  cord 
toward  the  lumbar  end  the  path  by  which  the  diffusion  takes  place  with 
least  resistance  is  not  the  shortest  one  between  the  two  groups  of  cells,  but 
a  path  toward  the  cephalic  end  of  the  cord,  so  that  the  impulses  tend  to  pass 
up  the  cord  on  one  side  and  down  on  the  other.1 

Strength  of  Stimulus. — In  a  reflex  response  the  strength  of  the  stimulus 
influences  the  extent  to  which  the  muscles  are  contracted,  the  number  of 
muscles  taking  part  in  the  contraction,  and  the  length  of  time  during  which 
the  contraction  continues.  That  the  strength  of  the  stimulus  influences  the 
extent  to  which  the  contraction  of  a  given  group  of  muscles  takes  place  is 
easily  shown  when,  for  example,  the  toe  of  a  reflex  frog  which  has  been  sus- 
pended is  stimulated  by  pinching  it  or  dipping  it  in  dilute  acid.  In  this  ease, 
if  the  stimulus  be  slight,  the  leg  is  but  slightly  raised,  whereas,  if  the  stimu- 
lus be  strong,  it  is  drawn  up  high.  In  the  same  way  by  altering  the  stimulus 
the  muscles  which  enter  into  the  contraction  may  be  only  those  controlling 
the  joints  of  the  foot,  whereas,  with  stronger  stimuli,  those  for  the  knee  and 
hip  are  successively  affected,  thereby  involving  a  much  larger  number  of 
muscles.  Here,  too,  we  infer  a  spread  of  the  incoming  impulses  which  is 
orderly,  since  the  several  joints  of  the  limb  are  moved  in  regular  sequence. 
1  Rosenthal  und  Mendelssohn  :  Neurologisches  Ccntralblatt,  1897,  Bd.  xvi.  S.  978. 


CENTRAL    NERVOUS   SYSTEM.  1>11 

The  responses  which  are  thus  obtained  are  not  spasmodic,  but  are  con- 
tractions of  muscles  in  regular  series,  giving  the  appearance  <>!'  a  carefully  co- 
ordinated movement — a  movement  that  is  modified  in  accordance  both  with  the 
strength  of  the  stimulus  and  its  point  of  application.  Moreover,  such  a  move- 
ment may  occur  not  only  once,  but  a  number  of  times,  the  leg  being  alter- 
nately flexed  and  extended  during  an  interval  of  several  seconds,  although 
the  stimulus  is  simple  and  of  much  shorter  duration. 

Continuance  of  Response. — The  continuance  of  the  response  after  the 
stimulus  has  been  withdrawn  must  be,  of  course,  the  result  of  a  long-continued 
chemical  change  at  some  point  in  the  pathway  of  the  impulse,  and  it  appears 
probable  by  analogy  with  the  results  obtained  from  the  direct  stimulation  of 
the  central  cortex,  or  the  spinal  cord,  that  in  these  cases  the  stimulating 
changes  are  taking  place  (p.  188)  in  the  central  cells  or  efferent  cells1  as  well 
as  in  the  skin  supplied  by  the  afferent  nerves. 

Latent  Period. — It  has  been  observed  that  in  the  case  of  a  reflex  frog — 
that  is,  a  frog  prepared  as  described  above,  with  the  spinal  cord  separated 
from  the  brain — an  interval  of  varying  length  elapses  between  the  application 
of  a  stimulus  and  the  appearance  of  a  reaction.  The  modifications  of  the 
interval  according  to  variations  in  the  stimulus  have  been  carefully  studied. 
When  dilute  acid  applied  to  the  skin  is  used  as  a  stimulus,  this  latent  interval 
decreases  as  the  strength  of  the  acid  is  increased.  When  separate  electrical 
or  mechanical  stimuli  are  employed,  the  reaction  tends  to  occur  after  a  given 
number  of  stimuli  have  been  applied,  although  the  time  intervals  between  the 
individual  stimuli  may  be  varied  within  wide  limits.  The  experimental 
evidence  for  electrical  stimuli  shows  that  the  time  intervals  may  range 
between  0.05  second  and  0.4  second,2  while  the  number  of  stimuli  required 
to  produce  a  response  remains  practically  constant. 

Summation  of  Stimuli. — A  single  stimulus  very  rarely  if  ever  calls  forth 
a  reaction  if  the  time  during  which  it  acts  is  very  short,  and  hence  there  has 
developed  the  idea  of  the  summation  of  stimuli,  implying  at  some  part  of  the 
pathway  a  piling  up  of  the  effects  of  the  separately  inefficient  stimuli  to  a 
point  at  which  they  ultimately  become  effective.3 

The  details  of  the  changes  involved  in  this  summation  and  the  place  at 
which  the  changes  occur,  are  both  obscure,  but  it  w  /uld  seem  most  probable 
that  summation  is  an  expression  of  changes  in  the  relations  between  the  final 
twigs  of  the  afferent  elements  and  the  cell-bodies  of  the  central  or  efferent 
elements,  which  permit  the  better  passage  of  the  impulse  from  one  element  to 
the  other,  for  the  evidence  strongly  indicates  that  the  course  of  the  impulse 
can  be  interrupted  at  these  junctions. 

The  foregoing  paragraphs  have  been  concerned  mainly  with  changes 
occurring  in  the  afferent  portions  of  the  pathway.  Next  to  be  considered  is 

1  Birge:  Archivfilr  Anatomieund  Physiologic  (Physiol.  Abthl.),  1882,  S.  484. 

2  Ward  :   Ibid.,  1880. 

3  Gad  uud  Goldscheider:  "  Ueber  die  Summation  vou  llautreizen,"  Zeitschrift  fur  klinische 
Medicin,  1893,  Bd.  xx.  Hefte  4-6. 


212  AN-  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  amount  of  central  nervous  matter  which  must  be  present  in  the  frog's 
spinal  cord  in  order  that  the  reactions  can  take  place. 

Reactions  from  Fractions  of  the  Cord. — If  the  construction  of  the  cord 
was  strictly  segmental  (a  condition  nearly  approached  in  some  worms  and 
arthropods),  in  the  sense  that  each  segment  contained  the  associated  nerves 
for  a  given  band  of  skin  and  muscle,  there  should  be  no  disturbance  on 
dividing  the  cord  into  its  anatomical  segments ;  and  practically  among  the 
invertebrates,  where  the  ganglionic  chain  is  thus  arranged,  the  single  segments 
can  perform  alone  all  the  reactions  of  which  they  are  capable  under  normal 
conditions.1  In  such  invertebrates  the  only  change  effected  by  the  combina- 
tion of  the  segments  is  that  of  co-ordinating  in  time  and  in  intensity  the 
reactions  of  the  series.  If,  on  the  other  hand,  the  segments  of  the  cord  were 
more  or  less  dependent  upon  one  another,  and  not  physiologically  equivalent, 
modifications  of  various  degrees  would  arise  according  to  the  segments  isolated. 
It  has  been  found  that  the  spinal  cord  of  the  frog  may,  under  special  condi- 
tions, be  reduced  to  three  segments  and  reactions  still  be  obtained. 

During  the  breeding  season  the  male  frog,  by  means  of  his  fore  legs,  clasps 
the  female  vigorously  and  often  for  days.  If,  at  this  season,  there  is  cut  out 
from  the  male  the  region  of  the  shoulder  girdle  bearing  the  fore  limbs  together 
with  the  connected  skin  and  muscles,  and  the  three  upper  segments  of  the 
spinal  cord,  then  an  irritation  of  the  skin  will  cause'  a  reflex  clasping  move- 
ment similar  to  that  characteristic  for  the  normal  male  at  this  season. 

Reactions  in  Other  Vertebrates. — It  must  not  be  thought,  however, 
because  it  is  the  custom  to  emphasize  the  reflex  activities  of  the  lower  verte- 
brates, and  to  show  that  these  reflexes  can  be  carried  out  even  by  fractions 
of  the  spinal  cord  alone,  that,  therefore,  the  spinal  cord  is  particularly  well 
developed  in  them.  Comparative  anatomy  shows  in  the  lower  vertebrates  a 
simplicity  in  the  structure  of  the  cord  quite  comparable  with  that  found  in 
the  brain,  and,  as  we  ascend  the  vertebrate  series,  both  parts  of  the  central 
system /increase  in  complexity.  In  this  increase,  however,  the  cephalic  divi- 
sion takes  the  lead ;  and  further,  by  means  of  the  fibre-tracts  from  it  to  the 
cord,  the  cell-groups,  in  the  cord  are  more  and  more  brought  under  the  influence 
of  the  special  sense-organs  which  connect  with  the  encephalon:  The  physio- 
logical reactions  of  the  higher  vertebrates  are  especially  modified  by  this 
latter  arrangement.  It  is,  therefore,  true  that  the  cord,  as  well  as  the  brain, 
is  in  man  more  complicated  anatomically  than  in  any  of  the  lower  forms,  and 
this  is  true  in  spite  of  the  fact  that  the  independent  reactions  of  the  human 
cord  are  so  imperfect. 

When  an  amphioxus  is  cut  into  two  pieces  and  then  put  back  in  the  water, 
a  slight  dermal  stimulus  causes  in  both  of  them  locomotor  .movements,  such 
as  are  made  by  the  entire  animal. 

When  a  shark  (Scyllium  eaniculai)  is  beheaded,  the  torso  swims  in  a  co-or- 
dinated manner  when  returned  to  the  water.  Separation  of  the  cord  from  the 

1  Loeb:  Einleitung  in  die  vergkichende  Gehimphysiologie  vr  <  nde  Psychologie,  Leip- 

zig, 3899. 


CENTRAL    NERVOUS  SYSTEM.  213 

brain  does  not  deprive  a  ray  (Torpedo  oculata)  of  the  power  of  perfect  loco- 
motion. The  same  is  true  of  the  ganoid  fish.  In  the  case  of  the  cyclostome 
fish  (Petromyzon)  the  beheaded  trunk  is,  in  the  water,  inactive,  yet,  on  gentle 
mechanical  stimulation,  it  makes  inco-ordinated  responses;  but,  put  in  a  bath 
formed  by  a  3  percent,  solution  of  picro-sulphuric  acid,  locomotion  under 
the  influence  of  this  strong  and  extensive  dermal  stimulus  is  completely  per- 
formed. In  the  case  of  the  eel  the  responsiveness  even  to  the  picro-sulphorio 
acid  bath  is  evident  in  the  caudal  part  of  the  body  alone.  In  the  bony  fish  this 
capability  of  the  spinal  cord  to  control  locomotion  has  not  been  observed.1 

In  these  experiments  the  central  system  is  represented  by  the  entire  spinal 
cord  with  the  associated  nerves,  or  by  some  fraction  of  it ;  but  so  simple, 
constant,  and  independent  are  the  reactions  of  the  cord  under  normal  condi- 
tions that  a  strong  stimulus  is  able  to  elicit  the  characteristic  responses  from 
even  a  fragment  of  the  system.  The  higher  we  ascend  in  the  vertebrate 
series  the  less  evident  do  the  independent  powers  of  the  cord  become. 

Tarchanow 2  has  shown  that  beheaded  ducks  can  still  swim  and  fly  in  a 
co-ordinated  manner,  and  among  mammals  (dog  and  rabbit)  Goltz  and  Ewald  3 
and  others  have  demonstrated  that  if  the  lumbar  region  be  separated  from  the 
rest  of  the  cord  by  a  cut  and  the  animal  allowed  to  recover  from  the  opera- 
tion it  will  with  proper  care  live  for  many  months,  and  not  only  are  the  legs 
responsive  to  stimulation  of  the  skin,  but  the  reflexes  of  defecation  and  urina- 
tion are  easily  induced  by  slight  extra  stimulation.  An  instructive  reaction 
occurs  when  such  an  animal  is  held  up  so  that  the  hind  legs  hang  free. 
When  thus  held,  the  legs  slowly  extend  by  their  own  weight  and  then  are 
flexed  together.  The  reaction  becomes  rhythmic  and  may  continue  for  a  long 
time.  It  is  assumed  in  this  case  that  the  stretching  of  the  skin  and  tendons 
due  to  the  weight  of  the  pendent  legs  acts  as  a  stimulus,  and  in  consequence 
the  legs  are  flexed.  This  act  in  turn  removes  the  stimulus,  and  as  a  result 
they  extend  again,  to  be  once  more  stimulated  and  drawn  up. 

In  man,  as  a  rule,  death  rapidly  follows  the  complete  separation  of  any 
considerable  portion  of  the  cord  from  the  rest  of  the  central  system,  especially 
if  the  separation  be  sudden,  as  in  the  case  of  a  wound.  But  Gerhardt4  has 
recorded  the  retention  of  the  reflexes  in  a  case  of  compression  of  the  cord  by 
a  tumor,  the  case  having  been  under  observation  for  four  and  a  half  years ; 
and  Hitzig,5  a  case  in  which  a  total  separation  between  the  last  cervical  and 
first  thoracic  segments  had  been  survived  for  as  long  as  seven  years.  The 
principal  reaction  to  be  observed  in  such  cases  is  a  contraction  of  the  limb 
muscles  in  response  to  stimulation  of  the  skin,  such  as  a  drawing  up  of  the 
legs  when  the  soles  of  the  feet  are  tickled.  No  elaborate  reflexes  are,  how- 
ever, retained  such  as  would  be  necessary  in  acts  of  locomotion. 

1  Steiner  :    Die  Fanctionen  des  Centralnervensystems  und  ihre  Phylogenese,  2te  Abth.,   "Die 
Fische,"  1888. 

2  Tarchanow:  Pfliiger's  Archiv,  1885,  Bd.  xxxiii. 
8  Coltz  und  Ewald:  Ibid,,  1896,  Bd.  Ixiii. 

4  Neurologisches  Centralblatt,  1894,  S.  502.  5  Loc.  cit. 


214  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

It  thus  appears  that  the  reflex  responses,  namely,  simple  reactions  unac- 
companied by  consciousness,  are  in  man  mainly  given  by  the  unstriped 
muscle-tissue  and  by  glands,  and  only  in  a  minor  degree  by  the  striped 
muscles.  Moreover,  while  the  typical  reflex  is  a  reaction  over  which  we 
cannot  exercise  direct  control,  the  normal  individual  has  some  power  over 
many  of  these  reactions ;  for  example,  the  impulse  to  micturition  or  defeca- 
tion can  be  thus  delayed,  respiration  arrested,  and,  in  some  instances,  so 
remote  a  reaction  as  the  beat  of  the  heart  either  accelerated  or  slowed  at  will. 

It  is  of  interest  to  note  that  many  reflexes  which  in  the  young  are  not 
controlled,  as  micturition,  for  instance,  become  so  gradually,  a  change  most 
probably  dependent  on  the  growth  of  axones  from  the  cephalic  centres  into 
the  cord,  thus  subjecting  the  cord-cells  to  a  new  set  of  impulses  which  modify 
their  reactions.  That  such  is  the  case  is  indicated  by  the  fact  that  extreme 
fright  or  anaesthetics,  which  diminish  the  activities  of  the  higher  centres,  often 
cause  these  reactions  to  take  place  involuntarily.  Other  reflexes  are  present 
in  early  life,  but  disappear  later ;  such  are  the  sucking  reflex  of  the  infant, 
and  the  remarkable  clinging  power  of  the  hands,  by  which  a  young  child  is 
enabled  to  hang  from  a  bar,  thus  supporting  the  weight  of  its  entire  body, 
often  for  several  minutes.  This  last  capacity  soon  begins  to  wane,  and  usually 
disappears  by  the  second  month  of  life.1 

Co-ordination  of  the  Efferent  Impulses. — Incessantly  the  efferent  im- 
pulses pass  out  from  the  cord  to  the  muscles  and  glands.  With  each  fresh 
afferent  impulse  those  which  go  out  are  modified  in  strength  and  in  their 
order,  but  just  how  they  shall  be  co-ordinated  is  dependent  on  so  many  and 
such  delicate  conditions  that  even  in  the  simplest  case  the  results  are  to  be 
predicted  only  in  a  general  way. 

The  attempt  to  determine  the  spread  of  the  impulse  in  the  cord  by 
observing  the  order  in  which  the  various  muscles  of  the  thigh  and  leg  con- 
tract in  response  to  thermal  stimuli  was  made  by  Lombard.2  In  a  reflex 
frog  the  tendons  of  the  leg  and  thigh  muscles  were  exposed  at  the  knee,  and 
each  attached  to  a  writing-rod  in  so  ingenious  a  manner  that  simultaneous 
records  of  fifteen  muscles  could  sometimes  be  obtained.  The  stimulus  was  a 
metal  tube,  filled  with  water  at  47°-61°  C.,  which  was  applied  to  the  skin. 
Under  these  conditions,  it  was  remarkable  that  a  continuous  stimulus  was 
often  followed,  not  by  a  single  contraction  of  the  muscles,  but  by  a  series  of 
contractions,  suggesting  that  in  the  central  system  the-eelU-were  roused  to  a 
discharge  and  then  for  a  time  concerned  with  the  preparation  for  sending  out 
new  impulses,  and  that  during  this  latter  period  the  muscles  were  relaxed. 

Apparently  a  high  degree  of  uniformity  in  the  conditions  was  obtained  in 
these  experiments,  but  at  the  same  time  the  reactions  were  far  from  uniform, 
in  either  the  latent  time  of  contraction  or  the  order  in  which  the  contraction 
of  the  several  muscles  followed,  although  certain  muscles  tended  to  contract 
first,  and  certain  series  of  contractions  to  reappear.  The  co-ordination  of  the 

Robinson:  Nineteenth  Century,  1891. 

2  Archivfiir  Anatomie  und  Physiologic,  1885. 


CENTRAL   NERVOUS  SYSTEM.  215 

contractions  is  therefore  variable  in  time,  even  under  these  conditions.  These 
variations  are  probably  due  either  to  the  fact  that  the  impulses  are  not  dis- 
tributed in  the  centre  in  the  same  manner  on  cadi  occasion  ;  or  if  they  are 
thus  distributed,  the  central  and  efferent  cells  vary  from  moment  to  moment 
in  their  responsiveness.  That  these  cells  should  so  vary  is  easy  to  compre- 
hend, for  all  the  cell-elements  in  such  a  reflex  frog  are  slowly  dying.  In  this 
process  they  are  undergoing  a  destructive  chemical  change,  and  with  these 
destructive  changes  are  generated  weak  impulses  sufficient  to  cause  their 
physiological  status  continually  to  vary,  thus  modifying  the  effects  of  any 
special  set  of  incoming  impulses  acting  upon  them. 

It  is  not  to  be  overlooked  also  that  the  dissection  of  the  muscles  tested, 
and  the  removal  of  the  skin  about  them,  deprived  the  spinal  cord  of  the 
incoming  impulses  due  to  the  stretching  of  the  skin  by  the  swelling  of  the 
contracting  muscles  and  disturbed  the  order  and  intensity  of  such  sensory 
impulses  as  come  in  from  the  tendons  and  the  muscles  themselves.  The 
observations  of  both  Bickel l  and  Hering 2  show  that  these  impulses  are  not 
necessary  for  accurate  reflex  movements  of  the  frog's  leg,  and  thus  weaken 
the  force  of  the  suggestion  just  made.  However  much  these  impulses  may 
add  to  the  regularity  of  the  muscular  responses,  Lombard  concludes  that 
the  discharge  of  one  efferent  cell  is  not  necessary  in  order  that  another 
efferent  cell  may  discharge,  but  that  each  discharging  cell  stands  at  the  end 
of  a  physiological  pathway  and  may  react  independently. 

Purposeful  Character  of  Responses. — When  the  muscular  responses  of 
a  reflex  frog  to  a  dermal  stimulus  are  studied,  they  are  seen  to  have  a  pur- 
poseful character,  in  that  they  are  often  directed  to  the  removal  of  irritation. 
This  is  demonstrated  by  placing  upon  the  skin  on  one  side  of  the  rump  a 
small  square  of  paper  moistened  with  dilute  acid.  As  a  result,  the  foot  of 
the  same  side  is  raised  and  the  attempt  made  to  brush  the  paper  away ;  if  the 
first  attempt  fails,  it  may  be  several  times  repeated.  When  the  irritation  has 
been  removed  the  frog  usually  becomes  quiet.  If  the  leg  of  the  same  side 
be  held  fast  after  the  application  of  the  stimulus,  or  if  the  first  movements 
fail  to  brush  away  the  acid  paper,  then  the  leg  of  the  opposite  side  may  be 
contracted  and  appropriate  movements  be  made  by  it.  Emphasis  has  been 
laid  by  various  physiologists  upon  reactions  of  this  sort  as  showing  a  capa- 
bility of  choice  on  the  part  of  the  spinal  cord,  thus  granting  to  the  cord 
psychical  powers.  Against  such  a  view  it  must  be  urged  that  the  movements 
of  the  leg  on  the  side  opposite  to  the  stimulus  do  not  occur  until  after  the 
muscles  of  the  leg  on  the  same  side  have  responded.  When  these  responses 
are  inefficient  because  the  leg  is  prevented  from  moving  or  because  they  fail 
to  remove  the  stimulus,  the  prime  fact  remains  that  the  stimulus  continues  to 
act  and  the  diffusion  of  the  impulses  in  the  cord  goes  on,  involving  in  either 
case  the  nerve-cells  controlling  the  muscles  of  the  opposite  leg.  The  adjust- 
ment of  the  reaction  of  the  leg,  on  whichever  side  it  occurs,  is,  however,  far 

1  Bickel  :  Pjluger's  Archiv,  Bd.  Ixvii. 

*  Hering:  Archiv  fur  experimentelle  Pathologic  und  Pharmakologie,  Bd.  xxxviiL 


216  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

from  precise ;  and  although  the  movements  of  the  leg,  when  the  stimulus  is 
applied  far  up  on  the  rump,  differ  from  those  which  follow  the  application 
of  the  stimulus  to  the  lower  part  of  the  thigh,  yet  in  either  case  they  are  very 
wide,  and  in  both  cases  the  foot  is  brushed  across  a  large  part  of  both  the 
rump  and  leg.  Considering,  therefore,  the  rather  general  character  of  these 
movements,  and  the  fact  that  the  movements  of  the  opposite  leg  only  follow 
after  a  continued  stimulus  to  the  leg  of  the  same  side  has  produced  an  inef- 
fective response,  it  is  best  to  explain  the  result  by  the  diffusion  of  the  impulses 
within  the  cord,  leaving  quite  to  one  side  the  psychical  element.  Such  reflex 
actions  are  in  a  high  degree  predictable,  but  in  reality  this  has  li+tle  signifi- 
cance, since  there  is  but  one  general  movement  that  a  frog  in  such  a  condition 
can  make,  whether  the  stimulus  be  applied  to  the  toes  or  the  rump — namely, 
the  flexion  of  the  leg — so  that  under  these  circumstances  the  prediction  of  the 
kind  of  movement  is  a  simple  matter.  The  extent  of  the  contraction  is  related 
to  the  intensity  of  the.  stimulus,  and  is  in  turn  dependent  on  the  excitability 
of  the  central  system,  which  can  be  increased  or  diminished  in  various  ways. 
The  modification  of  the  reaction  as  dependent  on  the  location  of  the  stimulus 
can  be  in  a  measure  predicted,  but  the  modification  is  wanting  in  precision 
just  in  so  far  as  the  movements  themselves  are  wanting  in  this  quality. 

Reflexes  in  Man. — In  the  normal  individual  reflexes  involving  striped 
muscles  are  found  in  the  tendon  reflexes,  of  which  the  knee-kick  is  an  exam- 
ple, in  winking,  and  the  whole  series  of  reflex  modifications  of  respiration, 
such  as  coughing,  sneezing,  and  the  like. 

The  activities  of  the  alimentary  tract  are  examples  of  reflex  actions  involv- 
ing the  contraction  of  muscles  in  deglutition,  defecation,  and  similar  peristaltic 
movements  in  other  hollow  viscera.  These  muscle-fibres  are  for  the  most  part 
unstriped.  So,  too,  micturition,  the  cremaster  reflex,  emission  and  vaginal 
peristalsis,  and  the  reactions  of  parturition  are  to  be  classed  here.  Moreover, 
the  entire  vascular  system  is  controlled  in  this  manner,  the  contraction  and 
distention  of  the  small  arteries  being  in  a  large  measure  in  response  to  stimuli 
originating  at  a  distance ;  while  as  a  third  group,  we  have  the  glands,  the 
activity  of  which  is  almost  entirely  reflex.  For  the  discussion  of  the  various 
reflexes  mediated  by  the  cranial  nerves,  the  reader  is  referred  to  the  special 
sections  dealing  with  the  cardiac,  vasomotor,  and  respiratory  centres  in  tho 
bulb  and  the  pupillary  centres  in  the  mid-brain. 

Periodic  Reflexes. — Not  all  reflexes  are  to  be  obtained  from  the  same 
animal  with  equal  intensity  at  all  times.  In  general,  frogs  in  the  spring-time 
and  in  early  summer,  after  reviving  from  their  winter  sleep,  are  highly 
irregular  in  their  reflex  responses — so  irregular  that  students  are  advised 
not  to  attempt  the  study  of  these  reactions  at  this  season.  On  the  other 
hand,  it  is  during  the  spring  that  mating  occurs,  and  during  this  period  the 
male  clasps  the  female  and  exhibits  the  peculiar  reflex  which  has  already 
been  described.  Comparable  with  this  variation  in  the  frog  must  be  the 
changes  which  occur  in  the  spinal  cords  of  migratory  birds,  which,  both  in 
the  spring  and  in  the  fall,  are  capable  of  such  extended  flights,  or  in  the 


hibernating 
vari;  tlu-ir  h:: 

Variations  in  Diffusibility. — The 
impul-  s  is  d 

:u   the  r  on    the  phy 

in  which 
Sis  happ  i-l.      It  d  that  by  m 

- 
.•linin  aiul  drug-  with  a  similar  pi  ,1  have  i, 

Influence  of  Strychnin. — The  exp-  study,  uing 

shows  the  followi  V  trog  poisoned  1>\  the  inj. 

Imnvn  ;  -rain  is  intaet  or  lias 

the  injection.     T  -mid   to  have  accumulated  in  the 

tin-   <pinal  cord.1     The   p  lit  in  th 

timnlus  will  cause  an  extended  and   prolonged 
•n  ot'  t!  the   diffusion   ot'  imp 

'.ent   to  an  nnnsual  degree.     '1 

appl  uin  to  the  spinal  cord  has   been   carefully  studied   by 

i  and  Mnirhead.-     When  the  Mrvehnin  si>lution  was  applied  locally 
nt  of  the  spinal  cord  of  a  brain  i 

:  of  the  arms  produced  tetan  \s  nf 

i  he  poison  luui  acted  tor  a  time,  of  th  runk 

and  i  ier  hand,  stimulation  of  the  lc_ 

ill.      Siiu-e.  in  order  to  can-   ••ontraetion  of  th- 
«n  trolling  the  mi  -rust  bed 

when  the  stimulus  was  applied  to  the  arm  region  tl 

cells  .iiise  a  tetanic  spasm,  while  in   tli  :i  the 

stim  ipplied  t«>  the  ICL  d  onlv  slightly — the  alt< ' 

in   the  cord   prodm-ed   by  the  drug   must  atleet  some  other  orronp  than   these 

nns  of  the  leirs  «-«>tdi!  --d  l>y  the 

-    :n  of  the  arm,  the  application  ug  to  the 

iial  enlargement  only,  it  iiat  the  ct-nt 

by  the  dorsal  root-fibres  in   the  braehial  region   to 
the   lumbar  enlargement,  art-   pmbablv   atlVct^d  :  and,  further, 

cells  that  the  drng   mi, 

aloiu  vhieh  the  di  -plication 

of  tl  »  between  the 

• 

= 

within  tl:  with 

which  tli- 

>r  only  in  on  now  be  determined. 

i.  ix. 
5  .!/• 


218  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

The  diffusion  of  impulses  in  the  central  system  depends  anatomically  not 
only  on  the  amount  of  branching  among  the  axones  of  the  individual  central 
cells,  but  also  on  the  association  of  many  cells  together,  so  as  to  accomplish 
a  wide  distribution  of  the  impulses.  In  the  case  of  the  afferent  elements, 
as  we  have  seen,  the  diffusion  depends  on  the  branching  of  the  axones  alone. 

Peripheral  Diffusion. — Turning  next  to  the  efferent  system,  we  find  the 
conditions  for  diffusion  dependent  on  the  arrangement  of  several  cells  in 
series.  When  a  group  of  efferent  cells  discharges,  we  know  from  the 
arrangement  of  the  ventral  roots  that  the  impulses  leave  the  cord  mainly 
along  the  fibres  which  comprise  these  roots ;  but  where  the  lateral  root  i& 
present  they  may  also  pass  out  over  it,  as  well  as  over  the  few  efferent  fibres 
found  in  the  dorsal  roots.  These  axones  carrying  the  outgoing  impulses  have 
two  destinations :  (1)  The  voluntary  or  striped  muscle-fibres ;  (2)  the 
sympathetic  nerve-cells,  grouped  in  masses  to  form  the  vagrant  ganglia  (see 
Fig.  93). 

When  the  impulses  are  thus  sent  out  there  is  in  the  case  of  motor  nerves 
no  diffusion,  the  effect  being  limited  to  the  peripheral  distribution  of  the 
efferent  axones,  by  way  of  which  the  impulses  leave  the  central  system. 
The  fibres  going  to  the  voluntary  muscles  form,  however,  but  one  portion, 
which  has  just  been  indicated  as  group  (1).  The  connections  of  the  remain- 
ing group  (2),  passing  to  the  sympathetic  ganglia,  are  still  to  be  examined. 

Sympathetic  System. — Associated  with  the  cerebro-spinal  system  by  the 
efferent  axones,  and  by  these  alone,  is  the  series  of  vagrant  ganglia  and  also- 
of  peripheral  plexuses  containing  ganglion-cells,  which  taken  together  form 
the  sympathetic  system.1  This  system  is  composed  of  neurones  always 
monaxonic,  but  sometimes  with,  and  sometimes  without  well-marked  den- 
drites.  The  cells  are  more  or  less  grouped  in  ganglia,  and  these  ganglia 
interpolated  between  the  efferent  axones  of  the  spinal  nerve-roots  on  the 
one  hand  and  the  peripheral  plexuses  or  terminal  tissues  on  the  other.  The 
number  of  cells  in  the  ganglia  is  greater  than  the  number  of  spinal  root  axones 
going  to  them,  and  hence  their  interpolation  in  the  course  of  the  ventral  root- 
fibres  increases  the  number  of  pathways  toward  the  periphery,  as  is  shown  in 
Fig.  93.  In  speaking  of  the  fibres  concerned,  it  is  desirable  to  distinguish 
between  the  pre-ganglionic,  or  those  originating  in  the  medullary  centres  and 
passing  to  the  ganglia ;  and  the  post-ganglionic  fibres,  or  those  originating  in 
the  cells  of  the  ganglia  and  passing  to  the  periphery. 

Following  the  histological  observations  of  Gaskell,2  previously  quoted,  and 
the  physiological  studies  of  Langley,3  an  outline  of  the  relations  of  the  sym- 
pathetic cells,  based  on  the  arrangement  found  in  the  cat,  is  briefly  as  follows : 

Pre-ganglionic  fibres — i.  e.,  those  growing  out  of  cell-bodies  located  in  the 

1  Gaskell  :  Journal  of  Physiology,  1885,  vol.  vii.  ;  von  Kolliker,  "  Ueber  die  feinere  Anatomie 
und  die  physiologische  Bedeutnng  des  sympathischen  Nervensystems,"  Verhandlungen  Gesell- 
schaft  deutscher  Naturforscher  und  Aertze,  194,  Allgemeiner  Theil,  1894.  2  Loc.  cit. 

3  Langley  :  "A  Short  Account  of  the  Sympathetic  System,"  Physiological  Congress,  Berne,. 
1895. 


.. 


. 

' 
• 


• 


CENTRAL    NERVOUS  SYSTEM.  219 

cord — arise  from  the  first  thoracic  to  the  fourth  or  fifth  lumbar,  and  from  these 
segments  only  (Gaskcll).  The  fibres  are  mcdullatcd.  Lan^ley's  experiments 
indicate  that  no  sympathetic  cell  sends  a  branch  to  any  other  sympathetic 
cell,  but  other  observers  do  not  admit  his  results  as  conclusive.  It  has  been 
shown  that  the  pre-ganglionic  fibres  are  interrupted  in  the  ganglia.  The 
post-ganglionic  fibres  are  in  part  medullated,  though  sometimes  medullation 
occurs  only  at  intervals,  but  in  the  main  they  are  gray  or  unmedullated. 

The  cerebro-spinal  axones  end  in  the  ganglia  in  such  a  manner  that  the 
branches  of  the  pre-ganglionic  axone  are  distributed  to  a  number  of  the 
ganglion  cell-bodies,  and  these  cells  in  turn  send  their  axones  either  directly 
to  the  peripheral  structures  controlled  by  the  sympathetic  elements  or  to  the 
plexuses  such  as  are  to  be  found  in  the  intestine  and  about  the  blood-vessels. 

The  same  pre-ganglionic  fibre  may  have  connections  with  several  cells  in 
one  ganglion,  or,  by  means  of  collaterals,  connect  with  one  or  more  cells  in  a 
series  of  ganglia  (Langley). 

Manner  of  Diffusion. — It  has  been  found  that  while  the  cells  in  a  sympa- 
thetic ganglion  are  so  arranged  that  one  pre-ganglionic  fibre  may  be  in 
connection  with  a  group  of  cells,  and  thus  the  impulses  which  pass  out  of  the 
ganglion  be  more  numerous  than  those  which  entered  it,  yet  the  several 
groups  of  cells  within  the  ganglion  are  not  connected.  In  the  peripheral 
plexuses  there  appears  to  be  a  different  arrangement.1 

It  has  been  observed  upon  stimulation  of  the  branches  of  the  coeliac  plexus 
in  the  dog  that  the  several  branches,  though  unlike  in  size,  bring  about  nearly 
the  same  quantitative  reaction  in  the  constriction  of  the  veins,  from  which  we 
infer  that  though  entering  the  peripheral  plexus  by  different  channels,  the 
impulses  find  their  way  to  the  same  elements  at  the  end,  owing  to  a  multi- 
plicity of  pathways  within  the  plexus.2 

Experiments  with  strychnin  on  the  more  proximal  sympathetic  ganglia  do 
not  show  any  increased  diffusibility  following  the  application  of  the  drug ;  but, 
on  the  other  hand,  Langley  and  Dickinson  3  have  shown  that  nicotin  applied 
to  the  superior  cervical  sympathetic  ganglion  of  the  cat  produces  a  condition 
whereby  electrical  stimulation  below  the  ganglion,  which  in  the  normal  animal 
is  followed  by  dilatation  of  the  pupil,  is  without  effect.  Since  the  application 
of  the  drug  to  the  nerve-fibres  on  either  side  of  the  ganglion  is  ineffective, 
when,  at  the  same  time,  the  application  to  the  ganglion  itself  is  effective,  it  i& 
inferred  that  the  drug  acts  by  altering  some  peculiar  relation  existing  within 
the  ganglion,  and  the  relation  which  is  assumed  to  be  thus  modified  is  that 
between  the  fibres  terminating  in  the  ganglion  and  the  cells  which  they  there 
control.  The  passage  of  the  efferent  impulses  through  other  sympathetic 
ganglia  is  likewise  blocked  by  nicotin. 

Evidence  for  Continuous  Outgoing-  Impulses. — Under  normal  condi- 
tions striped  and  unstriped  muscular  tissues  are  always  in  a  state  of  slight 

1  Berkeley:  Anatomischer  Anzeiger,  1892. 

2  Mall:  Archivf.  Anatomie  u.  Physiologic,  1892. 

3  Proceedings  of  the  Royal  Society,  1889,  vol.  xlvi. 


220  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

contraction  or  tonus.     When  the  nerves  controlling  any  such  set  of  muscles 
are  cut,  or  their  central  connections  injured,  the  muscles  at  first  relax. 

If  a  frog  be  hung  up  vertically  after  removing  the  brain,  the  cord  remain- 
ing intact,  it  is  found  that  the  legs  are  slightly  flexed  at  the  hip  and  knee. 
If  now  the  sciatic  nerve  be  cut  upon  one  side,  the  leg  on  the  side  of  the 
section  hangs  straighter,  indicating  that  the  muscles  have  relaxed  a  little  as 
the  result  of  the  section  of  the  nerve ;  if,  in  the  same  animal,  the  smaller 
arteries  in  the  web  of  the  foot  be  examined  both  before  and  after  the  section, 
it  is  found  that  after  the  section  they  have  increased  in  diameter.  Con- 
versely, artificial  stimulation  of  the  peripheral  stump  causes  a  contraction 
of  the  vessels,  but  it  is  not  possible  in  so  rough  a  way  to  imitate  the  tonic 
contraction  of  the  skeletal  muscles. 

It  is  inferred  from  these  experiments  that  normally  there  pass  from  the 
central  system  along  some  of  the  nerve-fibres  impulses  which  tend  to  keep 
the  muscles  in  a  state  of  slight  contraction.  Destruction  of  the  entire  cord 
abolishes  all  outgoing  impulses,  ancl  produces  a  complete  relaxation  of  these 
muscles. 

Though  the  intensity  of  these  outgoing  impulses  is  normally  always  small, 
yet  it  is  subject  to  significant  variations.  The  difference  between  the  tone 
of  the  muscles  of  an  athlete  in  prime  condition  and  those  of  a  patient  recover- 
ing from  a  prolonged  and  exhausting  illness  is  easily  recognized,  and  this 
difference  is  in  a  large  measure  due  to  the  difference  in  the  intensity  of  the 
impulses  passing  out  of  the  cord.  Among  the  insane,  too,  the  variations  in 
this  tonic  condition  follow  in  a  marked  way  the  nutritive  changes  in  the 
central  system,  and  both  facial  and  bodily  expression  have  a  value  as  an 
index  of  the  strength  and  variability  of  those  impulses  on  which  the  tone  of 
the  skeletal  muscles  depends.  Indeed,  so  wide  in  the  insane  is  the  variation 
thus  brought  about  that  when  the  expressions  of  an  individual  at  one  time  in 
a  phase  of  mental  exaltation,  and  at  another  in  that  of  mental  depression,  are 
compared,  it  appears  hardly  possible  that  they  can  be  those  of  the  same  person. 

This  continuous  outflow  of  impulses  from  the  central  system  is  indicated 
also  by  the  continuous  changes  within  the  glands,  and  the  variations  in  these 
metabolic  processes  according  to  the  activities  of  the  central  system. 

Rigor  Mortis. — Even  in  the  very  act  of  dying  the  influence  of  these 
impulses  can  be  again  traced.  The  death  of  the  central  nerve-tissues  being 
expressed  as  a  chemical  change,  causes  impulses  to  pass  down  the  efferent 
nerves,  and  these  impulses  modify  those  chemical  changes  which,  in  the 
muscles  of  a  frog's  leg,  for  example,  lead  to  rigor  mortis.  It  thus  happens 
that  a  frog  suddenly  killed  and  then  left  until  the  onset  of  rigor,  will  under 
ordinary  circumstances  show  rigor  at  about  the  same  time  in  both  legs.  If, 
however,  the  sciatic  nerve  on  one  side  be  cut  immediately  after  the  death  of 
the  animal,  the  beginning  of  rigor  in  that  leg  is  much  delayed,  thus  showing 
that  the  nervous  connection  is  an  important  factor  in  modifying  the  time  of 
this  occurrence  (Hermann). 

The  Nervous  Background. — We  return  now  to  the  conditions  which 


CENTRAL   NERVOUS  SYSTEM.  221 

modify  the  spread  of  the  impulses  within  the  central  system,  when  this  sys- 
tem is  represented  by  the  spinal  cord  of  a  reflex  frog.  Admitted  I  v,  then-  is 
in  the  rase  chosen  but  a  fraction  of  the  central  system.  Jt  has  been  shown  that 
all  incoming  impulses  tend  to  spread  over  a  large  part  of  the  central  system. 
In  a  reflex  frog,  therefore,  the  cord  is  cut  off  from  the  remote  effect  of  im- 
pulses which  normally  enter  the  system  by  way  of  cells  located  in  the  portion 
removed.  Moreover,  in  the  complete  nervous  system  the  incoming  impulses 
tend  to  be  transmitted  to  the  cephalic  end,  and  in  some  measure  give  rise  to 
impulses  returning  within  the  central  system  and  affecting  the  efferent  cells. 
In  a  fragment  of  the  central  system  like  the  cord  such  impulses  taken  up  by 
the  central  cells  must  pass  so  far  as  the  axones  are  intact;  but  as  these  for  the 
most  part  end  at  the  level  of  the  section,  such  impulses  are  lost,  in  the  physi- 
ological sense,  at  that  point. 

The  fact,  therefore,  that  the  experiments  with  spinal  reflexes  are  con- 
ducted on  a  portion  of  the  central  system  has  two  important  physiological 
consequences.  In  the  first  place,  there  are  wanting  incoming  impulses,  direct 
or  indirect,  from  the  portion  removed ;  on  the  other  hand,  through  the  sec- 
tion of  the  afferent  axones,  in  their  course  within  the  central  system,  there  is 
a  direct  diminution  in  the  number  of  the  pathways  by  which  the  impulses 
arriving  at  the  cord  may  be  there  distributed.  It  is  most  probable  that  in 
the  frog,  at  least,  the  reduction  of  the  central  mass  does  not  so  much  dimin- 
ish the  number  of  pathways  by  which  the  impulses  may  be  immediately 
distributed  by  way  of  the  afferent  and  central  elements,  as  it  diminishes  the 
number  of  impulses  which  by  way  of  the  portion  removed  arrive  at  the 
efferent  cells  and  modify  their  responsiveness. 

The  modification  of  the  responsive  cells  tinder  more  than  one  impulse  is 
well  illustrated  by  an  experiment  of  Exner : l  A  rabbit  was  so  prepared  that 
an  electric  stimulus  could  be  applied  to  the  cerebral  cortex  at  a  point  the 
excitation  of  which  caused  contraction  of  certain  muscles  of  the  foot.  One 
of  these  muscles  was  attached  to  a  lever  so  that  its  contraction  could  be 
recorded,  and  a  second  electrode  applied  to  the  skin  of  the  foot  oyerlying  the 
muscle.  The  discharging  efferent  cells  in  the  cord  were  in  this  case  subject 
to  impulses  from  two  directions,  one  from  the  cortex  and  one  from  the  skin 
of  the  foot.  With  a  current  of  given  strength  stimulation  of  the  cortex  alone 
caused  a  contraction  of  the  muscle,  and  stimulation  of  the  skin  of  ilie  foot 
alone,  a  similar  contraction.  When  both  were  stimulated  simultaneously  the 
extent  of  the  contraction  was  greater  than  when  either  was  stimulated  alone. 
If  now  the  strength  of  the  stimulus  applied  to  the  skin  of  the  paw  was  so 
reduced  that  alone  it  was  inefficient,  then  a  stimulus  from  the  cortex  which 
produced  a  reaction  as  indicated  by  the  first  cortical  stimulus  in  Fig.  94 
(-4,  a),  put  the  efferent  cells  in  such  a  condition  that  the  stimulus  from  the 
skin  (A,  6,  Fig.  94),  applied  within  0.6  of  a  second,  produced  a  second  con- 
traction of  the  muscle,  although  alone  the  stimulus  from  the  skin  had  proved 
inefficient.  Here  the  first  efficient  stimulus  from  the  cortex  had  rendered 
1  Exner:  Archivfilr  die  gesammte  Physiologic,  Bd.  xxvii. 


222  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  discharging  cell  for  a  short  period  of  time  more  excitable.  In  the  same 
figure  the  record  shows  that  if  a  longer  interval — here  more  than  three  seconds — 
be  allowed  to  elapse,  then  the  second  stimulus  from  the  skin  remains  im'tti- 
cient.  A  similar  relation  between  the  two  incoming  impulses  is  also  found 
to  hold  when  the  stimulus  from  the  skin  is  made  to  precede.  The  curv<  />, 
Fig.  94,  shows  the  results  when  both  stimuli  are  inefficient.  In  this  the 
stimuli  (I)  and  a)  produce  no  effect  when  given  several  seconds  apart,  but  when 
they  occur  within  a  short  interval  (l*f  and  a') — in  this  case  0.13  of  a  second 
— a  contraction  of  the  muscle  follows.  These  various  experiments,  taken 
together,  show  in  a  beautiful  way  that  in  the  cases  chosen  the  two  sets  of 
impulses  tend  to  reinforce  each  other,  whether  they  are  efficient  or  inefficient, 
and  without  regard  to  the  order  in  which  they  come. 

This  relation  between  the  discharging  cell  and  those  by  which  it  is 
stimulated  can  be  illustrated  in  still  another  way.  It  was  observed  by 
Jendrassik  l  that  when  a  patient  was  being  tested  for  the  height  of  his  kmv- 
kick,  a  voluntary  muscular  contraction,  or  an  extra  sensory  stimulus,  occur- 


Movement  of  paw. 


Wiin  illation  of  cortex.        Aa' 


b'      "  paw. 


Time  in  seconds. 


* 


» 


r\rVHrHr\r\r\r-\j 


FIG.  94.— To  show  the  reinforcing  influence  of  stimuli  applied  to  the  cerebral  cortex  and  to  the  skin 
of  the  paw,  on  the  movements  of  the  paw  of  a  rabbit  (Exner).  The  arrows  indicate  the  direction  in 
which  the  curves  are  to  be  read.  In  curve  A  the  cortical  stimulus  at  o  causes  a  movement  of  the  paw. 
Dermal  stimulus,  within  a  second,  at  b  causes  a  movement  of  the  paw.«  Cortical  stimulus  at  o'  causes  a 
movement  of  the  paw.  Dermal  stimulus  several  seconds  later  at  V  is  ineffective.  In  curve  If  dermal 
stimulus  at  b  is  ineffective.  The  cortical  stimulus  at  a  several  seconds  later  is  also  ineffective.  The 
dermal  stimulus  at  V  is  ineffective,  but  if  followed  within  0.18  second  by  a  cortical  stimulus  at  a' a  move- 
ment of  the  paw  occurs. 

ring  about  the  same  time  that  the  tendon  was  struck,  had  the  effect  of 
increasing  the  height  of  the  kick.  This  relation  was  studied  in  detail  by 
Bowditch  and  Warren,2  and  they  were  able  with  great  exactness  to  measure 
the  interval  between  the  contraction  of  the  muscle  used  for  reinforcement 
and  the  time  at  which  the  tendon  was  struck.  The  curve  shown  in  Fig.  95 
represents  the  results  of  these  experiments.  It  indicates  that,  up  to  0.4  of  a 
second,  the  closer  together  these  two  stimuli  occur  the  greater  the  reinforce- 
ment. At  an  interval  of  0.4  of  a  second  no  effect  is  produced  by  the  muscu- 
lar contraction.  Increasing  the  interval  only  very  slightly  has,  however,  the 
effect  of  greatly  diminishing  the  height  of  the  knee-kick — i.  e.,  decreasing 
the  strength  of  the  discharge  of  the  efferent  cells — and  this  effect  is  not  lost 
until  the  interval  is  increased  to  1.7  seconds,  when  the  voluntary  muscular 
contraction  ceases  to  modify  the  response.  A  given  efferent  cell  is  thus 
modified  in  its  discharge  according  to  the  several  stimuli  that  act  upon  it. 

1  Deutsches  Archiv  fur  klinisehe  Medizin,  Bd.  xxxiii. 
»  Journal  of  Physiology,  1890,  vol.  xi. 


Effects  of  Afferent  Impulses.- 

. 

!!<•<!. 

.ito  a  sol; 
sponsive; 

En  this  cai  on  of  tl  of 

-pcciaj    fi 

•lie  .-kin,  and    th  .11   of  those  inipi  ~es  a 

•iiition    in    central    P-JV.I,-  which    can    thus   be  a<-« 

by  cutting  oil'  the  afferent  im:  the  skin 

•  \v  diminution  in  the<c  inmnl-cs  althoiiLfh  all 


40- 

30- 

20- 

K) 

0 

10-1 

EO- 

30- 


V  M 


ft    ,  - 


or  0.2-  o>r 


0:7- 


ur 


millimeters,  the  amount  by  which  the  "  reinforced  "  knee-kick  vark<l  from  the 
normal,  the  ;  nich  is  represented  by  the  horizontal  line  at  0,  "normal."    '\  tervals 

.£  between  the  clinching  of  the  hand  (which  constituted  the  reinforcement)  and  the  tap 
tendon  are  marked  below.    The  reinforcement  is  greatest  when  the  two  events  are  nearly  simulta 

nterval  of  0.4"  it  amounts  to  nothing;  during  the  next  0.6"  the  height  of  the  kick  is  actually 
diminished  the  longer  the  interval,  after  which  the  negative  reinforcement  tends  to  disappear;   and 
allowed  to  elapse  the  height  of  the  kick  ceases  to  be  affected  by  the  clinching  of  the  hand 
•  rren). 

slow  changes  are  much  more  likely  to  be  accompa  some  sort  of 

reby  other  afferent  impulses  in  a  measure  take  the  place  of 

been  suppressed.     The  loss  of  these  impi  ic-h  rouse 

'isuallx  nclition  than    i  iiitri- 

*«  kept  in  view. 

Inhibition.  —  Evidence  if  Dilating  to  show  that  all  the  a  -lies 

of  t!  -heir  nerves  in  two  oppo- 

•esses   in  the  gland.-  res,  or  muscles  <•: 

-es,  nerves    which   cause  inhibition   ;n   . 

' 


:  A  nmltBe  Patkologit  trmakologif,,  l& 

h      !  ..y,  1899. 


224  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

chemical  changes  in  the  inhibited  muscle  of  the  heart  are  peculiar  and 
different  from  those  occurring  during  excitation.  For  instance,  Gaskell* 
found  that  a  positive  variation  of  the  muscle  current  occurred  in  the  hearts 
of  both  the  tortoise  and  crocodile  upon  stimulating  the  peripheral  end  of  the 
vagus ;  while  stimulation  of  the  accelerator  nerves  caused  the  usual  negative 
variation.  Further,  Gaskell  pointed  out  that  during  the  inhibition  of  the 
heart-muscle  the  anabolic  processes  were  in  excess,  so  that  the  cessation  of 
inhibition  was  followed  by  an  increase  in  the  strength  of  the  heart-beats. 

If  we  now  turn  to  the  observations  bearing  on  inhibition  in  the  central 
nervous  system,  there  are  to  be  found  numerous  experiments,  of  which  the 
following  is  a  type :  Let  one  leg  of  a  reflex  frog  be  stimulated  by  pinching 
it,  or  by  dipping  the  toe  in  weak  acid  :  a  withdrawal  of  the  stimulated  leg 
will  follow.  Now  repeat  the  experiment,  at  the  same  time  pinching  or  other- 
wise stimulating  the  skin  on  the  opposite  leg.  It  will  then  be  found  that 
either  the  latent  period  of  the  reacting  leg  is  much  prolonged  or  that  the 
reaction  fails  completely.  This  is  a  very  simple  example  of  a  type  of 
inhibition  which  is  continually  occurring. 

The  inhibitory  effects  are,  however,  not  limited  to  the  motor  responses  of 
the  central  system.  It  is  an  observation  of  the  ancients  that  the  greater 
obscures  the  lesser  pain,  and,  in  a  general  way,  all  strong  sensations  prevent 
the  appreciation  of  the  weaker  ones,  whether  they  be  in  the  terms  of  the 
same  or  of  a  different  sense. 

Within  the  central  nervous  system  very  remarkable  examples  of  inhibitory 
of  lenomena  have  been  investigated,  chiefly  by  Sherrington.2  Boubnoff  and 
wleidenhain3  were  the  first  to  record  the  observation  that  under  certain 
Conditions  stimulation  of  the  cerebral  cortex  might  cause  a  relaxation  of 
Fsome  extensor  muscles  of  the  limbs  when  these  were  in  a  state  of  tonic 
contraction. 

Sherrington  was  able  to  show  that  the  stimulation  of  the  cortical  area  for 
the  flexors  of  the  arms  also  gave  rise  to  impulses  leaving  the  cortex  and 
causing  a  (inhibition)  relaxation  of  the  antagonistic  extensors. 

On  stimulating  the  cortical  area  for  the  extensor  muscles  a  corresponding 
relaxation  of  the  flexors  could  be  observed:  Thus  the  cortical  area  for  the 
contraction  of  a  given  group  of  muscles  coincides  with  the  area  for  the  inhi- 
bition of  the  group  antagonistic  to  it.  Sherrington  has  also  demonstrated  the 
important  role  of  this  inhibitory  process  in  mediating  muscular  co-ordination 
shown  in  movements  of  the  eye.  When  all  the  muscles  of  the  eye  are  para- 
lyzed, the  eyeball  held  by  the  connective  tissues  about  it  looks  straight 
ahead.  Sherrington  cut  the  nerves  to  all  the  muscles  of  the  left  eyeball 
(monkey)  except  the  external  rectus.  Under  these  conditions  the  eye,  when 
at  rest,  looked  toward  the  left.  Stimulation  of  the  cortical  centers,  which 
cause  a  conjugate  deviation  of  both  eyes  to  the  right,  was  followed  by  a 

1  Gaskell :  Journal  of  Physiology,  vol.  vii. 

2  Sherrington  :  76id.,  vol.  xvii. 

3  Boubnoff  und  Heidenhain:  Pftiiger's  Archiv,  xxvi. 


CENTRAL    NERVOUS   S  VST  KM.  225 

movement  of  the  operated  eye  toward  the  median  plane  (ilic  ri^lit  i,  and  to 
(he  position  in  which  it  would  he  held  by  the  elastic  tissues  alon<-.  This 
conld  he  explained  only  through  a  relaxation  or  inhibition  of  the  external 
rcctns  muscle,  as  a  consequence  of  the  cortical  stimulation.  Further  experi- 
ments support  the  explanation,  and  also  show  that  the  cells,  the  activity  of 
which  is  thus  inhibited,  must  lie  below  the  cerebral  cortex,  lor  the  inhibition 
follows  when  the  fibre-bundles  below  the  cortex  are  directly  stimulated,  the 
cortex  having  been  first  removed. 

The  general  bearing  of  these  results  is  of  the  greatest  importance.  As 
has  been  pointed  out  by  Hughlings-Jackson,1  damage  of  any  sort  to  a  portion 
of  the  nervous  system  may,  in  the  simplest  case,  decrease  the  activity  of  the 
group  of  neurones  controlled  by  the  damaged  part  by  cutting  off  the  stimulat- 
ing impulses  from  them  ;  or,  on  the  other  hand — and  this  is  often  overlooked— 
it  may  permit  them  to  become  abnormally  active  by  the  stoppage  of  some 
impulses  exerting  an  inhibitory  control.  Further,  whether  impulses  from  a 
given  set  of  cells  shall  prove  stimulating  or  inhibitory  depends  on  the  o///o- 
impulses  affecting  the  receiving  cell  group,  and  on  the  time  relations  between 
these  several  sets.  This  consideration  serves  to  indicate  the  complex  rela- 
tions which  may  underlie  the  manifestations  of  disease  in  the  central  nervous 
system. 

As  to  the  mechanism  for  these  inhibitory  reactions,  it  can  be  safely  said 
that  for  the  most  part  the  effects  are  not  dependent  on  the  existence  of  a 
special  class  of  inhibitory  nerves,  and  the  most  we  can  think  of  structurally 
is  a  different  but  not  necessarily  constant  dendritic  pathway  for  the  cellu- 
lipetal  impulses  causing  inhibition. 

0.    REACTIONS  INVOLVING  THE  ENCEPHALON. 

On  attempting  to  distinguish  between  a  voluntary  and  a  reflex  act  from  the 
physiological  standpoint  we  find  the  chief  difference  to  be  that  the  voluntary 
act  is  not  predictable,  because,  according  to  the  capabilities  of  the  animal,  it 
may  be  more  variable  in  form  than  is  the  reflex  response,  and  also  because, 
instead  of  necessarily  occurring  within  a  short  interval  after  the  stimulus,  as 
does  the  reflex,  the  voluntary  response  may  be  delayed  even  for  years. 

Reflexes  have  been  illustrated  by  the  reactions  from  a  portion  of  the  spinal 
cord.  It  is  to  be  remembered,  however,  that  any  of  the  sensory  cranial  or 
spinal  nerves  can  serve  as  a  pathway  for  the  afferent  impulses,  and  any  of  the 
groups  of  efferent  cells  situated  in  the  ventral  horns  or  their  homologues  in 
the  brain  stem,  can  carry  the  efferent  impulses  needed.  Further,  it  must  be 
remembered  that  it  is  these  same  afferent  cells  which  always  furnish  the  first 
set  of  impulses,  and  the  efferent  cells  controlling  the  muscles  and  glands 
which  furnish  the  last  set  of  impulses  in  both  reflex  and  voluntary  reactions. 
The  processes  then  which  distinguish  the  two  forms  of  reactions  must  take 
place  in  the  central  cells.  We  turn,  therefore,  to  the  nervous  connections  of 
the  encephalon  with  the  cord,  since  it  is  by  means  of  these  connections  that 

1  Hugh  lings-  Jackson  :  Lancet,  1898,  vol.  i. 
VOL.  II.— 15 


226 


AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


the  impulses  travel  to  the  many  series  of  central  cells  which  are  concerned  in 
the  simplest  voluntary  responses. 

For  the  most  complex  voluntary  reactions  the  entire  central  system  is 
necessary,  and  especially  the  cortex  of  the  cerebral  hemispheres,  while  it  has 

already  been  shown  that  the  impulses 
which  cause  reflex  actions  can  make 
their  circuit  in  a  very  limited  portion 
of  the  spinal  cord.  In  the  case  of 
voluntary  reactions  the  impulses  take 
a  longer  pathway  and  involve  a  larger 
series  of  central  nerve-elements,  since 
from  the  point  at  which  they  enter  the 
system  they  must  pass  to  the  cephalic 
end  and  back  again  to  the  efferent 
elements.  At  the  same  time,  in  a 
voluntary  reaction,  a  greater  number 
of  impulses  combine  to  modify  the 
discharge  from  the  efferent  cells. 

In  order  that  the  encephalon  may 
be  included  in  the  pathway  of  the  im- 
pulses entering  the  cord,  it  is  necessary 
that  pathways  formed  by  axones 
should,  on  the  one  hand,  extend  up 
to  the  encephalon,  and,  on  the  other, 
back  from  it  to  the  cord. 

Fig.  96  indicates  how  the  first 
part  of  this  path  is  composed  of  the 
afferent  elements  of  the  dorsal  spinal 
nerve-roots.  The  long  paths  in  the 
dorsal  funiculi  of  the  cord  are  formed 
by  the  ascending  branches  of  the  af- 
ferent axones,  and  these  terminate,  for 
the  most  part,  about  the  cell-bodies 
which  form  the  nuclei  of  the  dorsal 
funiculi  at  the  junction  of  the  cord 
and  bulb.  From  these  nuclei  a  second  series  of  axones  passes  out,  decussates 
at  once,  and  then  the  axones  pass  forward  in  the  medial  lemniscus  to  find 
a  second  ending  in  the  ventral  cell  masses  of  the  thalamus,  or  possibly  to 
continue  up  to  the  cortex.  From  this  point  a  third  group  of  neurones,  with 
their  cell-bodies  in  the  thalamus,  send  out  their  axones  to  the  cerebral  cortex. 
The  cranial  afferent  nerves,  which  are  not  nerves  of  specific  sensations 
(i.  e.,  the  fifth,  the  vestibular  portion  of  the  eighth,  the  ninth,  and  tenth), 
probably  have  corresponding  connections  in  the  central  system. 

The  impulses  which  are  brought  in  by  the  afferent  fibres  also  pass,  in  a 
large  measure,  to  cells  in  the  dorsal  horn  of  the  spinal  cord  by  way  of  the 


FIG.  96.— Schema  showing  the  smaller  pathway 
of  the  sensory  impulses.  On  the  left  side,  S,  S', 
represent  afferent  spinal  nerve-fibres ;  C,  an  afferent 
cranial  nerve-fibre.  This  fibre  in  each  case  termi- 
nates near  a  central  cell,  the  axone  of  which 
crosses  the  middle  line  and  ends  in  the  oppo- 
site hemisphere.  The  interruption  of  the  larger 
pathway  in  the  thalamus  is  not  indicated  (van 
Gehuchten). 


CENTRAL   NERVOUS  SYSTEM.  227 

collaterals  and  the  ascending  branches  of  the  allen-nt  a  \ones,  \\hi.-h  end 
before  they  reach  the  nuclei  of  the  dorsal  fimieuli.  The  cells  in  the  dorsal 
horns  send  their  axones  in  large  numbers  across  the  cord  to  the  lateral 
columns  of  the  opposite  side,  to  reach  the  thalamus  through  the  medial 
lemniscus,  and  thence  to  the  cortex. 

Of  the  many  disputed  points  in  this  pathway,  the  most  important  relates 
to  the  interruption  of  the  axones  of  the  lemniscus  in  the  ventral  portion  of  the 
thalamus.  The  recent  researches  of  Tschermak1  indicate  that  probably  there 
are  two  groups  of  neurones  concerned,  one  of  which  sends  its  axones  without 
interruption  to  terminate  in  the  cortex,  while  the  axones  from  the  other  are 
interrupted  at  the  level  of  the  thalamus.  The  latter  group  is  the  larger  and 
probably  the  more  important  for  the  general  reactions  of  the  central  system. 

The  pathways  which  are  here  sketched  have  been  worked  out  mainly  by 
the  study  of  degenerations,  in  large  part  resulting  from  experimental  lesions. 

When  the  dorsal  roots  are  crushed  or  sectioned  between  the  spinal  gan- 
glion and  the  cord,  the  prolongation  of  the  afferent  fibre  within  the  cord 
degenerates  throughout  its  entire  extent.  The  degeneration  extends  in  the 
dorsal  columns  down  the  cord  two  or  three  centimeters  from  the  level  of  the 
section,  and  also  up  the  cord  as  far  as  the  nuclei  of  the  dorsal  columns  located 
at  the  commencement  of  the  bulb.  If  the  section  is  made  near  the  caudal 
end,  the  degeneration  may  in  consequence  run  through  the  entire  length  of 
the  cord.  Moreover,  it  occurs  mainly  on  the  side  of  the  cord  to  which  the 
sectioned  nerves  belong.  Take,  for  example,  the  area  of  degeneration  caused 
by  the  section  in  a  dog  of  the  dorsal  roots  on  the  left  side  between  the  sixth 
lumbar  and  second  sacral  nerves.  The  degeneration  in  the  lower  lumbar 
region  is  represented  in  Fig.  97,  a,  in  the  upper  lumbar  region  in  6,  and  in 
the  thoracic  region  in  c  and  d.  The  section  e  passes  through  the  cervical 
enlargement.  On  passing  cephalad  the  area  of  degeneration  becomes  smaller. 
This  is  interpreted  to  mean  that  all  along,  between  the  caudal  and  cephalic 
limits,  fibres  are  given  off  from  the  main  bundle  to  the  intermediate  levels 
of  the  cord.  Here  is  evidence  of  an  arrangement  that  is  always  to  be  kept 
in  view.  Though  a  number  of  fibres  among  those  degenerating  after  section 
of  the  dorsal  roots  may  run  the  longer  course  to  the  bulb,  the  larger  portion 
run  a  short  or  an  intermediate  course,  and  are,  therefore,  distributed  at  dif- 
ferent points  between  the  termini.  Injury  to  the  dorsal  roots  at  di  Hi  •rent- 
levels  shows,  moreover,  that  the  fibres  from  a  given  level  which  run  the 
length  of  the  dorsal  columns  do  not  mingle  indiscriminately  with  those  from 
other  levels,  but  form  a  bundle ;  and  as  that  bundle  passes  cephalad  in  the 
cord,  it  tends  to  lie  nearer  the  middle  line.  Hence  in  the  upper  cervical 
cord,  where  the  bundles  from  all  levels  are  present,  a  cross-section  >hmvs  the 
bundles  which  entered  lowest  down  to  be  located  nearest  the  dorsal  surface 
and  the  median  septum. 

From  these  relations  it  is  evident  that  comparatively  few  of  the  dorsal 
root-fibres  run  the  entire  length  of  the  dorsal  fimiculi,  since  the  majority 
1  Tschermak:  Archivfur  Anatomieund  Physiologic,  Anat.  Abthl.,  1898,  S.  291-400. 


228  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

terminate  somewhere  between  this  extreme  limit  and  their  point  of  en- 
trance. 

Since  the  fibres l  in  the  dorsal  funiculi  of  the  cord  degenerate  on  destruc- 
tion of  the  dorsal  roots,  it  is  inferred  that  they  must  be  morphologically  con- 
tinuous with  certain  fibres  in  the  roots,  and,  since  the  dorsal  roots  are  afferent 
pathways,  they  too  must  form  part  of  the  afferent  pathway  in  the  cord. 

The  continuation  of  the  other  paths  for  the  afferent  impulses  must,  how- 
ever, be  formed  by  the  axones  of  the  central  cells  with  which  the  dorsal 
root-fibres  connect  as  they  terminate  at  the  several  levels  of  the  cord. 


FIG.  97.— The  sections  are  from  five  levels  of  the  spinal  cord  of  a  dog.  The  dorsal  roots  on  one  side 
had  been  sectioned  in  two  groups :  first,  the  twenty-eighth,  twenty-seventh,  and  twenty-sixth  spinal 
nerves  ;  and  second,  the  twenty-second  and  twentieth  :  a,  shows  a  schematic  picture,  representing  a 
cross-section  of  the  spinal  cord  taken  just  below  the  level  of  the  twenty-second  spinal  root.  The  black 
spot  represents  the  principal  bundle  of  degenerated  fibres  as  it  appears  in  the  dorsal  column.  At  this 
level  the  bundle  is  rather  near  the  median  septum,  but  if  sections  further  caudad  were  examined  in  series 
it  would  be  found  that  the  bundle  constantly  approached  the  dorsal  horn,  and  finally  fused  with  it  at  the 
level  where  the  injured  nerves  joined  the  cord.  If,  on  the  other  hand,  a  section  be  taken  from  the  level 
between  the  twenty-second  and  the  twentieth  nerves— that  is,  after  passing  the  level  at  which  the  second 
group  of  sectioned  nerves  joins  the  cord— there  are  to  be  seen  two  bundles  of  degenerated  fibres  marked 
by  black  spots  in  the  sections  (b,  c,  d).  The  last  bundle  to  enter  the  cord,  and  the  one  lying  nearer  the 
dorsal  horn,  is,  of  course,  formed  by  the  degenerating  fibres  from  the  second  group  of  roots.  In  the  sec- 
tions c,  d,  e,  taken  respectively  at  the  level  of  the  eighteenth  nerve,  the  middle  of  the  thoracic  cord  and 
the  cervical  enlargement,  it  is  seen  that  both  degenerated  bundles  grow  smaller ;  that  they  shift  toward 
the  median  septum  and  approach  one  another ;  and,  finally,  that  they  completely  fuse  in  the  cervical 
region  (e). 

Degeneration  after  Hemisection  of  Cord. — Upon  hemisection  of  the 
cord  involving  one  lateral  half,  the  ascending  fibres  which  degenerate  appear 
in  the  dorsal  columns,  in  the  dorso-lateral  ascending  tract,  and  in  the  ventro- 
lateral  ascending  tract.  The  number  of  degenerated  fibres  is  large  on  the 
side  of  the  lesion,  but  on  the  opposite  side  there  are  also  degenerated  fibres  in 
all  these  localities,  although  they  are  by  no  means  so  numerous.  It  is  inferred 
that  all  the  fibres  which  thus  degenerate  form  paths  for  the  afferent  impulses. 

The  impulses  which  come  in  over  a  dorsal  root  on  one  side  can,  therefore, 
find  their  way  cephalad  either  by  the  direct  continuations  of  the  dorsal  root- 
fibres  running  in  the  dorsal  column  mainly  on  the  side  of  the  lesion  or 
through  the  interpolation  of  central  cells,  the  axones  of  which  appear  degen- 

1  The  bundles  of  "endogenous  fibres"  not  arising  from  spinal  ganglion-cells  are  neglected 
here. 


CENTRAL   NERVOUS  SYSTEM.  229 

erated  in  both  lateral  columns,  but  more  numerously  on  the  side  of  the 
lesion.1 

The  tracts  which  undergo  secondary  degeneration  after  this  treatment 
include,  therefore,  those  formed  by  the  axones  arising  from  central  cells. 
These  neurones  have  their  cell-bodies  arranged  in  columns  running  the  length 
of  the  cord.  In  the  neighborhood  of  these  columns  some  of  the  dorsal  root- 
fibres  terminate.  In  the  bulb  we  are  familiar  with  such  groups  of  cells,  well 
marked  as  the  "nuclei  of  the  dorsal  funiculi  or  columns,"  and  the  corresponding 
cells  in  the  cord,  though  far  less  clearly  segregated,  are  the  homologues  of 
those  in  the  bulb.  If  this  is  granted,  then  the  fibres  which  are  outgrowths 
from  these  central  cell-groups,  whether  in  the  cord  or  bulb,  are  also  homologous. 

Corroborative  of  what  has  been  said  on  the  subject  of  afferent  pathways 
in  the  cord  are  the  results  of  Pelizzi.2  He  studied  dogs,  making  use  of  the 
method  of  Marchi,  whereby  the  nerve-sheaths  of  fibres  beginning  to  degene- 
rate or  the  nutrition  of  which  is  disturbed  give  a  characteristic  reaction.  He 
found,  after  hemisection  of  the  cord,  the  same  lesions  that  have  been  described 
above,  with  the  addition  that  the  changes  could  also  be  followed  in  some  of 
the  fibres  of  the  ventral  roots.  More  significant,  however,  is  the  fact  that 
section  of  the  lumbar  and  sacral  dorsal  roots,  without  direct  injury  to  the 
cord,  gave  rise  to  modifications  of  the  medullary  sheaths,  detectable  by  the 
method  of  Marchi,  in  all  the  localities  just  named. 

A  distinction  must  be  made  at  this  point.  Secondary  degeneration  in  the 
central  system  means  eventual  destruction  of  the  severed  fibre.  The  method 
of  Marchi  shows  a  characteristic  change  in  fibres  entering  upon  this  degenera- 
tion, but  this  method  also  shows  changes  in  the  sheaths  of  elements  which 
are  only  physiologically  connected  with  those  about  to  undergo  secondary 
degeneration,  but  which  themselves  are,  as  a  rule,  not  ultimately  destroyed. 
Under  the  usual  conditions  of  experiment,  complete  degeneration  is  confined 
within  the  morphological  limits  of  a  single  cell-element,  but  the  physiological 
changes  in  the  cells  overstep  this  limit,  as  shown  by  Marches  reaction. 

Physiological  Observations  on  Afferent  Pathways. — Making  use  of 
the  fact  that  strong  stimulation  of  the  sensory  fibres,  such  as  those  in  the 
sciatic  nerve,  causes  a  rise  in  blood-pressure,  Woroschiloff 3  sought  to  block 
the  passage  of  the  impulses  causing  this  reaction  by  section  of  the  cord  in 
different  ways  in  the  upper  lumbar  region  of  the  rabbit.  It  appears  that  in 
this  animal  the  reaction  was  most  diminished — that  is,  stimulation  of  the 
sciatic  produced  least  rise  in  the  blood-pressure — when  the  lateral  columns 
of  the  cord  had  been  cut  through  ;  and  that  the  effect  of  section  of  the  lateral 
column  on  the  side  opposite  to  that  on  which  the  stimulus  was  applied  was 
greater  than  the  following  section  of  the  column  on  the  same  side.  These 
experiments  form  a  very  definite  part  of  the  evidence  which  directs  our 
attention  to  the  lateral  columns  of  the  cord  as  a  principal  afferent  pathway. 

1  Kohnstaram:  Neurologisches  CentralblaU,  1900,  S.  242. 

2  Archives  ilaliennes  de  Biologie,  1895,  t.  xxiv. 

3  Berichte  der  math.-phys.  Classe  d.  k.  Gesellwh.  d.  Wissen.  zu  Leipzig,  1874. 


230  AN  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 

The  physiological  observations  of  Gotch  and  Horsley 1  indicate  that  when 
in  a  monkey  a  dorsal  root  is  stimulated  electrically  80  per  cent,  of  the 
impulses  pass  cephalad  on  the  same  side  of  the  cord,  while  the  remainder 
cross.  Of  the  20  per  cent,  that  cross,  some  15  per  cent,  pass  up  in  the 
dorsal  columns.  This  leaves  only  5  per  cent,  of  the  impulses  to  pass  up  by 
the  contra-lateral  columns.  These  experiments,  therefore,  give  less  impor- 
tance to  the  lateral  columns  than  was  to  be  expected  from  the  observations 
of  Woroschiloff.  The  dorso-ventral  median  longitudinal  section  of  the  cord 
in  the  monkey  (sixth  lumbar  segment) 2  shows  an  ascending  degeneration  in  a 
small  part  of  the  dorsal  area  of  the  direct  cerebellar  tracts  and  of  the  ventro- 
lateral  tracts,  as  well  as  in  the  columns  of  Goll.  This  would  indicate  that 
the  section  had  cut  fibres  which  crossed  the  middle  line  and  ran  cephalad  in 
these  localities. 

Osawa3  found  that  when  the  cord  in  a  dog  was  hemisected  (in  the  upper 
lumbar  or  lower  thoracic  region)  the  animal  showed  for  the  most  part  no 
permanent  disturbance  of  sensation  or  motion. 

If  the  cord  was  first  hemisected  on  one  side,  and  later  on  the  other  side,  the 
second  hemisection  being  made  a  short  distance  above  or  below  the  first,  sen- 
sation and  motion  persisted  behind  the  section,  although  they  were  somewhat 
damaged.  After  three  hemisections,  alternating  at  different  levels,  there  still 
remained  a  trace  of  co-ordinated  movement  possible  to  the  hind  legs,  although 
the  sensibility  of  the  parts  could  not  be  clearly  demonstrated.  The  path 
thus  marked  out  for  some  afferent  impulses  is  certainly  a  tortuous  one,  and 
at  present  not  readily  to  be  explained.  It  must  be  remembered,  however, 
that  our  information  concerning  the  short  pathways  in  the  cord  is  very  slight. 

Nerves  of  Common  Sensation. — In  order  to  analyze  the  afferent  path- 
ways still  further,  we  next  inquire  whether  among  the  dorsal  nerve-roots 
which  pass  between  the  cord  and  periphery  there  are  separate  nerve-fibres 
for  each  of  the  modes  of  sensation  represented  by  pressure,  heat,  cold,  pain, 
and  the  muscle-sensations.  The  data  available  for  determination  of  this 
question  are  not  of  the  best,  but  are  still  of  some  value. 

The  number  of  dorsal  root  nerve-fibres  on  both  sides  was  estimated  (in  a 
woman  twenty-six  years  of  age)  by  Stilling  to  be  approximately  500,000.4 
Stilling's  estimate  for  the  ventral  root  fibres  in  the  same  individual  was 
300,000. 

The  area  of  the  skin  in  a  man  of  62  kilograms  (136  pounds),  and  twenty- 
six  years  of  age,  was  found  by  Meeh  to  be  1,900,000  square  millimeters.5 

From  the  study  of  the  nerves  going  to  the  muscles  of  the  dog,  Sherring- 

Croonian  Lectures  :  Philosophical  Transactions  of  the  Royal  Society,  1891. 

Griinbaura  :  Journal  of  Physiology,  1894,  vol.  xvi. 

Untersuchungen  uber  die  Leitungsbahnen  im  Riickenmark  des  Hundes,  Strassburg,  1882. 

It  seems  probable  that  both  these  estimates  were  too  low. 

A  slight  correction  is  called  for  here,  owing  to  the  fact  that  the  area  of  skin  includes  that 
for  the  head,  while  the  sensory  nerves  enumerated  do  not  include  the  fibres  going  to  the  head. 
The  general  relations  given  below  would  not,  however,  be  significantly  modified  by  the  altera- 
tion of  the  data. 


CENTRAL   NERVOUS  SY8TXM.  231 

ton1  reports  that  from  one-third  to  one-half  the  number  of  tin -<•  muscular 
fibres  arise  from  the  dorsal  root  iran^lion,  and  are  the  re- fore  afferent  in  function. 

If  we  assume  that  two-fifths  of  the  number  of  sensory  fibres,  or  200,000, 
go  to  the  muscles  and  joints,  this  would  leave  but  .'*()(.),( KM)  sensory  iibn-s 
remaining,  or  one  nerve-fibre  to  innervate,  on  the  average,  about  six  square' 
millimeters  of  skin. 

The  experiments  on  tactile  and  temperature  discrimination  all  indicate 
that  the  innervation  of  the  skin  is  very  unequal.  The  average  distribution 
which  has  just  been  suggested  must  therefore  be  subject  to  local  modifications 
that  are  very  wide.  Woischwillo2  has  determined  that  in  man  the  skin  of 
the  arm  is  three  times  better  supplied  with  sensory  nerves  than  that  of  the 
leg.  In  both  arm  and  leg  the  relative  abundance  of  the  sensory  nerves 
increases  toward  the  extremity  of  the  limb.  This  increase  is  special  lv 
marked  in  the  leg.  Assuming,  however,  one  nerve-fibre  to  six  squa*re  milli- 
meters of  the  skin  to  be  the  average  relation,  it  becomes  a  serious  matter  to 
postulate  separate  groups  of  fibres  for  each  mode  of  dermal  sensation,  since 
each  time  a  new  set  of  fibres  is  admitted  the  area  of  the  skin  innervated  by 
any  other  set  with  a  given  function  is  thereby  increased. 

This  being  the  case,  there  are  good  anatomical  reasons  for  limiting  the 
number  of  categories  of  nerve-fibres. 

In  every  case  the  fibres  carrying  the  impulses  which  come  from  the  skin 
arise  as  outgrowths  of  the  spinal  ganglion-cells.  Trophic  nerves  as  a  special 
category  are  not  recognized,  nor  reflex  nerves,  the  functions  attributed  to  the 
latter  being  now  explained  by  the  collaterals  of  the  afferent  fibres.  At 
present  it  is  sometimes  maintained  that  there  must  be  special  nerves  for  pain, 
pressure,  heat,  and  cold.  The  evidence  for  those  of  pressure  and  heat  and 
cold  is  the  most  satisfactory. 

Pain. — Upon  severe  stimulation  of  the  skin  or  muscles  the  normal  person 
experiences  a  distinct  sensation  of  pain.  There  is,  however,  great  variation 
in  the  intensity  of  this  sensation  when  the  same  stimulus  is  applied  to  differ- 
ent persons. 

If  we  include  abnormal  persons,  it  is  found  that  while  in  a  few  cases  com- 
plete absence  of  painful  sensations  has  been  noted — the  other  sensations 
remaining  normal — there  are  at  the  other  end  of  the  scale  those  cases  in 
which  pain  is  produced  by  many  stimuli  which  would  not  have  this  effect  on 
persons  in  ordinary  health.  The  capability  of  a  given  stimulus  to  produce 
pain  is  therefore  subject  to  wide  variations  acpording  to  the  general  condition 
of  the  subject.3  The  same  stimulus  has  different  effects  in  a  given  individual 
according  to  several  circumstances.  Peripheral  irritation,  such  as  an  inflam- 
matory process  in  the  skin,  greatly  increases  the  intensity  of  the  pain  caused 
by  the  stimulation  of  the  nerves  supplying  the  locality.  Continued  stimula- 

1  Sherrington  :  Journal  of  Physiology,  1894-5,  vol.  xvii. 

2 "  Ueber  das  Verhaltniss  des  Kalibers  der  Nerven  zur  Haul  und  den  Muskeln  des  Men- 
schen,"  Inang.  Diss.  (Russian),  1883;  vide  Centrnlblatt  fur  Nervenheilkunde,  1883,  Bd.  vi. 
s  Strong  :  Psychological  Review,  1895,  vol.  ii.  No.  4. 


232  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

tion  of  the  sensory  nerves  of  the  muscles  and  viscera  has  the  same  effect.1 
Local  anesthetics,  such  as  cocaine,  may  reduce  the  sensibility  to  zero,  and 
the  same  follows  the  general  anesthesia  produced  by  chloroform,  ether,  nitrous 
oxide,  morphine,  and  similar  drugs.  Painful  sensations  are  distinct  and 
powerful  only  when  the  stimulus  is  applied  to  general  sensory  nerve-trunks — 
i.  e.}  those  mediating  cutaneous,  muscular,  and  visceral  sensibility — while  the 
nerves  which  mediate  the  special  sensations  of  light,  sound,  taste,  and  smell 
do  not  give  pain  even  on  excessive  stimulation. 

Limiting  our  observation,  therefore,  to  the  nerves  of  cutaneous  sensibility, 
it  is  found  in  exceptional  cases  that  the  sensations  of  pressure,  heat,  and  cold 
may  all  be  present  to  a  normal  degree,  and  yet  increasing  the  stimulus  be 
without  effect  in  causing  any  painful  sensations  whatever.  This  would 
represent  a  condition  of  complete  analgesia.  Moreover,  the  capacity  of  the 
skin  to  cause  abnormal  painful  sensations  upon  the  adequate  stimulation  of 
each  of  these  groups  of  nerves  may  be  associated  (in  lesions  of  the  central 
system)  with  any  one  group  alone,  the  abnormal  pain-sensations  thus  pro- 
duced being  either  excessive  or  deficient. 

We  advance  the  hypothesis,  therefore,  that  each  of  these  three  sensations, 
if  pushed  to  excess,  is  usually  accompanied  by  pain  of  gradually  increasing 
intensity.  Therefore  it  is  most  probable  that  these  nerves  when  slightly 
stimulated  mediate  their  proper  sensations,  but  when  this  stimulus  is  pushed 
to  excess  they  can  give  rise  to  pain  also,  and  that  in  the  last  instance  this 
sensation  of  pain  may  prove  exclusive  of  any  other.  If  this  view  is  correct, 
it  appears  improbable  that  special  pain-nerves  exist. 

As  various  experiments  show,  increasing  either  the  strength  of  the  periph- 
eral stimulus,  the  number  of  fibres  to  which  it  is  applied,  or  the  irritability 
of  the  terminals  of  the  fibres,  will  assist  in  arousing  painful  sensations.  In 
the  last  analysis  the  physiological  condition  for  pain  is  excessive  stimulation, 
which  by  all  analogy  must  mean  excessive  discharge  within  the  central 
system.  The  changes  following  this  discharge  into  the  central  system  are 
not  such  as  lead  to  co-ordinated  muscular  responses,  but  to  convulsive  reac- 
tions of  a  very  irregular  character.  Where  this  process  takes  place  in  the 
central  system  we  do  not  know.  As  to  normal  analgesia,  it  must  be  looked 
upon  as  dependent  on  a  condition  in  which  excessive  stimulation  cannot  be 
produced ;  and  we  find  this  condition  normally  present  only  in  the  case  of 
the  nerves  of  special  sense. 

Since  in  the  pathological  conditions  one  sort  of  sensibility  may  be  lost 
while  the  others  remain,  it  has  been  inferred  that  there  are  separate  fibres 
for  the  conveyance  of  each  sort  of  sensation.  This  idea  was  expressed  in  the 
law  of  the  specific  energies  of  nerves  as  formulated  by  Joannes  Miiller,  who 
pointed  out  that  in  many  cases  the  same  nerve  might  be  stimulated  in  any 
way — mechanically,  electrically,  or  chemically,  as  well  as  in  the  normal  physi- 
ological manner;  and  that  in  all  cases  the  mode  of  the  response  was  the  same — 
a  sensation  of  light  or  taste  or  contact,  as  the  case  might  be.  Hence  it  was 

1  Gad  und  Goldscheider  :  Zeitschrift  fur  klinische  Medicin,  Bd.  xx. 


CENTRAL    NERVOUS  SYSTEM.  233 

argued  that  the  mode  of  the  sensation  was  independent  of  the  kind  of  stimu- 
lus, but  dependent  on  the  nature  of  the  central  cells  among  which  the  idle  rent 
fibres  terminated.  It  will  be  seen,  however,  that  this  argument  does  not 
touch  the  character  of  the  nerve-impulses  in  any  two  sets  of  nerves,  and  we 
have  no  observations  by  which  to  decide  whether  the  nerve-impulses  pa:— in- 
along  the  optic  nerve-fibres  are,  for  example,  similar  or  dissimilar  to  th<>>r 
which  pass  along  the  auditory  fibres. 

If  the  nerve-impulses  are  always  all  alike,  there  seems  no  escape  from  the 
inference  that  separate  nerve-fibres  convey  the  impulses  destined  to  give  rise 
to  different  sensations.  At  the  same  time,  it  is  just  possible  that  the  nature 
of  the  impulses  and  of  the  resultant  sensation  is,  in  the  nerves  of  cutaneous 
sensibility,  determined  by  the  form  of  the  peripheral  stimulus,  and  that,  as  a 
consequence,  different  branches  of  the  same  nerve-fibres  may  be  conceived  of 
as  susceptible  to  different  forms  of  stimulation,  and  thus  the  two  different 
sensations  follow  from  the  partial  stimulation  of  the  same  nerve-fibres. 

The  second  possibility,  that  the  nerve  impulse  has  different  characters  in 
different  afferent  nerves,  and  further  may  be  modified  by  the  nature  of  the 
normal  stimulus  (pressure  or  temperature),  is  not  to  be  too  readily  rejected,  as 
Hering  at  least  argues  in  favor  of  such  a  view.1 

Pathway  of  Impulses  in  the  Spinal  Cord. — The  question  arises  how 
these  impulses  are  distributed  among  the  afferent  tracts  which  are  recognized 
in  the  cord,  and  whether  these  tracts  form  special  paths  for  the  impulses  that 
rouse  the  several  sensations  of  pressure,  temperature  (heat  and  cold),  and 
pain.  Since  it  is  necessary  to  know  the  sensations  of  the  subject,  this  prob- 
lem can  be,  in  some  ways,  best  studied  in  man.  Here,  owing  to  wounds  or 
disease,  it  may  so  happen  that  some  of  these  sensations  are  lost  or  greatly 
diminished,  and  it  is  to  be  determined  whether  this  loss  is  constantly  associ- 
ated with  the  interruption  of  definite  tracts.  Unfortunately,  however,  the 
material  for  such  a  study  is  very  meagre. 

In  man  the  typical  group  of  symptoms  following  hemisection  of  the  spinal 
cord  above  the  lumbar  region  has  long  been  known  as  Brown-Sequard's 
paralysis.  The  clinical  observations  on  cases  suffering  from  such  a  lesion 
have  been  recently  summarized  by  Oppenheim  2  as  follows  : 

1.  A  paralysis  of  the  homo-lateral  muscles.     In  the  case  of  the  leg,  the 
effects  are  most  intense  and  persistent  in  the  flexors  of  the  thigh  and  shank, 
and  the  extensors  of  the  foot. 

2.  When  the  two  sides  of  the  body  are  contrasted,  there  appears  to  be  a 
homo-lateral  hyperaBsthesia,  accompanied  by  contra-lateral  anesthesia. 

3.  As  to  the  several  forms  of  sensation,  the  following  may  be  stated : 

(a)  The  muscle  sensations  (Bathyasthesia,   Oppenheim)  :    the   defect   is 
never  contra-lateral ;  sometimes,  however,  it  is  bilateral,  but  in  most  cases  is 
homo-lateral. 

(b)  The  contact  sensations  are  very  often  not  affected  at  all — sometimes 

1  Hering:   "Zur  Theorie  der  Nerventhatigkeit,"  Akademischer  Vortrag,  Leipzig,  1899. 
'Oppenheim  :  Archivfur  Physiologic,  Physiol.  Abthl.,  Suppl.  Bd.,  1  Heft,  July,  1899. 


234  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

slightly,  and  in  the  latter  case  the  hypersesthesia  may  appear  on  either  or 
both  sides. 

(c)  The  prominent  and  almost  constant  sensory  symptom  is  the  contra- 
lateral  loss  of  the  sensation  for  pain  and  temperature. 

On  the  basis  of  a  case l  in  which  the  lateral  columns  of  the  cord  and  the 
gray  matter  of  both  horns  on  the  same  side  were  the  seat  of  damage,  and  in 
which  there  was  a  total  loss  of  pain  on  the  opposite  side  of  the  body  without 
impairment  of  tactile  sensibility,  it  may  be  inferred  that  the  pain-impulses 
cross  soon  after  entering  the  cord,  and  pass  cephalad  by  some  path  lying 
within  the  damaged  area. 

A  second  case 2  is  recorded  in  which  a  stab-wound  divided  all  of  one-half 
of  the  cord  plus  the  dorsal  column  of  the  other  half.  There  was  here  a  loss 
of  sensibility  to  pain  on  the  side  opposite  the  lesion,  together  with  the  loss  of 
tactile  sensibility  on  both  sides,  pointing,  therefore,  to  the  dorsal  columns  as 
the  paths  for  the  tactile  impulses.  The  experiments  on  the  lower  animals 
contradict  this  conclusion. 

The  observations  of  Turner3  on  monkeys,  in  which  hemisection  of  the 
cord  had  been  made  in  the  lumbar  and  thoracic  regions  indicate  that  all  sen- 
sory impulses  cross  immediately  after  entering  the  cord,  yet  section  in  the 
cervical  region  showed  that  the  impulses  roused  by  touching  the  skin  pass  in 
part  on  the  same  side  of  the  cord  as  the  section,  the  other  sensory  impulses 
being,  however,  completely  crossed. 

On  the  other  hand,  from  his  work  on  hemisection  of  the  thoracic  cord  of 
the  monkey  at  different  levels,4  Mott  found  the  disturbance  of  sensibility  of 
all  forms  mainly  on  the  side  of  the  section. 

Hemisection  of  the  Cord. — From  experiments  on  monkeys  and  a  few 
cats  Schafer  reports  the  following  physiological  changes  after  hemisection  of 
the  spinal  cord  in  animals  :  "  In  the  first  few  days  complete  motor  paralysis 
of  all  parts  supplied  with  nerves  below  the  section.  The  limb  or  limbs  on 
the  paralyzed  side  swollen  and  warm  (vasomotor  paralysis)  and  lessened  out- 
flow of  lymph  and  the  skin  dry  (diminution  of  sweat).  Knee-jerk  exag- 
gerated. Sensation  not  lost  on  the  same  side  as  the  lesion,  but  at  first  appears 
dulled.  (There  is  a  difficulty  in  arriving  at  a  clear  decision  on  account  of 
the  motor  paralysis  rendering  the  animal  unable  to  move  the  limb.)  After 
a  few  days,  unmistakable  signs  of  feeling  and  localizing  even  a  slight  touch, 
and  this  long  before  the  motor  paralysis  has  passed  off.  The  animals  gener- 
ally disregard  a  clamp-clip  on  the  skin  of  the  paralyzed  limb,  but  not  always ; 
this  phenomenon  usually  lasts  until  the  return  of  movement  in  the  muscles 
of  the  limb.5  I  have  seen  no  signs  of  paralysis  either  motor  or  sensory  on 

1  Gowers  :  Clinical  Society's  Transactions,  1878,  vol.  xi. 

2  Miiller  :  Beitrdge  zur  pathologische  Anatomic  und  Physiologic  des  Ruckenmarkes,  Leipzig,  1871. 

3  Brain,  1891.  4  Mott :  Journal  of  Physiology,  1891,  vol.  xvii. 

5  It  will  be  seen  that  my  observations  on  this  point  agree  generally  with  those  of  Mott 
(Phil.  Trans.  B.  1892),  although  ray  conclusions  are  somewhat  different.  Mott,  in  my  opinion, 
lays  too  much  stress  on  the  results  of  the  clip  test  (Schafer). 


CENTRAL    NERVOUS  SYSTEM.  235 

the  side  opposite  to  the  hemisection  in  any  case  in  which  this  has  been  strictly 
confined  to  the  one  half  of  the  cord.1  Sometimes  the  adjacent  posterior 
column  of  the  other  half  is  injured,  and  in  that  event  there  is  impairment  of 
sensation  for  a  time  on  both  sides  below  the  lesion.  The  motor  paralysis,  at 
first  complete,  becomes  gradually  incomplete,  and  finally  is  difficult  or  impos- 
sible to  determine.  But  purely  voluntary  movements  are  not  recovered  <>r 
but  very  imperfectly,  although  all  the  ordinary  associated  movements  of  the 
limb  are  recovered.  After  about  three  or  four  weeks  it  is  difficult  to  detect 
any  sort  of  paralysis,  but  the  limb  which  has  been  paralyzed  is  thinner  than 
the  other." 

If  the  hemisection  is  made  above  the  level  of  the  eighth  cervical  nerve, 
the  pupil  on  the  same  side  is  relatively  contracted  and  remains  so.  The 
dilator  fibres  and  the  pilomotor  fibres  in  the  cervical  sympathetic  do  not 
degenerate,  but  remain  excitable.  The  pupil  reacts  to  light  and  shade  in 
spite  of  its  being  persistently  smaller  than  the  other.  Excitation  of  the 
motor  cortex  of  the  opposite  cerebral  hemisphere  produces,  as  a  rule,  no  move- 
ments in  the  limbs  which  have  been  paralyzed,  even  if  the  associated  move- 
ments have  long  returned. 

As  will  be  seen  from  the  foregoing  paragraphs  bearing  on  the  aiferent 
pathways  found  in  the  spinal  cord  of  man  and  the  higher  mammals,  the  evi- 
dence for  the  path  of  the  cutaneous  impulses  is  decidedly  contradictory. 

In  addition  to  the  cutaneous  impulses  there  are  the  sensory  impulses  from 
the  viscera,  muscles,  and  tendons,  which  find  their  path  cephalad  probably 
along  the  direct  cerebellar  tract  as  well  as  by  the  long  pathways  in  the  dorsal 
columns.  After  hemisection  of  the  cord  the  "muscular"  sensations  are 
usually  lost  on  the  side  of  the  section,  and  the  observations  of  Tschermak, 
already  mentioned,  point  to  the  long  fibres  in  the  dorsal  funiculi  as  the  path- 
way for  the  impulses  from  the  muscles  and  joints. 

Indeed,  the  lack  of  good  evidence  for  the  conduction  of  any  impulses — 
save  those  from  the  muscles  and  joints — by  long  tracts  in  the  cord,  has  led 
Starr2  to  suggest  that  the  dermal  impulses  are  transmitted  by  short  path- 
ways through  the  cord. 

Since,  then,  the  dorsal  and  lateral  columns  of  the  cord  appear  to  contain 
the  chief  afferent  paths  for  the  sensory  impulses,  the  next  step  in  following 
the  pathway  is  to  find  the  terminations  of  these  tracts,  whether  long  or  short. 
Of  the  latter  nothing  can  be  said.  The  long  tracts  in  the  dorsal  columns  are 
connected  with  the  nuclei  of  those  columns  (nuclei  of  Goll.and  of  Burdach) 
on  the  same  side.  The  cells  of  these  nuclei  send  their  axones  cephalad  ;  in 
part  they  decussate  in  the  sensory  crossing  and  contribute  to  the  formation 
of  the  lemniscus,  by  way  of  which  they  pass  either  directly  to  the  cerebral 
cortex  about  the  central  gyri,  or  reach  this  only  after  interruption  in  the 

1  Ferrier  and  Turner  (Brain,  1891)  describe  loss  of  sensibility  in  the  opposite  hind  limb  in 
the  monkey.     Brown-Sequard,  as  is  well  known,  obtained  this  result  in  the  rabbit.     (See  also 
Ferrier,  Functions  of  the  Brain,  and  Croonian  Lectures,  1890.) 

2  Starr :  Transactions  of  the  American  Neurological  A  ssociation,  Twenty-third  Meeting,  1897,  p.  7. 


236  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

thalamus.  It  will  be  remembered  that  these  fibres  of  the  dorsal  columns 
are  physiologically  joined  with  the  contra-lateral  thalamus  and  hemisphere. 
In  part,  however,  the  axones  from  the  dorsal  nuclei  enter  the  cerebellum  by 
the  inferior  peduncle  of  the  same  side,  and  we  shall  refer  to  this  when  con- 
sidering the  cerebellum. 

Cranial  Nerves. — We  shall  next  consider  ybriefly  the  relations  of  the  sev- 
eral afferent  cranial  nerves,  beginning  with  the  vagus  and  working  cephalad. 

Nervus  Vagus  (Tenth  Nerve). — The  nucleus  of  termination  for  the  afferent 
fibres  of  the  tenth  nerve  (vagus)  are  shown  in  Fig.  91.  The  afferent  fibres 
of  this  nerve  are  found  to  convey  impulses  which  arise  in  the  pharynx,  oesoph- 
agus, stomach,  liver,  pancreas,  spleen,  larynx,  bronchi,  and  lungs.  Further, 
this  nerve  contributes  afferent  fibres  to  the  nervus  laryngeus  superior.  The 
location  of  the  nucleus  of  termination  (N.  alee  cinerse)  falls  within  the  area  of 
the  noeud  vital.  Concerning  the  axones  of  the  neurones  forming  the  nucleus 
of  termination,  it  can  only  be  said  that  they  are  continued  cephalad  in  the 
medial  lemniscus  and  the  fasciculus  longitudinalis  medial  is. 

Nervus  Glossopharyngeus  (Ninth  Nerve). — The  ninth  nerve  (glossopharyn- 
geus,  Fig.  91)  is  represented  in  the  bulb  by  the  tractus  solitarius,  the  fibres 
of  which  find  their  principal  nucleus  of  termination  in  the  cell-group  lying 
just  to  the  medial  side  of  the  tract.  The  neurones  of  this  nucleus  send  their 
axones  cephalad  by  way  of  the  medial  lemniscus.  The  afferent  fibres  of  N. 
glossopharyngeus  mediate  general  sensations  for  the  tonsils  and  pharynx,  the 
tympanic  cavity  and  Eustachian  tube,  while  by  way  of  the  ramus  lingualis 
it  innervates  the  taste-organs  of  the  posterior  part  of  the  tongue  and  those  in 
the  pharynx.  In  addition  to  these  fibres  mediating  the  sense  of  taste,  patho- 
logical evidence  points  to  some  additional  fibres  with  the  same  function  (not 
belonging  to  the  ninth  nerve)  which  reach  the  bulb  by  way  of  the  fifth  nerve 
and  the  nervus  intermedius.  The  nuclei  of  termination  for  these  three  nerves 
are  very  close  to  one  another  in  the  bulb,  and  hence  the  innervation  of  a 
special  sense-organ  from  three  cranial  nerves,  which,  in  the  first  instance, 
seems  anomalous,  becomes  more  intelligible  when  it  is  recognized  that  the 
nuclei  concerned  are  practically  continuous. 

Nervus  Intermedius. — In  this  connection  the  afferent  fibres  in  the  nervus 
intermedius  (of  Wrisberg)  should  be  mentioned.  These  fibres  arise  from  the 
cell-bodies  of  the  ganglion  geniculatum,  enter  the  bulb  between  the  super- 
ficial origin  of  the  seventh  and  the  vestibular  root  of  the  eighth  nerve,  and, 
running  caudad  along  the  dorso-medial  tip  of  the  ascending  root  of  the  fifth, 
finally  terminate  with  the  fibres  of  the  glossopharyngeus  in  the  cells  of  ter- 
mination found  along  the  tractus  solitarius.  The  longitudinal  extension  of 
these  fibres  of  the  nervus  intermedius  in  the  bulb  closely  matches  that  of  the 
nucleus  of  the  eighth  nerve,  and  at  the  periphery  the  fibres  from  the  genicu- 
late  ganglion  are  distributed,  in  part  at  least,  with  those  of  the  seventh  nerve.1 

Nervus  Auditorius  (Eighth  Nerve). — A.  Cochlear  Root. — The  eighth  nerve 

1  Van  Gehuchten  :  "  Eecherches  sur  la  Terminaison  centrale  des  Nerfs  sensible  pe'riphe'ri- 
ques— le  Nerf  interme'diare  de  Wrisberg."  Le  Niuraxe,  Mars,  1900,  t.  i.  fasc.  L 


CENTRAL   NERVOVS  SYSTEM.  237 

goes  to  the  inner  ear.  'Flu1  oochlear  portion  of  the  inner  ear  mediates  sensa- 
tions of  sound  and  is  connected  with  the  bulb  by  means  of  the  nervus  coch- 
leae; the  cochlear  branch  of  the  eighth  nerve.  The  cell-bodies  of  the  nervus 
cochleae  are  located  in  the  spiral  ganglion  of  the  cochleae,  which  is  homologous 
with  the  dorsal  root  ganglion  of  n  spinal  nerve.  The  ganglion  cells  are 
bipolar  or  diaxonic,  one  axone  passing  toward  the  organ  of  Corti  in  the 
cochlea,  and  the  other  toward  the  bulb. 

On  reaching  the  bulb,  the  nerve  formed  by  the  latter  ax  ones  enters  in  a 
large  measure  the  nucleus  nervi  cochleae  ventralis,1  and  to  a  less  extent  the 
nucleus  nervi  cochleae  dorsalis.  According  to  Held,2  some  of  the  root-fibres 
entering  the  ventral  nucleus  may  be  continuous  as  far  as  the  superior  quad- 
rigemina,  reaching  that  level  by  way  of  the  trapezoideum,  the  superior  olive, 
the  lateral  lemniscus,  and  the  colliculus  inferior ;  to  all  of  which  gray  masses, 
including  the  nucleus  nervi  cochleae  dorsalis,  these  axones  may  give  collat- 
erals. Further,  some  fibres  may  terminate  in  any  of  the  localities  reached 
by  the  collaterals.  Besides  the  direct  continuations  of  the  afferent  axones  by 
way  of  the  ventral  nucleus,  each  one  of  the  localities  mentioned  above,  includ- 
ing both  the  dorsal  and  ventral  cochlear  nuclei,  contains  cell-bodies  forming, 
on  the  one  hand,  nuclei  of  termination,  and  on  the  other  by  their  axones  con- 
tinuing the  auditory  pathway  even  to  the  cerebral  cortex  (Held).  A  group 
of  central  cells  with  their  bodies  in  the  nucleus  nervi  cochleae  dorsalis  send 
their  axones  across  the  floor  of  the  fourth  ventricle,  forming  the  striae  acusticae. 
These  axones  in  part  decussate  with  the  corresponding  fibres — the  crossing 
occurring  in  the  raphe — and  then  either  as  direct  or  crossed  fibres  find  their 
way  cephalad  by  the  same  path  (with  some  additions)  as  that  described  in 
connection  with  the  ventral  nucleus. 

B.  Vestibular  Root. — Quite  separate  from  the  cochlear  is  the  vestibular 
division  of  the  eighth  nerve,  and  this  separateness  is  a  strong  argument  against 
the  suggestion  sometimes  made  that  the  portions  of  the  labyrinth  innervated 
by  the  vestibular  nerve,  may  also  mediate  sensations  of  sound.  The  best 
evidence  shows  the  nerve  to  convey  those  impulses  from  the  macula  acustica 
utriculi  and  the  cristae  ampullares,  which  are  largely  utilized  in  the  mainten- 
ance of  the  equilibrium  and  in  arousing  the  sensations  of  the  movement  of 
the  body  as  a  whole.  The  peripheral  neurones  which  give  rise  to  the  vestib- 
ular fibres  have  their  cell-bodies  collected  in  the  vestibular  ganglion.  The 
peripheral  axones  of  the  ganglion-cells  end  among  sensory  epithelium  of  the 
parts  just  named,  while  the  central  axones,  forming  a  larger  root  than  that 
associated  with  the  cochlea,  join  the  bulb  at  the  caudal  edge  of  the  pons,  the 
vestibular  root  lying  to  the  cephalic  side  of  the  cochlear  root.  Having 
entered  the  bulb,  the  axones  divide,  after  the  manner  of  dorsal  root  fibres, 
into  an  ascending  and  a  descending  branch,  which  find  their  nuclei  of  termina- 
tion in3  (1)  the  nucleus  nervi  vestibuli  spinalis  (the  radix  descendens);  (2)  in 

1  Barker :   The  Nervous  System  and  its  Constituent  Neurones,  1899,  pp.  544-555. 

2  Held:  Archivfiir  Physiologie,  Anat.  Abth.,  Leipzig,  1893. 

3  Barker:   The  Nervous  System  and  its  Constituent  Neurones,  1899,  p.  627,  et  seq. 


238  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  nucleus  nervi  vestibuli  medialis ;  (3)  the  nucleus  nervi  vestibuli  lateralis 
(nucleus  of  Deiters) ;  and  (4)  the  nucleus  nervi  vestibuli  superior.  Finally, 
among  other  connections  are  to  be  specially  mentioned  those  with  the  cere- 
bellum (nuclei  fastigii,  nucleus  dentatus,  and  the  cerebellar  cortex).  Nothing 
definite  is  known  concerning  the  pathways  by  which  the  impulses  entering 
over  the  radix  vestibularis  reach  the  cortex. 

Nervus  Trigeminus  (Fifth  Nerve). — The  neurones  of  this  nerve  have  their 
cell-bodies  located  in  the  ganglion  semilunari  (Gasseri),  and  the  peripheral 
axones  act  as  the  nerves  of  common  sensation  for  the  skin  of  the  face  and 
tongue  and  the  mucous  membranes  of  the  mouth.  The  central  axones  which 
branch  on  reaching  the  bulb  send  their  shorter  divisions  cephalad  for  a  little 
distance,  and  the  longer  caudad,  in  both  cases  finding  cells  of  reception  in 
the  region  of  the  substantia  gelatinosa,  and  in  the  latter  instance  extending 
caudad  at  least  as  far  as  the  first  segment  of  the  spinal  cord.  Possibly,  one 
set  of  neurones  of  the  fifth  passes  directly  into  the  cerebellum.  The  path- 
way from  the  nucleus  of  termination  to  the  cortex  has  not  been  determined. 

Second  Nerve,  Optic. — As  has  long  been  recognized,  the  optic  nerve,  so 
called,  is  a  cerebral  tract  morphologically  equivalent  to  such  tracts  as  con- 
nect any  portion  of  the  cerebral  cortex  with  a  primary  centre,  the  retina 
being  in  part  the  representative  of  the  cerebrum ;  and  the  pulvinares,  the 
quadrigernina,  and  geniculata  externa  being  the  primary  centres. 

At  the  chiasma  where  the  two  optic  nerves  come  together  their  fibres 
intermingle,  and  then  emerge  as  the  optic  tracts,  which  contain  not  only  the 
fibres  connected  with  the  retina,  but  others  added  from  the  superposed  parts 
of  the  brain,  and  forming  the  commissures  of  Meynert  and  von  Gudden. 

In  the  rabbit  it  was  shown  by  von  Gudden1  that  in  the  chiasma  the 
majority  of  the  fibres  forming  one  optic  nerve  pass  to  the  tract  of  the 
opposite  side,  but  that  a  portion  of  the  fibres  remains  in  the  tract  of  the 
same  side. 

This  was  inferred  because  removal  of  one  eyeball  caused  in  young  rab- 
bits a  degeneration  in  the  associated  optic  nerve  and  also  in  both  optic  tracts — 
most  marked,  however,  in  the  tract  of  the  side  opposite  to  the  lesion.  Con- 
versely, the  section  of  one  optic  tract  causes  a  degeneration  in  both  optic 
nerves,  the  nerve  of  the  side  opposite  to  the  lesion  being  most  affected,  and  a 
smaller  degeneration  appearing  in  the  nerve  of  the  same  side  (see  Fig.  98). 

In  the  fish,  amphibia,  reptiles,  and  birds — except  the  owls 2 — the  decussa- 
tion  appears  to  be  complete.3  For  the  partial  decussation  in  the  owls  the 
evidence  is  physiological.  This  distribution  of  the  optic  fibres  was  associated 
by  von  Gudden  with  the  position  of  the  eyes  in  the  head.  The  extreme 
lateral  position  of  the  eyes  as  it  occurs  in  the  lower  mammals  permits  of  but 
little  combination  of  the  two  visual  fields ;  whereas  the  position  in  man,  in  a 

1  von  Gudden  :   Gesammelte  und  hinterlassene  Abhandlunyen,  Wiesbaden,  1889. 
'2  Fcrrier :   The  Croonian  Lectures  on  Cerebral  Localization,  London,  1890,  p.  70. 
3  Singer  and  Munzer  :  Denkschriften  der  math.-naturwiss.  Olasse  der  kais.  Akademie  der  Wissen- 
schaften,  1888,  Bd.  iv. 


CENTRAL    NERVOUS  SYSTEM. 


239 


frontal  plane,  permits  a  combination  of  the  fields  to  a  much  greater  degree. 
It  was  in  accordance  with  this  principle  that  partial  dccussntioii  of  these 
nerves  was  anticipated  by  von  Gudden  in  the  owl,  although  the  histological 
evidence  for  it  was  not  obtained  by  him. 

The  most  recent  researches  on  mammals  have  so  increased  the  number  in 
which  a  partial  decussation  occurs  that  we  are  justified  in  regarding  this 


/ 


f// 

Uncrossed.  /'  / 


[Crossed. 


\ 


C.Q 


/ 


FIG.  98.— Illustrating  the  relations  01  the  afferent  fibres  in  the  optic  nerve.  The  crossed  fibres  are 
indicated  by  solid  lines,  the  uncrossed  fibres  by  broken  lines :  N,  nasal  side  of  the  right  eye  ;  T,  temporal 
side  of  the  same ;  G.  E,  geniculatum  externum ;  P.  pulvinar ;  C.  Q.  quadrigeminum  anterius. 

arrangement  as  the  rule,  although  the  proportion  of  the  uncrossed  fibres  is 
small  in  those  mammals  in  which  the  eyes  are  placed  laterally.1 

In  man  the  evidence  from  degeneration  in  the  optic  nerve  points  to  the 
presence  of  a  crossed  and  an  uncrossed  bundle  of  fibres  in  each  optic  nerve, 
the  uncrossed  being  much  the  smaller  of  the  two  bundles.  The  contrary 

1  Cajal :  Die  Structur  des  Chiasma  opticum  nebst  einer  Allgemeinen  Theorie  der  Kreuzung  der 
Nervenbdhnen,  Leipzig,  1899. 


240  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

view  of  complete  decussation  has  been  maintained  by  Michel.1  The  central 
ends  of  the  afferent  optic  fibres  forming  an  optic  tract  are  for  the  most  part 
distributed  between  the  anterior  quadrigeminum,  the  geniculatum  extern  urn, 
and  the  pulvinar  of  the  same  side.  By  central  cells  located  in  these  latter 
structures  the  pathway  is  continued  to  the  occipital  cortex  of  the  hemisphere 
of  the  same  side,  their  axones  passing  in  the  occipital  end  of  the  internal 
capsule  and  forming  the  optic  radiation.  It  must  be  remembered,  however, 
that  between  the  cortex  and  the  primary  centres,  and  again  between  these 
centres  and  the  retina,  there  are  pathways  conducting  from  the  cortex  to  the 
primary  centres,  and  also/rom  the  primary  centres  to  the  retina.2 

As  the  result  of  partial  decussation  it  will  be  seen  that  the  relations  of  the 
two  retinae  to  the  cortex  is  this  :  The  nasal  or  crossed  bundle  of  the  contra- 
lateral  retina  and  the  temporal  or  uncrossed  bundle  of  the  retina  of  the  same 
side  come  together  in  the  optic  tract  of  one  side  and  are  associated  with  the 
occipital  lobe  of  that  side.  Hence  it  would  appear  that  hemianopsia  or 
blindness  in  the  corresponding  halves  of  the  two  eyes  following  a  lesion  of 
the  optic  pathway  anywhere  behind  the  chiasm  would  be,  in  some  measure, 
explained  by  this  anatomical  arrangement.  If  strictly  interpreted,  an  ap- 
proximately equal  number  of  fibres  would  be  expected  for  each  half  of  the 
retina.  Such,  however,  has  not  been  established  as  the  relation  between  the 
areas  of  the  bundles.  It  is  to  be  added,  nevertheless,  that  anatomical  arrange- 
ments such  as  decussations  are  probably  open  to  wide  individual  variations, 
and  hence  that  many  more  observations  are  required  before  we  can  say  what 
is  the  usual  relation  between  these  two  bundles. 

With  a  view  to  determining  the  exact  location  of  the  cortical  centres  in 
man,  many  observations  have  been  made.  The  cuneus  and  immediately  sur- 
rounding parts  of  the  cortex  are  those  most  concerned.  Henschen3  indicates 
the  calcarine  fissure  and  its  immediate  neighborhood  as  the  most  important 
locality.  Observations  on  the  arrest  in  the  development  of  the  cortex  due  to 
early  blindness  following  destruction  of  the  retina  in  the  case  of  the  blind 
deaf-mute  Laura  Bridgman,  show  the  entire  cuneus  to  be  the  central  and 
fundamental  portion,  while  the  associated  portions  extend  some  distance  on- 
to the  convex  surface  of  the  hemisphere.4 

First  Nerve. — Comparative  anatomy  indicates  that  the  parts  of  the  enceph- 
alon  mediating  the  sense  of  smell  are  most  closely  connected  with  the  cerebral 
hemispheres,  in  the  sense  that  phylogenetically  the  first  development  of  the 
cortex  was  in  connection  with  the  central  terminations  of  the  olfactory  tracts.5 
It  happens  in  man,  however,  that  although  the  cerebral  hemispheres  are  pro- 
portionately much  more  massive  than  in  the  lower  mammals,  yet  the  olfactory 
bulbs  and  tracts  are  at  the  same  time  but  poorly  developed.  The  pathway 

1  K olliker's  Festschrift,  Wiirtsburg,  1887. 

2  von  Monakow:  Archivfur  Psychiatrie,  1890,  Bd.  xx.  H.  3. 

3  Henschen  :  Klinische  und  anatomische  Beitrdqe  zur  Pathologic  des  Gehirns,  Upsala,  1890-92.. 
*  Donaldson  :   American  Journal  of  Psychology,  1892,  vol.  iv.  No.  4. 

5  Sir  William  Turner  :  Journal  of  Anatomy,  1890;  Edinger:   Anatomischer  Anzeiger,  1893. 


CENTRAL   NERVOUS  SYSTEM.  241 

of  the  olfactory  impulses  is  from  the  olfactory  area  in  the  nose  to  the  olfactory 
bulb  of  the  same  side,  thence  via  the  olfactory  tract  to  its  termination  in  front 
of  the  anterior  perforated  space,  one  branch  of  the  tract  passing  directly  into 
the  substance  of  the  gyrus  fornicatus  at  this  point,  and  the  other  going  into 
the  more  lateral  portion  represented  in  man  by  the  temporal  end  of  the  gyrus 
hippocampi.  The  cortical  areas,  together  with  the  olfactory  lobe  and  tract, 
form  the  rhineucephalon  of  the  comparative  anatomists.  It  has  been  shown, 
nevertheless,  by  Hill l  that  in  anosmic  mammals  the  fascia  dentata  alone 
varies  with  the  development  of  the  olfactory  apparatus.  The  experimental 
pathological  evidence  is  very  meagre  in  relation  to  these  nerves,  but,  on  the 
other  hand,  the  anatomical  evidence  is  of  the  best.2 

D.  LOCALIZATION  OF  CELL-GROUPS  IN  THE  CEREBRAL  CORTEX. 

The  foregoing  section  has  brought  to  light  the  fact  that  groups  of  incom- 
ing impulses  find  their  way  to  the  cerebral  cortex.  The  significance  of  this 
is  evident  only  when  in  response  to  those  impulses  arriving  at  the  cortex 
others  leave  it,  and  finally  affect  some  expressive  tissue  or  instrument  by  the 
aid  of  which  we  can  interpret  them.  Since  the  cerebral  hemispheres  with 
their  cortex  become  increasingly  developed  as  we  pass  up  the  mammalian 
series,  it  naturally  follows  that  the  pathways  connecting  the  cortex  with  the 
lower  parts  of  the  system  are  correspondingly  increased.  Using  the  reactions 
of  the  expressive  tissues  as  a  guide,  it  is  our  present  purpose  to  trace  the 
impulses  in  those  cases  in  which  the  cortex  forms  part  of  the  path.  We  turn, 
therefore,  to  the  study  of  those  parts  of  the  cerebral  cortex  the  direct  stimu- 
lation of  which  produces  impulses  that  pass  to  cell-groups  lying  more  or  less 
caudad  in  the  central  system. 

Earlier  Observations. — It  was  demonstrated  by  Fritsch  and  Hitzig  in 
1870 3  that  if  a  constant  current  is  applied  to  the  surface  of  the  dog's  cere- 
brum, it  is  possible,  by  interrupting  it,  to  obtain  movements  of  the  limbs 
and  face  when  the  electrodes  are  placed  on  the  parts  of  the  cerebral  cortex 
about  the  sulcus  cruciatus.  The  reaction  varies  according  to  the  place  of 
stimulation,  a  constant  relation  subsisting  between  the  two.  From  this  time 
on,  active  investigations  of  the  relations  thus  suggested  have  been  pursued, 
both  by  stimulating  small  areas  in  the  cortex  of  various  animals,  including 
the  monkey  and  man,  and  by  the  removal  of  various  parts  of  the  cerebral  hemi- 
spheres and  cortex,  together  with  the  study  of  the  effects  of  pathological  lesions 
in  man.  The  results  following  removal  of  the  parts  are  complicated  by  the 
transitory  effects  of  the  lesion,  and  can  best  be  treated  by  themselves  later  on. 
The  results  following  the  stimulation  of  the  cortex  are  the  simplest,  and  will 
next  be  described. 

Stimulation  of  the  Cortex. — The  common  method  of  experiment  is  to 

1  Philosophical  Transactions  of  the  Royal  Society,  1893,  vol.  clxxxiv. 

2  For  a  description  of  the  very  complicated  pathways  associating  the  olfactory  bulb  with 
the  other  portions  of  the  cerebrum,  the  reader  is  referred  to  Barker's  The  Nenous  System,  1899. 

3  Archivfur  Anatomie  und  Physioloyie,  1870. 

VOL.  II.— 16 


242 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


apply  the  faradic  current  by  means  of  fine  but  blunt  electrodes,  the  ends  of 
which  are  but  two  or  three  millimeters  apart,  to  the  exposed  surface  of  the 
cerebral  hemispheres,  the  pia  being  undisturbed.  Rabbits,  dogs,  and  monkeys 
have  been  the  animals  most  commonly  studied. 

If  the  current  be  slight,  its  application  for  one  or  more  seconds  causes  a 
response  in  the  shape  of  movements  of  muscles,  which  are  thrown  into  co-ordi- 
nated contraction.  The  contraction  continues  for  some  time  after  the  stim- 
ulus has  been  removed.  When  the  stimulus  is  very  strong,  instead  of  a  lim- 
ited and  co-ordinated  response,  there  may  be  a  widespread  contraction  of 


FIG.  99.— Brain  of  the  macaque  monkey,  showing  the  sensory  and  motor  areas.  In  the  sensory  region 
the  name  of  the  sensation  is  over  the  locality  most  closely  associated  with  the  corresponding  sense-organ ; 
in  the  motor  region  the  name  of  the  part  is  written  over  the  portion  of  the  cortex  which  controls  it.  The 
upper  figure  gives  a  lateral  view  of  the  hemisphere,  and  the  lower  a  dorsal  view  (Beevor  and  Horsley). 

many  muscles,  resembling  an  epileptic  convulsion.  This,  however,  occurs 
more  commonly  in  the  lower  than  in  the  higher  mammals.  On  the  other 
hand,  the  irritability  of  the  cortex  is  easily  reduced,  so  that  it  becomes  irre- 
sponsive, and  often  immediately  after  the  first  exposure  of  the  brain  there  is 
a  time  during  which  no  response  can  be  obtained. 

Deferring  for  a  moment  the  evidence  by  which  the  sensory  characters  of 
the  several  areas  have  been  established,  and  also  the  arrangements  within  the 
cortex  by  which  any  group  of  muscles  can  be  made  to  respond  to  stimuli 
arriving  at  any  sensory  area,  we  shall  follow  out  the  distribution  of  those 


CENTRAL    NERVOUS  SYSTEM. 


243 


cortical  cells  which,  on  direct  stimulation,  cause  contractions  of  the  skeletal 
muscles. 

The  results  here  presented  were  obtained  from  the  electrical  stimulation 
of  the  monkey's  brain  by  Beevor  and  Horsley l  (see  Figs.  99,  100).  These 
experimenters  explored  the  exposed  surface  of  the  hemisphere  with  the  elec- 
trodes, moving  them  two  millimeters  at  a  time,  and  at  each  point  noting  the 
muscle-group  first  thrown  into  contraction. 

As  the  result  of  many  observations  on  the  monkey,  it  is  possible  to  map 
out  the  cerebral  cortex  in  the  following  way  :  The  surface  of  the  hemispheres 
is  divided  into  regions  (motor  and  sensory  regions),  which  are  the  largest 
subdivisions.  These  are  subdivided  into  areas  for  the  muscle-groups  belong- 
ing to  different  members  of  the  body — arms,  head,  trunk,  etc. — as  well  as 
those  areas  within  which  all  the  impulses  from  a  given  sense-organ  reach  the 


PoF. 


FIG.  100.— Mesial  surface  of  the  brain  (monkey).  The  localization  of  motor  functions  is  indicated 
along  the  shaded  portion  of  the  marginal  gyrus.  The  location  of  the  visual  area  is  indicated  at  the  tip  of 
the  occipital  lobe,  and  the  location  of  the  olfactory  area  at  the  tip  of  the  temporal  (Horsley). 

cortex.  The  areas  in  turn  are  subdivided  into  centres,  comprising  the  groups 
of  cells,  which,  for  example,  control  the  smaller  masses  of  muscle  belonging 
to  a  given  segment  of  a  limb,  or  in  the  visual  area  constitute  those  cells  espe- 
cially connected  with  one  part  of  the  retina.  There  is  thus  a  motor  region, 
the  stimulation  of  which  gives  rise  to  the  more  evident  bodily  movements. 
Within  this  are  several  subdivisions,  the  stimulation  of  one  of  which  is  fol- 
lowed by  movements  of  groups  of  muscles — for  instance,  those  controlling  the 
arm — and  within  such  an  area  in  turn  come  the  smaller  centres,  or  those  the 
stimulation  of  which  is  first  followed  by  movements  at  one  joint  only. 

The  physiological  characters  of  these  cortical  motor  centres  have  been 
determined  by  the  following  observations  : 

If  a  vertical  incision  be  carried  around  such  a  centre  so  as  to  isolate  it 
from  the  other  parts  of  the  cortex,  the  characteristic  reactions  still  follow  the 
stimulation  of  it,  indicating  that  the  special  effect  can  be  produced  by  the 
1  Beevor  and  Horsley  :  Philosophical  Transactions  of  the  Royal  Society,  1888-90. 


244          AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

passage  of  impulses  from  the  point  of  stimulation  toward  the  infracortical 
structures.  If,  in  addition,  a  cut  be  made  below  the  cortex  and  parallel  with 
its  surface,  then  stimulation  of  the  cortex  above  this  section  is  ineffective,  thus 
indicating  that  the  impulses  pass  from  the  cortex  directly  into  the  substance 
of  the  hemisphere  along  certain  nerve-tracts,  which  by  this  operation  were 
sectioned.  Further,  if  the  bit  of  cortex  thus  separated  from  the  underlying 
white  substance  be  removed  and  the  faradic  current  be  applied  to  the  white 
substance  beneath,  a  reaction  of  the  same  type  and  involving  the  same  mus- 
cles can  be  obtained,  although  it  differs  from  that  to  be  gotten  from  the  cor- 
tex itself,  in  the  first  place  by  being  less  co-ordinated,  in  the  second  by  con- 
tinuing only  so  long  as  the  stimulus  lasts,  and  in  the  third  place  by  giving 
rise  to  less  intense  electrical  changes  connected  with  the  passing  impulse. 
By  careful  exploration  the  bundle  of  fibres  which  is  thus  picked  out  can  be 
followed,  as  the  brain  substance  is  cut  away,  through  the  internal  capsule  and 
the  cerebral  peduncles. 

These  facts  taken  together  lead  to  the  conclusion  that  when  the  cortex  is 
stimulated  the  impulses  concerned  in  producing  the  muscular  contractions 
traverse  cell-bodies  at  the  point  of  stimulation,  and  are  transmitted  thence 
through  the  underlying  fibres.  We  shall  see  later  that  this  direct  course 
probably  does  not  represent  the  sole  pathway  for  these  impulses. 

Course  of  the  Descending1  Impulses. — The  course  of  the  impulses  is 
next  inferred  from  the  relation  between  the  removal  of  different  parts  of  the 
cortex  and  the  consequent  secondary  degenerations  throughout  the  length  of 
the  central  nervous  system.  When  the  part  of  the  cortex  removed  is  taken 
from  the  motor  area  then  the  degeneration  occurs  in  the  internal  capsule  and 
in  the  callosum.  The  path  of  the  fibres  forming  outgrowths  of  the  cortical 
cells  can  be  followed  thence  through  the  crusta  and  pyramids  to  the  spinal 
cord. 

After  removal  of  the  motor  region  of  one  cerebral  hemisphere  the  degen- 
eration is  mainly  in  the  internal  capsule  and  crusta  of  the  same  side,  though 
by  way  of  fibres  crossing  in  the  callosum  it  may  be  traced  to  the  other  side 
also.  At  the  decussation  of  the  pyramids  the  fibres  occupying  the  internal 
capsule  of  the  same  side  as  the  lesion  for  the  most  part  cross  the  middle  line. 
The  portion  which  remains  uncrossed  passes  as  the  direct  pyramidal  tract  of 
the  ventral  columns  in  man,  while  the  crossed  bundle,  which  is  much  the 
larger,  lies  in  the  dorsolateral  field  of  the  lateral  column,  forming  the  crossed 
pyramidal  tract.  Since  the  observations  of  Pitres1  in  1881-82  evidence  has 
been  accumulating  to  show  that  in  man  a  lesion  of  the  motor  cortex  of  one 
cerebral  hemisphere  is  followed  by  a  degeneration  of  the  crossed  pyramidal 
tract  on  both  sides  of  the  cord.  Of  course,  the  degeneration  in  the  hetero- 
lateral  tract  is  much  the  larger  of  the  two.  That  the  fibres  degenerating  in 
the  homolateral  tract  remain  on  the  same  side  throughout  their  entire  course 
is  shown  by  the  physiological  experiments  of  Wertheimer  and  Lapage 2  on 

1  ProgrZs  medicale,  Paris,  1882,  x.  528. 

2  Archives  de  Physiologic,  1897,  No.  1,  p.  168. 


CENTRAL    NERVOUS  SYSTEM. 


245 


dogs,  and  by  the  studies  of  Mellus l  on  secondary  degenerations  occurring  in 
the  cord  after  very  limited  lesions  of  the  motor  cortex  of  monkeys. 

The  direct  pyramidal  tracts  are  well  marked  only  in  man.  They  usually 
disappear  in  the  mid-thoracic  region,  having  entered  the  gray  substance  by 
way  of  the  ventral  commissure,  in  which  they  undergo  decussation.  The 
crossed  pyramidal  tract  shows  the  greatest  diminution  in  area  after  passing 
caudad  of  the  cervical  and  lumbar  enlargements  respectively,  and  hence  it  is 
inferred  that  the  pyramidal  Hbres  largely  terminate  at  these  levels  of  the 
cord. 


FIG.  101.— Schema  of  the  projection-fibres  within  the  brain  (Starr) ;  lateral  view  of  the  internal  cap- 
sule :  A,  tract  from  the  frontal  gyri  to  the  pons  nuclei,  and  so  to  the  cerebellum  ;  B,  motor  tract ;  C,  sen- 
sory tract  for  touch  (separated  from  B  for  the  sake  of  clearness  in  the  schema) ;  D,  visual  tract ;  E,  audi- 
tory tract ;  F,  G,  H,  superior,  middle,  and  inferior  cerebellar  peduncles ;  J,  fibres  between  the  auditory 
nucleus  and  the  inferior  quadrigeminal  body ;  K,  motor  decussation  in  the  bulb;  Vt,  fourth  ventricle. 
The  numerals  refer  to  the  cranial  nerves.  The  sensory  radiations  vare  seen  to  be  massed  toward  the 
occipital  end  of  the  hemisphere. 

Sherrington  has  put  forward  the  view  that  the  pyramidal  fibres  recross  in 
the  cord,  these  recrossing  fibres  being  derived  in  large  part  from  a  division 
of  the  pyramidal  fibres  into  two  branches,  one  of  which  may  cross  to  the 
opposite  side  of  the  cord,  while  the  other  continues  its  first  course.  Such 
dividing  fibres  he  designates  as  "  geminal  fibres ;"  and  the  number  of  them 
is  by  no  means  small. 

The  observations  of  Sherrington  were  made  on  monkeys  (Macacus)  and 

1  Proceedings  of  the  Royal  Society,  London,  1894  and  1895. 


246  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

dogs,  and  probably  the  arrangement  of  these  fibres  in  man  is  similar.  The 
observations  are  particularly  significant  as  gi\ring  another  anatomical  basis 
for  the  control  of  the  movements  in  both  halves  of  the  body  from  each 
cerebral  hemisphere. 

The  continuous  degeneration,  coupled  with  the  histological  evidence  for 
the  absence  of  intervening  nerve-cells,  indicates  that  the  cell-bodies  in  the 
cortex  have  axones  that  extend  all  the  way  to  the  cell-groups  of  the  spinal 
cord,  even  as  far  as  the  sacral  region.  The  usual  picture  of  the  final  connec- 
tions of  the  pyramidal  fibres  shows  the  collaterals  as  coming  into  contact 
with  the  large  cells  in  the  ventral  horns  of  the  cord.  On  the  ground  of 
recent  experiments  on  monkeys  and  cats,  Schafer1  denies  the  existence  of 
such  a  direct  association  of  the  two  sets  of  elements.  He  finds  that  a  large 
mass  of  collaterals  which  degenerate  when  the  pyramidal  tract  is  interrupted 
end  about  the  large  cells  in  the  column  of  Clarke. 

Returning  to  consider  the  arrangement  of  these  cells  in  the  cortex,  we  find 
that  the  axones  of  one  group  of  these  cortical  cells  pass  to  the  cell-groups  in 
the  cervical  enlargement,  while  those  from  others  pass  to  the  groups  in  the 
lumbar  enlargement.  It  thus  happens  that  if  the  spinal  cord  be  cut  across 
in  the  middle  of  the  thoracic  region,  and  then  the  leg  area  (see  Fig.  78)  be 
stimulated,  an  electrometer  applied  to  the  cut  end  of  the  cord  will  show  the 
passage  of  nerve-impulses,  because  the  electrometer  is  applied  to  a  tract  of 
fibres  on  their  way  to  the  lumbar  enlargement,  and  the  fibres  originate  from 
cortical  cells  within  the  region  stimulated. 

When,  however,  the  cortical  stimulus  is  made  in  the  arm-area,  the  electrom- 
eter being  applied  as  before,  no  electric  change  occurs,  for  the  axones  of 
the  cells  in  the  arm  terminate  in  the  part  of  the  cord  containing  the  cell- 
groups  which  control  the  muscles  of  the  arm,  and  these  groups  all  lie  cephalad 
to  the  point  of  section  of  the  cord.  It  is  evident,  therefore,  that  the  arrange- 
ment is  a  comparatively  simple  one — namely,  an  extension  of  the  axones  of 
the  several  groups  of  cortical  cells  from  the  different  areas  for  the  leg,  arm, 
face,  etc.,  to  the  axial  cell-groups  which  control  the  muscles  of  these  parts, 
and  which  are  situated  in  the  cord. 

The  cortical  cells  in  the  motor  region  belong  to  the  group  of  central  cells 
— i.  e.,  their  axones  never  leave  the  central  system — and  hence  they  are  en- 
gaged in  distributing  impulses  within  it.  To  the  axial  cell-groups  in  the  cord 
they  bring  impulses,  and  therefore,  from  the  standpoint  of  these  latter,  may 
be  considered  as  afferent,  Avhereas,  owing  to  the  fact  that  they  carry  impulses 
away  from  the  cortex,  they  are  sometimes  called  efferent.  Just  how  these 
two  sets,  the  cortical  and  the  cord  elements,  are  numerically  related  still 
requires  to  be  worked  out.  According  to  one  estimate,  there  are  for  the  arm, 
trunk,  and  leg,  in  man,  79,111  pyramidal  fibres  in  each  half  of  the  cord,  or 
158,222  in  the  entire  cord.2  The  number  of  fibres  in  the  pyramidal  tracts 
indicates*  that  there  certainly  is  not  one  fibre  for  each  cell  in  the  axial  cell- 

1  Journal,  of  Physiology,  vols.  xxiii.  and  xxiv.;  Proceedings  of  the  Physiological  Society. 
3  Blocq  et  Ozanoff:   Gaz.  des  HopiL,  1892. 


CENTRAL    NERVOUS  SYSTEM.  247 

groups,  because  the  number  of  pyramidal  fibres  is  very  much  less  than  is  the 
number  of  cells  which  they  control.  This  discrepancy  is  in  some  measure 
relieved  by  the  formation  of  "geminal"  fibres  already  described.  Moreover, 
the  branching  of  the  pyramidal  fibres  near  their  termination  is  very  probable, 
and  the  most  plausible  view  at  present  is  that  each  pyramidal  fibre  by  means 
of  its  collaterals  controls,  perhaps  indirectly,  a  considerable  number  of  cord 
cells,  and  probably  the  cells  controlled  by  any  one  fibre  form  a  more  or  less 
compact  group. 

Mapping-  of  the  Cortex. — Having  sketched  the  relations  of  the  pyramidal 
cells  forming  the  motor  region  of  the  cerebral  cortex  to  the  parts  lying  below, 
we  turn  to  study  the  arrangement,  size,  subdivisions,  and  comparative  anatomy 
of  this  region,  and  then  to  examine  the  relation  of  it  to  the  other  parts  of  the 
cortex.  The  observations  here  quoted  are  those  on  the  monkey  only. 

On  glancing  at  Fig.  99  it  is  evident,  first,  that  the  areas  for  the  face  and 
leg  are  widely  separated  from  each  other,  that  the  arm-area  lies  between 
them,  and  that  the  area  for  the  trunk,  though  less  schematically  placed,  is 
located  between  that  for  the  arm  and  leg.  This  arrangement  is  more  typi- 
cally represented  on  the  mesial  (Fig.  100)  than  on  the  convex  surface  of  the 
hemisphere,  and  in  the  former  locality  the  serial  order  of  the  cortical  areas 
corresponds  with  the  order  of  the  muscle-groups  which  they  control. 

The  Size  of  the  Cortical  Areas. — Evidently  there  is  no  direct  relation 
between  the  extent  of  a  cortical  area  and  the  mass  of  muscles  which  it  con- 
trols. Certainly  in  man  the  mass  of  muscles  in  the  leg  is  three  times  greater 
than  that  in  the  arm,  and  this  latter  many  times  greater  than  that  of  the  face 
and  head ;  yet  it  is  for  the  last  area  that  the  greatest  cortical  extent  is  found. 
Mass  of  muscle  and  extent  of  cortical  area  do  not  therefore  go  together. 

When  the  movements  effected  by  the  muscles  represented  in  these  several 
areas  are  considered,  we  find  that  such  movements  become  more  complex  and 
more  accurate  as  we  approach  the  head,  and  it  therefore  accords  with  the  facts 
to  consider  the  extent  of  the  motor  areas  as  correlated  with  the  refinement 
of  the  movements  which  they  control — a  relation  which  may  depend  even 
more  on  the  multiplication  of  the  pathways  bringing  in  impulses  than  on 
those  which  send  them  out. 

Subdivision  of  Areas. — The  areas  which  have  been  described  are  further 
subdivided,  the  subdivisions  in  the  arm-area  being  the  clearest.  Here  it  is 
found  that  the  stimulation  of  the  upper  part  of  the  arm -area  gives  rise  to 
movements  which  start  at  the  shoulder,  while  stimulation  at  the  lower  part 
of  this  area  gives  rise  to  movements  first  involving  the  fingers,  and  especially 
the  thumb.  The  centres  from  which  these  several  reactions  may  be  obtained 
occupy,  as  Fig.  99  shows,  narrow  fields  across  the  cortex  in  a  fronto-occipital 
direction.  Moreover,  the  centre  for  the  most  proximal  joint  of  the  arm  is 
farthest  removed  from  that  for  the  most  distal,  while  the  intermediate  joints 
are  represented  by  their  several  centres  lying  in  regular  order  between  these 
two.  A  similar  arrangement  appears  in  the  subdivisions  of  the  cortex  con- 
trolling the  leg,  and  in  the  face-area  as  well. 


248 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


FIG.  102.— Horizontal  section  of  the  human  cere- 
brum, showing  the  internal  capsule  on  the  left 
side :  F,  frontal  region ;  6,  knee  of  the  capsule ; 
NC,  NC,  caudate  nucleus ;  NL,  lenticular  nucleus ; 
O,  occipital  lobe  ;  TO,  thalamus ;  X,  X,  lateral  ven- 
tricle. In  the  internal  capsule  the  letters  indicate 
the  probable  position  of  the  bundles  of  fibres  which 
upon  stimulation  give  rise  to  movements  of  the 
parts  named  or  which  convey  special  sets  of  in- 
coming impulses  :  E,  eyes  ;  H,  head  ;  T,  tongue ;  M, 
mouth;  L,  shoulder;  B,  elbow;  D,  digits  ;  A,  abdo- 
men ;  P,  hip  ;  K,  knee ;  U,  toes ;  S,  temporo-occip- 
ital  tract ;  OC,  fibres  to  the  occipital  lobe ;  OP,  optic 
radiation  (based  on  Horsley). 

leg-area.    In  the  orang-utang,2  and  to 

1  Mann  :  Journal  of  Anatomy  and  Physiolo( 

2  Beevor  and  Horsley  :  Proceedings  of  the 


Interpreting  these  facts  in  the  terms 
of  nerve-cells  and  their  arrangement, 
it  appears  that  in  the  shoulder-centre 
the  axones  of  the  cortical  cells  that 
discharge  downward  affect  predomi- 
nantly the  efferent  cell-groups  which 
in  the  spinal  cord  directly  control  the 
muscles  of  the  shoulder,  and  that  a 
similar  arrangement  obtains  for  the 
other  centres  in  this  region  with  the 
corresponding  cell-groups  in  the  cord. 
The  stimulation  of  the  different  por- 
tions of  the  internal  capsule  where  it 
is  composed  of  bundles  of  fibres  com- 
ing from  the  motor  region  shows  (ob- 
servations on  orang-utang)  that  the 
fibres  running  to  the  several  lower 
centres  are  there  aggregated  and 
arranged  in  the  same  order  as  the 
cortical  centres  from  which  they  arise 
(see  Fig.  102). 

Separateness  of  Areas  and  Cen- 
tres.— As  we  ascend  in  the  mammalian 
series  there  is  an  increase  in  the  per- 
fection with  which  cells  forming  the 
several  centres  are  segregated,  though 
the  areas  in  the  different  orders  tend 
to  hold  the  same  relative  positions.1 

Figs.  103, 104  give  the  localizations 
obtained  in  the  rabbit's  brain  by  stim- 
ulation (Mann).  The  various  areas 
occupy  a  large  proportion  of  the  cortex, 
and  in  some  cases  come  very  close  to- 
gether, so  that  they  are  not  easily  sepa- 
rated by  experiment. 

In  the  lower  monkeys  (Macacus 
sinicus)  these  cell-groups  are  segre- 
gated, so  that  those  associated  with  the 
cervical  portion  of  the  cord  and  form- 
ing the  arm-area  are  much  more  to- 
gether and  quite  separate  from  those 
associated  with  the  lumbar  region,  the 
a  greater  extent  in  man,  a  further  sepa- 

gy,  1895,  vol.  xxx. 

Royal  Society,  London,  1890-91,  vol.  xlviii. 


CENTRAL    NERVOUS   SYSTEM. 


249 


FIG.  103.— Rabbit's  brain,  dorsal  view.  The 
areas  indicated  are  those  the  stimulation  of  which 
causes  a  movement  of  the  parts  named  (Mann). 


FIG.  104.— Rabbit's  brain,  lateral  view.  The 
areas  indicated  are  those  the  stimulation  of  which 
causes  a  movement  of  the  parts  named  (Mann). 


FIG.  105.— Lateral  view  of  the  left  hemisphere  of  an  orang-utang,  showing  the  motor  area  about  the  cen- 
tral fissure  (Beevor  and  Horsley). 


FIG.  106.— Lateral  view  of  a  left  human  hemisphere,  showing  the  motor  areas  in  man.  The  schema 
is  based  on  the  observations  on  the  monkey,  on  pathological  records  (human),  and  on  direct  experiments 
on  man.  It  is  to  be  remembered  that  in  the  human  brain  the  excitable  localities  are  surrounded  by 
rather  extensive  areas  not  directly  excitable  (Dana). 

ration  occurs,  so  that  these  centres  come  to  be  surrounded  by  parts  of  the  cortex 
from  which  no  response  can  be  obtained  upon  direct  stimulation  (see  Fig.  105). 


250 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


By  a  few  direct  experiments  and  by  many  pathological  observations  some- 
thing is  known  of  the  motor  centres  in  the  human  cerebral  cortex.     When 


FIG.  107.— Mesial  view  of  a  human  hemisphere,  showing  motor  areas. 

Fig.  106. 


Formed  in  the  same  way  as 


the  results  are  plotted  they  give  a  distribution  such  as  is  shown  in  Fig.  106. 

At  the  same  time  all  such  figures  are  largely  based  on  results  obtained  from 

the  monkey.  It  is  here  seen  that 
the  two  central  gyri  are  the  princi- 
pal seat  of  these  areas,  and  that  it 
is  only  along  the  great  longitudinal 
fissure  separating  the  hemispheres 
that  the  motor  areas  extend  beyond 
this  limit  in  a  cephalo-caudad  direc- 
tion. Perhaps  the  relation  most 
worthy  of  remark  is  the  compara- 
tively small  fraction  of  the  cortex 
concerned  with  the  direct  control 
of  the  spinal  cord  cells.  The  motor 
areas  in  man  are  elaborated  not  so 
much  by  the  increase  in  the  number 
of  the  cells  controlling  the  lower 
centres,  as  by  an  increase  in  the 
number  of  those  cells  under  the  in- 
fluence of  which  these  areas  react. 
According  to  the  estimates  of  Miss 
Thompson,1  there  are  in  man  9200 
millions  of  cells  in  the  entire  cere- 
bral cortex  of  both  hemispheres. 
In  the  motor  region  about  the  cen- 
tral fissures  there  appear  to  be  only 
159,600  cells  concerned  in  the  pro- 
duction of  pyramidal  fibres  going 

to  the  cord.  The  relation  of  the  areas  in  a  frontal  section  is  shown  in  Fig  108. 
Multiple  Control  from  the  Cortex. — It  has  been  found  that  stimulation 
1  Helen  B.  Thompson :  Journal  of  Comparative  Neurology,  1899,  vol.  ix.  No.  2. 


FIG.  108.— Frontal  section  of  human  cerebrum  on 
the  left  side.    The  fibres  forming  the  internal  capsule 

( ),  the  callosum  ( ),  and  the  anterior 

commissure  (.  —  .  —  .  —  )  have  been  indicated  :  T,  cor- 
tical area  for  the  trunk ;  L,  cortical  area  for  the  leg;  A, 
cortical  area  for  the  arm ;  F,  cortical  area  for  the  face ; 
A,  anterior  commissure ;  C,  callosum  ;  CO,  optic  chias- 
ma ;  NC,  caudate  nucleus ;  NL,  lenticular  nucleus ;  R, 
fornix ;  TO,  thalamus ;  X,  lateral  ventricle. 


CENTRAL    NERVOUS  SYSTEM.  251 

of  the  cortex  in  the  region  of  the  frontal  lobes  marked  "  eye  "  (Fig.  99)  was 
followed  by  movements  of  the  eye.  Schafer l  has  shown  that  very  precise 
movements  of  the  eye  also  follow  the  stimulation  of  the  temporal  and 
various  parts  of  the  occipital  cortex.  Since  the  efferent  fibres  which  control 
the  muscles  concerned  start  from  the  cell-groups  forming  the  nuclei  of  the 
third,  fourth,  and  sixth  cranial  nerves,  it  would  appear  most  probable  that  in 
both  parts  of  the  cortex  there  are  located  cells  the  axones  of  which  pass  to 
those  groups  and  are  capable  of  exciting  them.  An  alternative  hypothesis 
— namely,  that  the  cortical  impulse  always  travels  first  to  the  cortical  cells  in 
the  frontal  lobe  and  thence,  by  way  of  them,  to  the  efferent  cell-groups — was 
at  one  time  considered,  for  the  latent  period  of  contraction  of  the  eye  muscles 
was  less  by  several  hundredths  of  a  second  when  the  stimulus  was  applied  in 
the  frontal  region  than  when  applied  elsewhere.  The  experiments  of  Schafer 
show,  however,  that  when  the  occipital  and  frontal  lobes  are  separated  from 
one  another  by  a  section  severing  all  the  association-fibres  the  reactions  can 
still  be  obtained  by  stimulation  in  the  former  locality — showing  that  the  con- 
nections of  the  two  cortical  areas  with  the  cell-groups  controlling  the  muscles 
of  the  eye  are  independent  of  each  other. 

This  instance  of  the  direct  control  of  the  same  efferent  cell-groups  from  dif- 
ferent areas  of  the  cortex  is  analogous  to  the  control  of  efferent  cell-groups  in 
the  spinal  cord,  either  by  impulses  coming  down  from  the  cerebrum  or  by 
those  entering  the  cord  through  the  dorsal  roots,  and  the  instance  here  cited 
is  typical  of  a  general  arrangement. 

Cortical  Control  Crossed. — Where  the  stimulation  of  the  cerebral  cortex 
causes  a  response  on  one  side  only,  that  response  is  on  the  side  opposite  to 
the  stimulated  hemisphere.  It  sometimes  happens,  however,  that  two  groups 
of  symmetrically  placed  muscles  both  respond  to  the  stimulus  applied  to  one 
hemisphere  only ;  but  these  cases — the  conjugate  movements  of  the  eyes, 
movements  of  the  jaw  muscles  or  those  of  the  larynx — usually  depend  on  the 
response  of  muscles  which  are  naturally  contracted  together. 

This  last  reaction  must  be  determined  by  the  arrangement  of  the  fibres  in 
the  cord,  since  in  lower  mammals  (dog  and  rabbit,  for  example)  it  is  not 
seriously  disturbed  by  the  removal  of  one  hemisphere. 

Here  should  be  added  the  very  important  observations  of  Sherrington 2 
already  mentioned  on  p.  224,  which  show  that  a  stimulus  which  applied  to 
the  cortex  will  cause  one  set  of  muscles,  the  flexors  of  the  arm,  for  example, 
to  contract,  causes  at  the  same  time  a  relaxation  of  the  antagonistic  muscles, 
thus  rendering  co-ordination  possible  in  the  movements  of  the  limb. 

Course  of  Impulses  Leaving  the  Cortex. — In  the  higher  mammals,  as 
well  as  in  man,  it  is  by  way  of  the  pyramidal  fibres  that  impulses  travel  from 
the  cortex  to  the  efferent  cell-groups  of  the  cord.  The  pyramidal  tracts  by 
definition  form,  in  part  of  their  course,  the  bundles  of  fibres  lying  on  the  ven- 
tral aspect  of  the  bulb,  caudad  to  the  pons,  ventrad  to  the  trapezium,  and 

1  Proceedings  of  the  Royal  Society,  1888,  vol.  xliii. 

2  Sherrington :  Journal  of  Physiology,  1897-1898,  vol.  xxii. 


252  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

between  the  olivary  bodies.  According  to  Spitzka,1  these  bundles  are  absent 
in  the  case  of  the  elephant  and  porpoise,  a  condition  which  is  correlated  with 
the  slight  differentiation  of  the  limbs,  which  are  not  modified  either  for  fine 
movements  or  for  tactile  purposes.  It  has  been  pointed  out,  too,  that  removal 
of  a  hemisphere  causes  in  the  dog  and  most  rodents  a  degeneration  of  other 
parts  of  the  cord  (dorsal  columns)  than  those  occupied  by  the  pyramidal  tracts 
in  man.2  The  fibres  passing  from  the  cortex  to  the  efferent  cell-groups  in  the 
cord  do  not,  therefore,  hold  exactly  the  same  position  in  various  mammals. 

Size  of  Pyramidal  Tracts. — It  has  been  clearly  shown  that  if  the  cross- 
sections  of  the  cords  of  the  dog,  monkey,  and  man  be  drawn  of  the  same  size, 
the  pyramidal  fibres  being  indicated,  then  the  area  of  this  bundle  is  propor- 
tionately greatest  in  man  and  least  in  the  dog,  the  monkey  being  inter- 
mediate in  this  respect.  The  relations  thus  indicated  are  evident — namely, 
that  the  number  of  fibres  controlling  the  cell-groups  in  man  is  the  largest, 
and  also  is  much  larger  than  that  in  the  lower  animals. 

The  relative  areas  of  the  pyramidal  tract  at  corresponding  levels,  the 
area  of  the  entire  cord  being  taken  as  100  per  cent.,  are  given  by  v.  Len- 
hossek 3  for  the  following  animals : 

Mouse 1.14  per  cent. 

Guinea-pig 3.0        " 

Kabbit 5.3        " 

Cat 7.76       " 

Man 11.87       " 

This  relation  is  to  be  carefully  noted,  for  with  it  is  correlated  the  degree 
of  the  disturbances  in  the  reactions  of  the  entire  nervous  system  following 
removal  of  parts  of  the  cerebrum,  the  effect  being  slight  when  the  cerebrum 
is  connected  with  the  cord  by  a  small  number  of  fibres,  and  serious  when  the 
connection  is  by  many  fibres,  as  in  the  case  of  man  and  the  highest  mammals. 

E.    LOCALIZATION  IN  THE   CEREBRAL  CORTEX  OP  THE  CELL-GROUPS 

RECEIVING  THE   AFFERENT  IMPULSES. 

Sensory  Regions. — If  an  attempt  is  made  to  unify  the  construction  of 
the  entire  cortex  by  bringing  the  motor  and  sensory  areas  under  a  common  law, 
it  must  be  based  on  the  fact  that  the  system  of  axones  bringing  impulses  to 
the  motor  region  forms  part  of  the  pathway  for  conducting  the  afferent  im- 
pulses from  the  skin  and  muscles  back  to  some  organ  controlled  by  the 
efferent  nerves.  To  Munk  4  is  due  the  credit  of  having  from  the  first  looked 
upon  the  responsive  cortex  as  marked  off  into  areas  within  which  certain 
groups  of  these  fibres  terminate,  so  that  apart  from  the  sensory  areas  named 
from  the  special  senses,  he  calls  the  area  which  controls  the  skeletal  muscles 
the  "  Fuhlsphare  "  or  body-sense  area,  on  the  assumption  that  in  it  end  the 

1  Journal  of  Comparative  Medicine  and  Surgery,  1886,  vol.  vii. 

a  von  Lenhossek :  Anatomischer  Anzeiger,  1889. 

3  Diefeiner  Ban  des  Nervensystems  im  Lichte  neuester  Forschungen,  Basel,  1893. 

*  Ueber  die  Functionen  der  Grosshirnrinde,  1881. 


CENTRAL    NERVOUS  SYSTEM.  253 

fibres  bringing  in  impulses  which  arise  through  the  stimulation  of  the  skin  and 
muscles.  It  has  been  suggested,  to  be  sure,  that  separate  localities  form  the 
seat  for  the  dermal  and  muscular  sensations.  Ferrier  indicated  the  limbic 
lobe,  especially  the  hippocampal  gyrus,  while  Horsley  and  Schiifer  argued 
for  the  gyrus  fornicatus.  At  present,  the  weight  of  evidence  is  in  favor  of 
the  location  of  the  centres  for  dermal  and  muscular  sensations  in  the  central 
gyri,  a  part  just  caudad  to  and  a  part  overlapping  the  area  stimulation  of 
which  causes  the  muscles  of  the  trunk  and  limbs  to  contract.  Both  in 
monkeys  and  in  man  defects  in  sensation  are  not  permanent  after  limited 
lesions  of  the  cortex,  but,  as  suggested  by  Mott,  the  wide  distribution  of  the 
incoming  impulses  would  explain  this  result. 

Thus  the  entire  portion  of  the  cortex  to  which  a  definite  function  can  be 
assigned  must  be  looked  upon  as  containing  fibres  which  bring  impulses  into 
it,  and  cell-bodies  which  by  their  discharge  send  impulses  to  other  divisions 
of  the  central  system  as  well  as  to  other  parts  of  the  cortex  itself.  All  parts 
of  the  cortex  having  assigned  functions  give  rise  on  stimulation  to  move- 
ments ;  but  in  the  case  of  the  sensory  areas,  so  called,  they  involve  the  con- 
tractions of  only  those  muscles  controlling  the  external  sense  organ,  as  the 
eyeball,  external  ear,  tongue,  and  nostrils.1  Though  physiologically  impor- 
tant, and  in  the  case  of  the  eye  reaching  a  high  degree  of  refinement,  they 
are  quantitatively  very  insignificant  when  compared  with  the  responses  to  be 
obtained  from  stimulating  the  ''  motor  region/'  from  which  contractions  of 
the  larger  skeletal  muscles  are  obtained.  Hence  the  usual  terms  "  sensory  " 
and  "  motor "  do  not  completely  characterize  the  corresponding  regions, 
though  they  emphasize  their  most  striking  features. 

Determination  of  the  Sensory  Areas.  — Using  as  a  guide  the  appearance 
of  the  medullary  sheaths  upon  the  projection-fibres  of  the  cerebral  cortex  of 
man  during  the  last  months  of  foetal  life  and  shortly  after  birth,  Flechsig2 
has  been  able  to  outline  the  sensory  areas  in  the  cortex  with  great  clearness. 

The  illustrations  from  Flechsig  (Figs.  109,  110)  show  the  parts  of  the 
brain  where  the  projection-fibres  can  be  determined  at  a  time  when  these 
fibres  constitute  all  or  almost  all  the  medullated  fibres  connecting  the  cortex 
with  the  stem  and  basal  ganglia.  By  thus  marking  out  in  color  on  the 
developing  cortex  the  portions  concerned,  there  are  seen  to  be  four  main 
areas :  First,  the  area  connected  with  the  olfactory  tract  (olfactory  area), 
involving  the  uncinate  gyrus,  the  gyrus  hippocampi,  and  the  part  of  the  gyrus 
fornicatus  nearest  the  callosum.  Second,  the  area  connected  with  the  optic 
radiation  (visual  area),  where  the  fibres  in  question  are  most  abundant  about 
the  calcarine  fissure.  They  appear,  however,  all  through  the  cuneus  and 
extend  to  the  cortex  which  surrounds  it  on  the  ventral  and  lateral  aspects  of 
the  occipital  lobe.  Third,  we  have  (auditory  area)  the  portion  of  the  cortex 
which  covers  the  transverse  gyri  in  the  Sylvian  fissure  and  the  first  temporal 
gyrus  where  the  former  join  it.  This  area  is  occupied  by  the  projection- 

1  Ferrier  :  Functions  of  the  Brain,  1876. 
"Flechsig:   Gehirn  und  Seele,  Leipzig,  1896. 


254 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


fibres  which  convey  the  impulses  arriving  over  the  auditory  nerve  (the  coch- 
lear  branch  of  the  eighth).     Finally  the  fourth  area  is  seen  (area  for  "  body- 


FIG.  109.— The  colored  portion  about  the  cuneus,  especially  that  more  deeply  colored  about  the  cal- 
carine  fissure,  shows  the  msual  area  as  seen  from  the  mesial  surface.  The  portion  comprising  the  deeply 
colored  tip  of  the  hippocampal  gyrus,  the  dorsal  portion  of  the  hippocampal  gyrus,  and  the  edge  of  the 
gyrus  fornicatus  through  its  entire  extent,  marks  the  olfactory  area.  The  remaining  portion,  occupying 
the  paracentral  gyrus  and  the  mesial  aspect  of  the  first  frontal  gyrus,  marks  the  mesial  extension  of 
the  body-sense  area.  The  uncolored  portions  of  the  cortex  form  the  association  centres  of  Flechsig.  F,  pes ; 
HS,  crura ;  Z,  pineal  body  ;  1,  corpus  albicans  ;  2,  chiasma ;  3,  anterior  commissure ;  4,  quadrigemina ; 
5,  callosum  ;  6,  fornix ;  7,  septum  lucidum  (from  Flechsig). 


FIG.  110.— The  colored  portion  at  the  tip  of  the  occipital  lobe  represents  the  postero-lateral  extension 
of  the  visual  area.  The  colored  portion  about  the  central  fissure  and  the  neighboring  parts  of  the  frontal 
lobe  represents  the  lateral  extension  of  the  body-sense  area.  The  colored  portion  about  the  caudal  end  of 
the  first  temporal  gyrus,  and  extending  over  the  transverse  gyri  within  the  Sylvian  fissure,  represents  the 
auditory  area.  In  all  three  areas  the  portions  most  deeply  colored  represent  the  areas  where  the  projec- 
tion-fibres are  most  abundant.  The  uncolored  portions  of  the  cortex  form  the  association  centres  of  Flechsig 
(from  Flechsig). 

sense  "),  which  is  most  richly  supplied  with  projection-fibres  about  the  central 
fissure,  in  the  two  central  gyri — but  also  extends  forward  on  the  lateral  sur- 


CENTRAL   NERVOUS  SYSTEM.  255 

face  to  include  part  of  all  the  frontal  gyri,  while  on  the  mesial  surface  the 
cortex  concerned  extends  forward  from  the  precuneus  over  more  than  half 
of  the  mesial  surface.  In  this  last  area  are  delivered  the  afferent  impulses 
from  the  skin,  muscles,  joints,  viscera,  and  the  lining  walls  of  the  alimentary 
tract.  Flechsig  points  out  that  the  projection-fibres,  according  to  which  these 
areas  have  been  defined,  are  composed  of  axones  bringing  impulses  to  the 
cortex,  and  hence  are  sensory,  in  the  usual  terminology.  The  areas  thus 
bounded  are  found  to  coincide  with  the  areas  which  (in  animals)  respond  to 
direct  stimulation  by  the  contraction  of  definite  groups  of  muscles. 

The  earlier  determinations  of  the  sensory  areas  in  man  were  made  from 
the  study  of  brains  modified  by  destructive  lesions  or  congenital  defects. 

The  cortical  centre  for  smell,  inferred  from  comparative  anatomy  and 
physiology  to  be  closely  connected  with  the  hippocampal  and  fornicate  gyri 
and  the  uncus,  has  been  similarly  located  in  man  on  the  basis  of  pathological 
observations  ;  but  the  evidence  lacks  precision.  Concerning  the  location  of 
taste  sensations,  very  little  is  known.  Both  of  these  senses,  it  must  be 
remembered,  are  insignificant  in  man,  and  hence  their  central  connections  are 
not  easily  studied. 

On  the  other  hand,  the  cortical  areas  for  hearing  and  sight  have  been 
located  with  much  more  precision  and  certainty. 

Damage  to  the  posterior  third  of  the  first  temporal  gyrus  and  the  asso- 
ciated gyri  transversi  causes  in  man  deafness  in  the  opposite  ear,  and  con- 
cordantly  conditions  of  the  ear  which  early  in  life  lead  to  deafness  and  deaf- 
mutism  are  accompanied  by  a  lack  of  development  in  these  gyri.1  Destruc- 
tion of  this  area  on  one  side  causes  slight  deafness  mainly  in  the  opposite 
ear,  while  complete  deafness  follows  a  cortical  lesion  only  when  it  is  double. 

In  the  case  of  the  visual  areas  in  man  there  is  the  same  sort  of  evidence, 
but  somewhat  more  exact.  The  destruction  of  the  area  represented  by  the 
cuneus  and  the  surrounding  cortex  (Figs.  109  and  110)  always  injures  vision, 
the  maximum  disturbance  following  injury  to  the  cortex  of  the  calcarine 
fissure.  Conversely,  the  failure  of  the  eyes  to  grow,  arrests  the  development 
of  this  portion  of  the  hemisphere. 

Hemianopsia. — It  is  found,  moreover,  that  injury  to  the  visual  area  in 
one  hemisphere  usually  produces  a  hemianopsia  or  partial  defect  of  vision  in 
both  retinae.  The  homonymous  halves  are  affected  on  the  same  side  as  the 
lesion  and  the  dividing  line  is  usually  vertical.  The  clinical  picture  corre- 
sponds to  a  semi-decussation  of  the  optic  tract  and  the  representation  of  the 
homonymous  halves  of  each  retina  in  both  hemispheres.  At  the  same  time 
the  relation  is  much  more  complicated  than  at  first  sight  appears,  for  the 
point  of  most  acute  vision  is  often  unaffected  in  such  casesl  This  peculiarity 
depends  apparently  on  the  fact  that  there  is  a  binocular  centre  for  macular 
vision  in  the  cortex  lining  the  sides  and  bottom  of  the  posterior  portion  of 
the  calcarine  fissure.2 

1  Donaldson :  American  Journal  of  Psychology,  1891-2,  vol.  iv. 

2  Laqueur  and  Schmidt :    Virc.how's  Archiv,  1899,  Bd.  158,  Heft  3,  S.  467. 


256 


AN  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 


In  the  case  of  neither  vision  nor  hearing  do  we  find  in  man  any  subcortical 
cell-groups  capable  of  acting  as  centres ;  that  is,  after  the  destruction  of  the 
appropriate  cortical  region  the  corresponding  sensations  and  reactions  to  the 
stimuli  which  arouse  these  sensations  are  completely  and  permanently  lost. 

From  these  facts,  therefore,  it  appears  that  the  impulses  which  give 
rise  to  visual  and  auditory  sensations  are  delivered  in  certain  parts  of  the 
cerebral  cortex,  and  unless  they  arrive  there  the  appropriate  sensations  are 
wanting. 

Association  Fibres  and  Association  Centres. — Common  experience 
shows  us  that  we  can  voluntarily  contract  any  group  of  muscles  in  response 
to  any  form  of 'stimulus — dermal,  gustatory,  olfactory,  auditory,  or  visual. 
When,  therefore,  the  hand  is  extended  in  response  to  a  visual  stimulus,  the 


FIG.  111.— Lateral  view  of  a  human  hemisphere,  showing  the  bundles  of  association-fibres  (Starr) : 
A,  A,  between  adjacent  gyri;  B,  between  frontal  and  occipital  areas;  C,  between  frontal  and  temporal 
areas,  cingulum;  D,  between  frontal  and  temporal  areas,  fasciculus  uncinatus;  E,  between  occipital  and 
temporal  areas,  fasciculus  longitudinalis  inferior ;  C,  N,  caudate  nucleus;  0,  T,  optic  thalamus. 

nerve-impulses  pass  first  to  the  visual  area,  and  then  in  an  indirect  manner 
arouse  the  cortical  cells  controlling  the  muscles  of  the  hand.  This  connec- 
tion of  the  two  areas  is  accomplished  through  the  so-called  association-fibres 
of  the  cortex.  These  fibres  are  formally  defined  as  those  which  put  into 
connection  different  parts  of  one  lateral  half  of  any  subdivision  of  the  central 
system  (see  Fig.  111). 

The  bundles  which  are  thus  shown  in  the  cerebral  hemisphere  must  be 
looked  upon  as  typical  of  the  arrangement  throughout  the  entire  cortex,  and, 
further,  the  arrangement  in  the  cortex  is  typical  of  that  in  other  parts  of  the 
central  system.  Anatomy  would  suggest,  and  pathology  would  bear  out  the 
suggestion,  that  it  is  by  these  tracts  that  the  impulses  travel  from  one  area  to 
another. 


CENTRAL    NERVOUS  SYSTEM.  257 

The  term  "  association  centres  "  is  applied  by  Flechsig '  to  those  portions 
of  the  cerebral  cortex  that  lie  between  the  sensory  centres  which  he  has  been 
able  to  demonstrate.  The  functions  of  the  association  centres  are  first  to 
furnish  pathways,  more  or  less  intricate,  between  the  several  centres,  and 
second,  to  retain  as  memories  previous  sense  impressions,  so  that  in  acting 
they  also  modify  the  impulses  sent  into  them,  and  by  these  modifications 
shade  and  adjust  to  an  almost  infinite  degree  the  form  of  the  final  response. 

On  looking  at  Figs.  109,  110,  we  note  two  well-defined  areas  :  (1)  that 
occupying  the  frontal  lobe  and  forming  the  great  anterior  association  centre, 
and  (2)  the  area  in  the  parieto-temporal  region  which  forms  a  second,  the 
posterior  association  centre.  The  third,  the  middle  association  centre  coin- 
cides with  the  Island  of  Reil,  and  is  much  less  in  evidence.  On  comparison 
it  will  be  seen  that  these  regions  correspond  to  what  have  been  called  the 
"latent  areas"  of  the  cortex,  because  no  evident  response  follows  the  direct 
stimulation  of  them.  When  we  compare  the  extent  of  these  association 
centres  in  man  with  that  in  other  mammals,  even  the  apes,  we  find  the 
human  brain  characterized  by  the  high  development  of  these  portions. 
Thus  Flechsig  feels  justified  in  speaking  of  these  association  centres  as  the 
"  organs  of  thought,"  and  in  pointing  out  how  by  means  of  them  the  incom- 
ing sense  impressions  are  made  to  interact  on  one  another,  and  in  combination 
with  the  memory  images  which  are  thus  aroused  give  rise  to  new  ideas. 

The  association  processes  carried  on  by  these  several  centres,  are  modified 
by  their  location,  so  that  the  several  centres  have  different  and  distinct  values. 
With  the  disturbance  of  these  association  centres  are  correlated  the  several 
sorts  of  mental  defects  which  have  been  gathered  under  the  term  aphasia. 

Aphasia. — The  development  of  the  ideas  bearing  on  this  subject  has  been 
slow.  After  the  publication  of  the  great  work  of  Gall  and  Spurzheim  (1810- 
19)  on  the  brain,  some  pathologists  (Bouillaud,  1825;  Dax,  1836),  especially 
in  France,  were  in  seach  of  evidence  touching  the  doctrine  of  the  localization 
of  function.  At  the  same  time  the  subject  of  phrenology,  as  put  forward  by 
Gall  and  Spurzheim,  was  not  in  good  repute,  and  anything  which  looked 
that  way,  even  in  a  slight  degree,  was  generally  scouted.  Broca,  however, 
published  (1861)  the  important  observation  that  when  the  most  ventral  or  the 
third  frontal  convolution  in  the  left  hemisphere  (often  designated  Broca's  con- 
volution) was  thrown  out  of  function,  the  power  of  expression  by  spoken 
words  was  lost.  For  this  reason,  the  name  of  "  speech-centre "  has  been 
applied  to  this  convolution. 

Since  this  discovery  which  links  the  neurology  of  the  first  part  of  the  cen- 
tury with  that  of  to-day,  and  also  forms  a  fundamental  observation  in  the 
modern  doctrine  of  cerebral  physiology,  many  steps  have  been  taken. 

It  was  early  observed  that  although  in  such  cases  the  capacity  for  spoken 
language  was  lost,  nevertheless  the  muscles  which  were  used  in  the  act  of 
phonation  were  by  no  means  paralyzed.  This  relation  is  due  probably  to  the 
fact  that  the  muscles  are  innervated  from  both  hemispheres  and  possibly  also- 

1  Flechsig  :  Loc.  cit. 
VOL.  II.— 17 


258 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


from  localities  outside  the  third  frontal  gyms.  Experiments  show  that  in 
animals  stimulation  of  the  cortex  in  the  region  corresponding  to  the  third 
frontal  gyrus  causes  contractions  of  many  of  the  muscles  employed  in 
speech.1 

The  interesting  observation  was  also  made  that  in  the  normal  right-handed 
person  the  muscles  of  phonation  could  not  be  co-ordinated  for  speech  from 
the  right  hemisphere  alone.  Thus  the  symmetrical  portion  of  the  right 
hemisphere  has  not  the  same  physiological  value. 

Besides  this  lesio«p,  which  involves  the  cortex  in  front  of  the  motor  region 
proper,  numerous  other  lesions — namely,  those  which  involve  the  tracts  run- 
ning between  the  areas  of  special  sensation  (vision  and  hearing,  for  example), 
and  the  motor  or  expressive  region — produce  corresponding  disturbances  (see 
Fig.  112). 

An  individual  in  whom  the  association  tracts  between  the  visual  and  motor 
areas  have  been  interrupted  can,  for  instance,  see  an  object  presented  to  him 
in  the  sense  that  he  gets  a  visual  impression ;  but  because  of  the  interruption 
of  the  association  fibres  the  object  is  not  recognized,  and  the  impulses  reach- 
ing this  sensory  area  elicit  no  re- 
sponse  from   those   muscles  the 
motor    centres     for    which    are 
located  outside  of  the  receiving 
cortex. 

Upon  attempting  to  picture 
the  anatomical  arrangement  in 
anything  like  the  completeness 
demanded  by  the  physiological 
reactions,  it  is  necessary  to 
postulate  the  existence  of  asso- 
ciation pathways  between  each 
area,  whether  sensory  or  motor, 
and  all  the  others.  This  arrange- 
ment is  to  be  regarded  as  modi- 
fied in  several  ways. 

In  the  first  place,  the  connec- 
tion between  a  given  sensory  and  a  given  motor  area  differs  widely  accord- 
ing to  the  areas  concerned.  The  connection,  for  example,  between  the  visual 
area  and  the  motor  area  for  the  arm  is  probably  represented  by  more  nerve- 
elements,  and  these  better  organized,  than  the  connection  between  the  gusta- 
tory area  and  that  for  the  movements  of  the  leg. 

When,  therefore,  it  is  said  that  such  connections  exist,  it  must  be  added 
always  that  the  nexus  is  different  for  the  several  regions  concerned,  and  what 
is  more,  that  in  man,  at  least,  it  is  different  for  the  two  hemispheres. 

Relative  Importance  of  the  Two  Hemispheres. — The   cerebral  cortex 


ii 


FIG.  112.— Lateral  view  of  a  human  hemisphere ;  cor- 
tical area  V,  damage  to  which  produces  "  mind-blind- 
ness"; cortical  area  H,  damage  to  which  produces 
"mind-deafness";  cortical  area  S,  damage  to  which 
causes  the  loss  of  audible  speech ;  cortical  area  W,  dam- 
age to  which  abolishes  the  power  of  writing. 


1  Semon  and  Horsley  :  Philosophical  Transactions  of  the  Royal  Society,  1890,  vol.  181. 
Deutsche  medicinische  Wochenschrift,  No.  31,  1890. 


Also 


CENTRAL   NERVOUS  SYSTEM.  259 

is  always  active  during  our  periods  of  consciousness,  and  it  is  to  be  thought 
of  as  a  region  over  which  the  focal  point  of  intensest  activity  is  continually 
shifting — this  focal  point,  wherever  it  may  be,  having  about  it  a  halo  of  less 
active  cells  as  extensive  as  the  cortex  itself. 

When  the  subject  is  right-handed,  it  appears  that  injury  to  the  left  cere- 
bral hemisphere  is  productive  of  more  disturbance  than  injury  to  the  right 
hemisphere.  At  the  same  time,  lesion  of  the  left  hemisphere  is  far  more  fre- 
quent than  that  of  the  right.  So  far  as  can  be  judged  from  experiments  on 
man,  the  higher  sense-organs,  the  eye  and  the  ear,  are  more  perfect,  physio- 
logically, on  the  right  side.  Since  the  connection  of  the  sense-organs  is  largely 
with  the  cortex  of  the  contra-lateral  hemisphere,  this  means  that  the  impulses 
going  mainly  to  the  left  hemisphere  are  better  differentiated  than  those  going 
to  the  right,  and  it  would  appear  to  be  easier  for  these  impulses  to  reach  a 
motor  area  in  the  same  hemisphere  than  to  reach  the  corresponding  area  on 
the  opposite  side.  It  is  further  true  that  in  right-handed  persons  the  cortical 
activities  of  the  left  hemisphere  in  the  region  of  the  body  sense-area,  must 
always  be  greater  than  those  of  the  opposite  hemisphere,  and  these  two 
circumstances  cannot  fail  to  have  a  profound  influence.  The  observations  of 
Flechsig1  on  the  pyramidal  tracts  also  show  that  these  tracts,  before  medulla- 
tion  at  least,  may  be  unevenly  developed  on  the  two  sides  of  the  cord,  and  the 
ease  of  control  may  thus  be  rendered  unequal — a  condition  which  must  be 
dominant  in  the  determination  of  the  side  of  the  body  which  shall  be  most 
exercised.  Be  this  as  it  may,  the  lesions  which  cause  aphasia  or  apraxia 
(inability  to  determine  the  meaning  and  use  of  objects),  are  predominantly 
in  the  left  hemisphere  in  persons  who  are  right-handed,  while  there  is 
some  evidence  that  the  right  hemisphere  is  more  important  in  left-handed 
persons. 

In  the  adult,  damage  to  one  hemisphere  is  usually  followed  by  a  perma- 
nent loss  of  function,  to  a  greater  or  less  degree,  but  this  loss  may  be  more 
transient  and  less  serious  when  the  lesion  occurs  in  the  very  young  subject, 
so  that  during  the  growing  period  the  sound  hemisphere  can  in  a  measure 
replace  the  one  that  has  been  injured. 

Assuming  this  general  plan  for  the  arrangement  of  the  cortex  to  be  cor- 
rect, it  is  evident  that  a  given  cell,  the  axone  of  which  forms  part  of  the 
pyramidal  tract,  must  in  the  human  cortex  be  subject  to  a  large  series  of 
impulses  coming  to  it  over  as  many  paths.  Schematically,  it  would  be  as 
represented  in  Fig.  113. 

The  discharging  cell  may  be  destroyed;  then,  of  course,  the  muscles  con- 
trolled by  it  become  paralyzed  for  voluntary  movements.  The  discharging 
cell  may,  however,  remain  intact,  but  the  pathways  by  which  impulses  arrive 
at  it  be  damaged.  This  is  the  type  of  lesion  which  produces  symptoms  of 
aphasia.  When  an  interruption  of  associative  pathways  occurs  some  one  or 
more  of  these  tracts  is  broken,  and  hence  the  discharging  cell  does  not  receive 
a  stimulus  adequate  to  cause  a  response. 

1  Leitunysbahnen  im  Gehrin  und  Ruckenmark,  1876. 


260 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


The  physiological  complexity  of  the  elements  in  any  part  of  the  central 
system,  either  when  different  portions  of  the  system  from  the  same  animal  or 
when  the  corresponding  portions  of  different  animals  are  compared,  depends 
on  the  number  of  paths  by  which  the  impulses  are  brought  to  the  discharging 
cells. 

Composite  Character  of  Incoming-  Impulses. — To  these  conclusions 
based  on  the  anatomy  are  to  be  added  others  suggested  by  clinical  observa- 
tions. In  order  that  a  patient  suffering  from  a  lesion  between  the  visual  and 
motor  areas  may  be  able  to  recognize  an  object  and  to  indicate  its  use,  it  is 
sometimes  necessary  that  the  object  shall  appeal  to  several  senses.  For 
example,  the  name  and  use  of  a  knife,  when  seen  alone,  may  not  be  recalled, 


FIG.  113.— Schema  showing  in  a  purely  formal  manner  the  different  sort  of  afferent  impulses  which  may 
influence  the  discharge  of  a  cortical  cell. 

but  when  it  is  taken  into  the  hand — that  is,  when  the  dermal  and  muscular 
sensations  are  added  to  the  visual  one — the  response  is  made,  though,  acting 
alone,  any  one  set  of  sensations  is  inadequate  to  produce  this  result. 

Just  where  the  block  occurs  in  such  a  case  it  is  not  possible  to  say  with 
exactness,  but  the  lesion  lies,  as  a  rule,  between  the  sensory  and  motor  areas 
concerned,  and  by  the  damage  to  the  pathway,  it  is  assumed  that  one  or  more 
groups  of  impulses  are  so  reduced  in  intensity  that  they  are  alone  insufficient 
to  produce  a  reaction  ;  and  therefore  it  is  only  when  the  impulses  from  several 
sources  are  combined  that  a  response  can  be  obtained. 

Variations  in  Association. — It  is  a  familiar  fact  that  individuals  differ 


CENTRAL   NERVOUS  SYSTEM.  261 

in  no  small  degree  in  the  acuteness  of  their  senses — i.  e.,  in  the  power  to  dis- 
criminate small  differences,  and  this,  too,  when  the  sense-organs  are  normal. 
Further,  the  powers  of  those  best  endowed  are  by  no  means  to  be  attained  by 
others,  however  conscientious  their  training.  Moreover,  the  sensory  path- 
ways differ  widely.  The  inference  is  fair,  therefore,  that  those  who  think  in 
terms  of  visual  images,  as  compared  to  those  who  think  in  auditory  images,  do 
so  by  virtue  of  the  fact  that  in  the  former  case  the  central  cells  concerned  in 
vision  are  distinctly  the  better  organized,  while  in  the  latter  case  it  is  those 
concerned  in  hearing. 

In  the  same  way,  the  power  of  expression  varies  in  an  equally  marked 
degree,  and  the  capacity  for  the  expression  of  ideas  by  means  of  the  hand,  in 
writing,  is  by  no  means  necessarily  equal  to  the  power  of  expression  by  means 
of  spoken  words.  In  the  former  case  we  have  the  results  of  the  play  of 
impulses  from  the  several  sensory  centres  on  the  motor  area  for  the  hand,  and 
this  is  reinforced  by  the  sight  of  that  which  has  been  written,  whereas  in  the 
latter  case  impulses  from  these  same  sensory  centres  play  upon  the  area  which 
controls  the  muscles  of  phonation,  and  this  reaction  is  reinforced  by  the  sound 
of  the  words  uttered.  Of  course,  in  the  case  of  a  defective,  like  a  blind-deaf- 
mute,  the  expression  of  thought  is  by  movements  of  the  fingers,  and  this  is 
reinforced  by  the  tactile  and  muscular  sensations  which  follow  these  move- 
ments. 

It  is  not  by  any  means  to  be  expected  that  the  anatomical  connections 
which  render  such  reactions  possible  will  be  equally  perfect  for  the  different 
sensori-motor  combinations,  or  the  same  combinations  in  different  persons, 
and  hence  the  powers  of  the  individual  will  be  modified  by  the  varying  per- 
fection of  these  paths.  From  this  it  also  follows  that  the  same  lesion  as 
grossly  determined,  will  not  produce  identical  results  in  the  two  persons,  for 
it  will  not  effect  the  damage  of  structural  elements  which  are  strictly  com- 
parable. 

Latent  Areas. — It  has  been  plain  from  an  examination  of  the  foregoing 
figures,  as  well  as  from  the  descriptions,  that  there  must  be  a  large  portion  of 
the  cortex  which,  so  far  as  has  been  observed,  may  be  called  latent.  These 
areas,  which  include  nearly  the  entire  ventral  surface  of  the  hemispheres,  a 
large  part  of  the  mesial  surface,  and  on  the  dorsal  and  lateral  aspects  a  large 
portion  of  the  frontal,  parietal,  and  temporal  lobes  together  with  the  island, 
certainly  require  a  word. 

These  last  correspond  with  the  " association  centres"  as  described  by 
Flechsig.  To  direct  stimulation  they  give  no  response.  From  any  one 
portion  of  the  latent  area  the  connections  are  not  massive  enough  to 
permit  of  impulses  which  will  cause  a  muscular  contraction,  and  hence 
these  impulses  coming  from  one  locality  to  a  discharging  cell  form  only  a 
fraction  of  the  impulses  which  control  it.  For  this  reason  the  significance 
of  these  parts  fails  to  be  clearly  evident  upon  direct  experiment. 

The  cortex  of  the  frontal  lobes  has  some  connections  with  the  nuclei  of 
the  pons,  and  so  with  the  cerebellum.  The  more  recent  experiments  on  th« 


262  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

functions  of  this  region  are  by  Bianchi ]  and  Grosglik,2  the  former  on  monkeys 
and  dogs  and  the  latter  on  dogs  alone. 

These  experimenters  found  that  the  removal  of  one  frontal  lobe  is  com- 
paratively insignificant  in  its  effects,  while  when  both  are  removed  the  change 
is  profound.  On  removing  the  frontal  lobe  on  one  side  only  there  is  no  dis- 
turbance of  vision,  hearing,  intelligence,  or  character.  There  do  occur  both 
sensory  and  motor  disturbances,  but  these  are  for  the  most  part  transient. 
On  the  side  opposite  to  the  lesion  there  is  in  the  limbs  a  blunting  of  all  sen- 
sations and  some  paresis.  Moreover,  there  is  a  hyperaesthesia  combined  with 
a  paresis  of  the  muscles  of  the  neck  and  trunk  which  move  these  parts 
away  from  the  side  of  the  lesion. 

These  several  effects  of  the  operation  tend  to  pass  off,  and  if  then  the 
remaining  frontal  lobe  be  removed  from  a  dog  or  monkey,  not  only  do  the 
symptoms  just  described  appear  on  the  other  side  of  the  body,  but  still  more 
fundamental  changes  occur.  A  ceaseless  wandering  to  and  fro,  such  as 
Goltz3  observed  in  those  dogs  in  which  the  anterior  half  of  the  brain  had 
been  removed,  characterizes  the  animals ;  curiosity,  affection,  sexual  feeling, 
pleasure,  memory,  and  the  capacity  to  learn  are  at  the  same  time  abolished, 
and  the  expressions  of  the  animal  are  those  of  fear  and  excessive  irritability. 
That,  therefore,  the  frontal  lobes  play  an  important  role  in  the  total  reactions 
of  the  central  system  is  amply  evident,  but  this  by  no  means  justifies  the 
conclusion  that  they  are  the  seat  of  the  intelligence. 

F.  COMPARATIVE  PHYSIOLOGY  OF  THE  DIVISIONS  OF  THE  ENCEPHALON. 

For  the  better  comprehension  of  the  conditions  found  in  man  and  the 
monkeys,  it  will  be  of  importance  to  briefly  review  the  comparative  physi- 
ology of  the  parts  of  the  encephalon  in  vertebrates  below  the  monkeys.  The 
encephalon  in  the  lower  vertebrates  is  usually  composed  of  a  very  much 
smaller  number  of  cells  than  is  found  in  that  of  man,  and  also  the  massing 
of  the  elements  toward  the  cerebral  cortex  and  in  connection  with  the  princi- 
pal sense-organs  has  gone  on  to  a  far  less  extent. 

For  the  determination  of  the  functions  of  the  several  parts  of  the  enceph- 
alon it  is  possible  to  employ  in  animals  the  method  of  removal  as  well  as  the 
method  of  stimulation.  The  doctrine  of  cerebral  localization  was  at  one  time 
crudely  expressed  by  the  statement  that  a  cortical  centre  was  one  the  stimu- 
lation of  which  produced  a  given  reaction,  and  the  removal  of  which  abolished 
this  same  reaction.  Goltz  soon  showed  that  in  the  dog  the  removal  of  even 
an  entire  hemisphere  did  not  cause  a  paralysis  of  the  muscles  on  the  opposite 
side  of  the  body,  although  others  had  shown  that  a  stimulation  of  certain 
portions  of  the  cortex  of  the  hemisphere  would  cause  the  muscles  to  contract. 
It  was  argued,  therefore — and  quite  rightly — that  the  cortical  centres  of  the 
dog  did  not  completely  answer  to  the  definition. 

1  Archives  italiennes  de  Biolagie,  1895,  t.  xii. 

2  Archivfilr  Anatomic  und  Physiologic,  1895. 
8  Ueber  die  Verichtungen  des  Grosshirns,  1881. 


CENTRAL   NERVOUS  SYSTEM.  263 

From  the  experimental  work  of  the  strict  localizationists  like  Hitzig,1 
Munk,2  and  Ferrier,3  and  from  the  work  of  those  who,  like  Goltz4  and  Loeb,5 
denied  a  strict  localization  in  the  cerebral  cortex,  several  important  points 
of  view  have  been  developed. 

In  the  first  instance,  anatomy  indicates  that  in  the  central  system  there 
are  but  few  localities  which  consist  only  of  one  set  of  cell-bodies,  together 
with  the  fibres  coming  to  these  bodies  and  going  from  them.  Almost  every 
part  has  both  more  than  one  set  of  connections  with  other  parts  and  also 
fibres  passing  through  it,  or  by  way  of  it,  to  other  localities.  Hence  in  re- 
moving any  part  of  the  hemispheres,  for  instance,  not  only  are  groups  of  cell- 
bodies  taken  away,  but  a  number  of  other  pathways  are  interrupted  at  the 
same  time,  and  thus  the  damage  extends  beyond  the  limits  of  the  part  re- 
moved. Moreover,  when  any  portion  of  the  central  system  has  been  removed 
there  is  a  greater  or  less  amount  of  disturbance  of  function  following  imme- 
diately after  the  operation ;  but  this  disturbance  partially  passes  away. 
There  are  thus  "temporary "  as  contrasted  with  " permanent "  effects  of  the 
lesion,  and  these  require  to  be  sharply  distinguished,  because  it  is  a  per- 
manent loss  which  is  alone  significant  in  these  experiments.  Finally,  it  has 
been  made  clear  that  neither  the  relative  nor  the  absolute  value  of  any 
division  of  the  central  system  is  fixed,  but  depends  on  the  degree  to  which 
centralization  has  progressed,  or,  to  use  the  more  common  measure,  the  grade 
of  the  animal  in  the  zoological  series,  both  expressions  implying  an  increase 
in  the  connections  between  the  cerebrum  and  the  lower  centres.  The  age  of 
the  animal  on  which  the  operation  has  been  made  is  also  of  no  small  impor- 
tance in  this  respect.  These  relations  can  be  illustrated  by  reference  to  several 
experiments. 

Removal  of  Cerebral  Hemispheres. — If  from  a  bony  fish  the  cerebral 
hemispheres  (including  the  corpora  striata  as  well  as  the  mantle)  be  removed, 
the  animal  apparently  suffers  little  inconvenience.  The  movements  are  un- 
disturbed ;  such  fish  play  together  in  the  usual  manner,  discriminate  between 
a  worm  and  a  bit  of  string,  and  among  a  series  of  colored  wafers  to  which 
they  rise  always  select  the  red  ones  first.6  In  these  fish  the  eye  is  the  con- 
trolling sense-organ,  and,  as  will  be  recognized  (see  Fig.  114),  the  operation 
has  by  no  means  damaged  the  primary  centres  of  vision. 

Quite  different  is  the  result  when  the  cerebrum  is  removed  from  a  shark.7 
In  this  case,  although  the  eyes  are  intact,  the  animal  is  reduced  to  complete 
quiescence ;  yet,  on  the  whole,  the  nervous  system  of  the  shark  is  rather  less 
well  organized  and  more  simple  than  that  of  the  bony  fish. 

The  astonishing  effect  produced  is  explained  by  a  second  experiment  (see 
Fig.  115). 

1  Untersuchungen  ueber  das  Gehirn,  Berlin,  1874. 

*  Ueber  die  Functionen  der  Grosshirnrinde,  Berlin,  1881. 
3  The  Functions  of  the  Brain,  London,  1876. 

*  Ueber  die  Verichtungen  des  Grosshirns,  Bonn,  1881. 

5  Archivfur  die  gesammte  Pkysiologie,  1884,  Bde.  33  n.  34. 

6  Steiner  :  Die  Functionen  der  Centralnervensystems,  1888.  7  Steiner  :  Loc.  cit. 


264 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


If  the  olfactory  tract  be  severed  on  one  side,  no  marked  disturbance  in 
the  reactions  of  the  shark  is  to  be  noticed ;  when,  however,  both  tracts  are 
severed,  the  shark  acts  as  though  deprived  of  its  cerebrum.  From  this  it 
appears  that  the  removal  of  the  principal  sense-organ,  that  of  smell,  is  the 


FIG.  114.— Schema  of  the  encephalon  of  a  bony  fish— embryonic  (Edinger).    The  vertical  black  line  marks 
off  the  structures  in  front  of  the  thalamus. 

real  key  to  the  reactions,  and  that  the  responsiveness  of  the  fish  is  reduced  in 
the  first  instance,  because  in  this  case  it  has  been  deprived  of  the  impulses 
coming  through  the  principal  organs  of  sense,  and  in  the  second,  the  removal 
of  the  cerebrum  contains  the  pathway  for  the  impulses  from  the  olfactory 
bulbs  to  the  cell-groups  which  control  the  cord. 


FIG.  115.— Schema  of  the  encephalon  of  a  cartilaginous  fish  (Edinger).    The  vertical  black  line  marks  off 
the  striatum  and  pars  olfactoria  which  lie  in  front  of  the  thalamus. 

I  Passing  next  to  the  amphibia  as  represented  by  the  frog,  there  are  several 
series  of  observations  on  the  physiological  value  of  the  divisions  of  the  cen- 
tral system.  Schrader 1  finds  the  following  :  Removal  of  the  cerebral  hemi- 
spheres only,  the  optic  thalami  being  uninjured,  does  not  abolish  the  sponta- 
neous activity  of  the  frog.  It  jumps  on  the  land  or  swims  in  the  water,  and 
1  Archivfur  die  gesammte  Physiologic,  1887,  Bd.  xli. 


CENTRAL    NERVOUS  SYSTEM. 


265 


changes  from  one  to  the  other  without  special  stimulation.  It  hibernates  like 
a  normal  frog,  retains  its  sexual  instincts,  and  can  feed  by  catching  passing 
insects,  such  as  flies  (see  Fig.  116).  A  frog  without  its  hemispheres  is  tin -re- 
fore  capable  of  doing  several  things  apparently  in  a  spontaneous  way.  Such 
frogs  balance  themselves  when  the  support  on  which  they  rest  is  slowly  turned, 
moving  forward  or  backward  as  the  case  demands,  in  order  to  maintain  their 
equilibrium.  In  doing  this  the  frog  tends  first  to  move  the  head  in  the  direc- 


D 


E 


FIG.  116.— Frog's  brain;  the  parts  in  dotted  outline  have  been  removed :  A,  brain  intact ;  B,  cerebral 
hemispheres  removed;  C,  cerebral  hemispheres  and  thalami  removed;  D,  cerebellum  removed;  E,  two 
sections  through  the  optic  lobes  ;  F,  two  sections  through  the  right  half  of  the  bulb  (Steiner). 

tion  opposite  to  the  motion  of  the  support,  and  then  to  follow  with  move- 
ments of  the  body.  If  the  optic  thalami  are  removed  (Fig.  116,  C),  the  power 
of  balancing  is  lost,  because,  although  movements  of  the  head  still  occur,  those 
of  the  body  are  abolished.  A  frog  thus  operated  on  and  deprived  of  the  hem- 
ispheres and  thalami  exhibits  the  lack  of  spontaneity  which  is  usually  described 
as  following  the  loss  of  the  hemispheres  alone,  but  which  is  not  a  necessary 
consequence  of  this  operation,  as  the  preceding  experiments  show. 

A  frog  possessed  of  the  mid-brain  and  the  parts  behind   it  (Fig.  116,  C) 


266  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

will  croak  when  stroked  on  the  back.  When  the  optic  lobes  have  been 
removed  this  reaction  becomes  more  difficult  to  obtain,  but  it  is  not  necessa- 
rily abolished,  neither  is  the  characteristic  fling  of  the  legs  in  swimming.  At 
the  same  time,  a  frog  with  its  optic  lobes  can  direct  both  its  jumping  and 
swimming  movements  according  to  light  stimuli  acting  through  the  eye,  jump- 
ing around  and  over  obstacles  which  form  a  shadow  in  its  path,  and  climbing 
out  of  the  swimming  tank  on  the  lighter  side.  This  power  is  lost  when  the 
optic  lobes  have  been  removed. 

When  the  anterior  end  of  the  bulb  (pars  commissuralis — Stieda)  has  been 
also  removed  then  the  frog  becomes  incessantly  active,  creeping  about  and 
not  coming  to  rest  until  he  has  run  himself  into  some  corner.  Schrader  found 
such  frogs  capable  of  clambering  over  the  edge  of  a  box  eighteen  centimeters 
high.  They  are  at  a  loss  when  the  edge  of  the  box  has  been  finally  attained, 
and  vainly  reach  into  space  from  this  position.  In  the  water  they  swim  "  dog- 
fashion,"  but  only  upon  special  stimulation  do  they  make  a  spring. 

If  more  of  the  bulb  is  removed,  the  bearing  of  the  frog  departs  more  and 
more  from  the  normal,  and  is  only  temporarily  regained  in  response  to  strong 
stimulation ;  nevertheless,  co-ordinated  movements  can  be  obtained  when  the 
bulb  down  to  the  calamus  scriptorius  has  been  removed,  and  only  when  the 
movements  of  the  arms  are  directly  affected  by  the  damage  of  the  upper  end 
of  the  cord  does  the  inco-ordination  become  constant. 

A  section  through  the  optic  lobes  at  a  (Fig.  116,  E)  puts  the  frog  in  a  con- 
dition similar  to  that  following  the  isolated  removal  of  the  lobes,  while  a  sec- 
tion at  6  has  the  curious  effect  of  causing  the  animal  to  move  backward  upon 
stimulation  of  the  toes. 

When  the  small  ridge  which  forms  the  cerebellum  in  the  frog  has  been 
removed  a  slight  tremor  of  the  leg-muscles  and  a  loss  of  precision  in  jumping 
are  the  only  defects  noted  (Fig.  116,  D).  These  results  hold  for  symmetrical 
removal  of  the  divisions  of  the  encephalon.  When  the  removal  is  unsym- 
metrical  in  the  inter-brain,  mid-brain,  or  bulb  (Fig.  116  F,  a  and  6),  there  is 
more  or  less  tendency  to  forced  positions  or  forced  movements. 

As  a  rule,  action  is  most  vigorous  on  the  side  of  the  body  associated  \vith 
the  greater  quantity  of  nerve-tissue.  This  relation  appears  as  a  natural  result 
of  the  greater  effectiveness  of  the  incoming  impulses  when  entering  a  larger 
group  of  central  cells.  Indeed,  the  removal  of  the  different  portions  of  the 
central  system  in  the  frog  is  accompanied  by  a  progressive  loss  in  responsive- 
ness, stronger  and  stronger  stimuli  being  required  to  induce  a  reaction.  This 
holds  true  down  to  the  anterior  end  of  the  bulb,  the  removal  of  which,  on  the 
contrary,  sets  free  the  lower  centres,  so  that  the  frog  becomes  incessantly 
active.  Just  how  this  release  is  effected  is  not  easy  to  explain,  but  further 
removal  is  again  followed  by  the  loss  of  responsiveness. 

Passing  next  to  the  bird,  as  represented  by  the  pigeon,  the  observations  of 
Schrader  are  the  most  instructive.1  The  removal  of  the  hemispheres  from 
the  bird  involves  taking  away  the  mantle  and  the  basal  ganglia,  the  chiasma 
1  Archiv  fur  die  gesammte  Physiologie,  1888,  Bd.  xliv. 


CENTRAL    NERVOUS  SYSTEM.  267 

and  the  optic  nerves  being  left  intact.  For  the  first  few  days  after  the 
operation  the  bird  is  in  a  sleep-like  condition.  Next  the  sleep  becomes 
broken  into  shorter  and  shorter  periods,  and  then  the  bird  begins  walking 
about  the  room.  From  the  beginning  its  movements  are  directed  by  vision  ; 
slight  obstacles  it  surmounts  by  flying  up  to  them,  larger  ones  it  goes  around. 
In  climbing,  its  movements  are  co-ordinated  by  the  sense  of  touch,  and  the 
normal  position  of  the  body  is  maintained  with  vigor.  The  birds  which  walk 
about  by  day  remain  quiet  and  asleep  during  the  night.  In  flying  from  a 
high  place  the  operated  pigeon  selects  the  point  where  it  will  alight,  and  pre- 
fers a  perch  or  similar  object  to  the  floor. 

A  reaction  to  sound  is  expressed  by  a  start  at  a  sudden  noise,  like  the 
explosion  of  a  percussion-cap. 

Pigeons  without  the  cerebrum  do  not  eat  voluntarily,  though  the  presence 
of  the  frontal  portions  of  the  hemispheres  is  sufficient  to  preserve  the  reac- 
tion. 

In  a  young  hawk  slight  damage  to  the  frontal  lobes  abolished  for  the  time 
the  use  of  the  feet  in  the  handling  of  food,  and  thus  abolished  in  this  way  the 
power  of  feeding  as  well  as  that  of  standing. 

With  the  loss  of  the  cerebrum  the  pigeon  does  not  lose  responsiveness  to 
the  objects  of  the  outer  world,  but  they  all  have  an  equal  value.  The  bird  is 
neither  attracted  nor  repelled,  save  in  so  far  as  the  selection  of  the  points 
toward  which  it  will  fly  is  an  example  of  attraction.  Sexual  and  maternal 
reactions  both  disappear,  and  neither  fear  nor  desire  is  evident. 

In  ascending  the  mammalian  series,  the  removal  of  the  cerebrum  becomes 
a  matter  of  increasing  difficulty.  The  reasons  for  this  are  several,  and  reside 
in  the  increasing  size  of  the  blood-vessels  and  the  nutritive  complications 
dependent  on  the  increase  in  the  mass  of  the  cerebrum,  as  well  as  in  the 
greater  physiological  importance  of  this  division.  Goltz l  has  been  able  by 
repeated  operations  to  remove  the  entire  cerebrum  of  a  dog,  and  still  to  keep 
the  animal  alive  and  under  observation  for  eighteen  months,  at  the  end  of 
which  time  the  animal,  though  in  good  health,  was  killed  for  further  exami- 
nation. This  dog  was  blind,  though  he  blinked  when  a  very  bright  light 
was  suddenly  flashed  in  his  face.  He  could  be  awakened  by  a  loud  sound, 
and  when  awake  responded  to  such  sounds,  when  intense,  by  shaking  the  head 
or  ears.  This  would  not,  however,  be  complete  proof  that  he  could  hear. 
The  sense  of  taste  was  so  far  present  that  meat  soaked  in  quinine  was  rejected 
after  tasting.  Tactile  stimuli  and  those  involving  the  muscle  sense,  as  in  the 
case  where  the  animal  was  lifted,  caused  him  to  struggle  and  to  bite  in  the 
direction  of  the  irritation.  These  reactions  were  modified  according  to  the 
locality  of  the  stimulus.  The  power  to  make  movements  expressive  of  pain 
was  still  present. 

On  the  motor  side  the  dog  was  capable  of  such  highly  complicated  acts 
as  walking,  standing,  and  eating,  and  in  these  operations  was  guided  by  the 
muscle-sense  and  that  of  contact.  The  sexual  instincts  were  lost,  but  the 

1  Archivfiir  die  gesammte  Physiologic,  Bd.  xli. 


268  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

animal  was  excessively  active,  and  became  more  and  more  excited  when 
ready  to  defecate  or  when  hungry. 

The  examination  of  the  brain  showed  that  in  front  of  the  mid-brain  the 
important  structures  had  been  removed  or  were  degenerated,  only  small  por- 
tions of  the  corpora  striata  remained,  mainly  parts  of  the  caudal  portions 
of  the  nucleus  caudatus.  The  frontal  portion  of  the  thalamus  had  been  re- 
moved and  the  nuclei  in  the  remainder  were  highly  atrophic,  so  that  the 
defects  were  due  to  a  removal  of  rather  more  than  the  cerebrum  proper. 

Emotions,  feelings,  conscious  sensations,  or  the  capacity  to  learn  were 
entirely  wanting  in  this  dog,  and  its  reactions  were  those  of  a  very  elaborate 
machine. 

If  we  compare,  now,  the  effects  of  the  removal  of  the  cerebral  hemispheres 
in  the  bony  fish,  the  pigeon,  and  the  dog,  we  see  that  the  results  of  the 
operation  are  progressively  more  disturbing  as  we  pass  up  the  series.  In 
the  higher  animals  the  effects  are  more  often  fatal,  the  disturbance  imme- 
diately following  is  much  more  severe,  the  return  of  function  slower,  and  the 
permanent  loss  greater.  As  a  partial  exception  to  the  above  statements  is 
the  observation  that  after  operation  the  general  health  of  pigeons  always 
declines,  and  it  is  not  possible  to  keep  them  alive  more  than  about  six  weeks. 
On  the  contrary,  a  dog  could  be  kept  in  good  health  for  some  eighteen 
months ;  but  there  is  this  difference  between  the  experiments,  that  the 
removal  in  the  case  of  the  dog  was  made  by  several  successive  operations. 

By  removal  of  the  cerebrum  the  higher  animal  tends  to  lose  just  those 
capacities  which  best  serve  to  distinguish  it  from  the  lower  forms.  When, 
therefore,  the  inquiry  is  made  why  the  results  obtained  in  the  dog  are  not 
obtainable  in  the  monkey  or  in  man,  there  are  several  replies.  In  the  first 
place,  no  such  extensive  experiments  have  been  made  on  monkeys  of  the 
right  age  and  under  equally  favorable  conditions.  If  the  mature  animal  is 
taken,  the  secondary  degenerations  are  so  massive  that  they  certainly  cause 
great  disturbance  in  the  remaining  part  of  the  system.  This  is  not  equivalent 
to  an  assertion  that  the  same  results  could  be  obtained  in  the  monkey  by  more 
extensive  experiments,  but  a  suggestion  of  one  difference  behind  the  results 
thus  far  reported.  There  is  no  reason  for  assuming  any  deep-seated  differ- 
ence in  the  arrangement  of  the  central  system  of  the  highest  mammals  as 
compared  with  that  in  the  lower.  Indeed,  in  some  human  microcephalic 
idiots  the  proportion  of  sound  and  functional  tissue  in  the  encephalon  is  less 
than  one-fourth  that  found  in  a  normal  person  ;  yet,  on  the  other  hand,  no 
normal  adult  could  lose  anything  like  the  amount  of  tissue  which  is  out  of 
function  in  these  microcephalic  brains  and  at  the  same  time  live. 

The  central  system,  therefore,  even  in  man,  is  to  be  looked  upon  as  pos- 
sessed of  some  power  to  adapt  itself  when  portions  have  been  lost,  but  this 
is  most  evident  when  the  defect  begins  early  and  develops  slowly. 

Keeping  the  cerebrum  still  in  view,  it  is  possible  to  go  into  further 
details.  In  forms  below  the  monkey  the  loss  of  portions  of  the  cerebral 
cortex  from  the  motor  area  is  accompanied  by  a  greater  or  less  paralysis  of 


CENTRAL    NERVOUS  SYSTEM.  269 

tin'  muscles  represented.  This,  however,  is  an  initial  symptom  only,  and 
gradually  disappears,  though  not  always  with  the  same  completeness.  In 
man,  of  course,  the  tendency  to  recover  is  least. 

The  anatomical  relations  behind  this  ditl'ereiice  are  the  following:  The 
efferent  cells  in  the  ventral  horns  are  dominated  principally  by  two  sets  of 
impulses,  those  arriving  directly  over  the  dorsal  roots  of  that  segment  in 
which  they  are  located,  and  those  coming  over  the  long  paths  by  way  of  the 
cerebral  cortex  and  pyramidal  tracts.  In  the  lower  mammals  this  second 
pathway  is  insignificant,  and  when  interrupted,  therefore,  the  disturbance  in 
the  control  of  the  ventral-horn  cells  is  but  slight.  Passing  up  the  scries, 
however,  this  pathway  tends  to  become  more  and  more  massive  and  important, 
as  the  figures  previously  given  show  (see  p.  252),  until  in  man  and  the 
monkey  a  damage  of  it  such  as  is  effected  by  injury  to  the  cortex  causes  a 
high  degree  of  paresis  if  not  permanent  paralysis,  because  by  this  injury  a 
large  proportion  of  the  impulses  is  thus  cut  off  from  the  efferent  cells. 

It  has  been  previously  shown  that  the  cortical  areas  do  not  vary  according 
to  the  mass  of  the  muscles  which  they  control.  Experiments  also  show  that 
it  is  the  fore-limbs  which  are  most  disturbed  in  their  reactions  when  the 
lesion  involves  the  cortical  centres  for  both  fore-  and  hind-limbs,  and  this  falls 
under  the  law  that  the  more  highly  adaptable  movements  (/.  e.,  those  of  the 
fore-limb  as  contrasted  with  those  of  the  hind-limbs)  are  most  under  the  con- 
trol of  the  cortex.  If  the  examination  be  restricted  to  the  fore-limb  alone,  it 
is  found  that  the  finger  and  hand  movements  or  those  of  the  more  distal  seg- 
ments are  in  turn  the  ones  most  disturbed.  Thus,  in  the  limbs  the  more 
distal  groups  of  muscles  are  those  best  controlled  from  the  cortex.  It  fol- 
lows, then,  that  for  the  arm,  paralysis  of  shoulder  movements  as  the  result 
of  cortical  lesion  is  least  complete,  while  as  we  travel  toward  the  extremity 
of  the  arm  the  liability  to  disturbance  of  its  function  as  the  result  of  cortical 
injury  increases  steadily. 

Turning  now  to  the  "  sensory  "  areas  of  the  cortex,  the  principles  under- 
lying their  physiological  significance  and  connections  appear  to  be  similar. 
The  lower  the  animal  in  the  vertebrate  series  the  more  probable  that  its  reac- 
tions can  be  controlled  by  the  afferent  impulses  which  have  not  passed  through 
the  cerebral  cortex. 

None  of  the  senses  except  vision  can  be  analyzed  sufficiently  to  bring  out 
the  significance  of  the  subdivisions  of  the  cortical  area ;  hence  the  illustra- 
tions are  taken  from  that  sense  alone. 

It  has  already  been  shown  that  without  cerebral  hemispheres  a  bony 
fish  can  distinguish  the  colors  of  wafers  thrown  on  water  and  discriminate 
between  a  bit  of  string  and  a  worm.  In  the  same  case,  a  frog  is  able  to  direct 
its  movements  and  to  catch  flies — i.  e.j  to  detect  objects  in  motion  and  react 
to  them  normally.  A  pigeon  can  direct  its  movements  in  some  measure,  and 
even  select  a  special  object  as  a  perch  ;  but  it  is  not  able  to  respond  to  the  sight 
of  food  or  its  fellows,  or  those  objects  which  might  be  supposed  to  excite  the 
bird  to  flight.  In  the  dog  the  vision  which  remains  permits  only  the  response 


270  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

of  blinking  when  the  eye  is  stimulated  by  the  flash  of  a  strong  light.  The 
progressive  diminution  in  the  response  which  follows  visual  stimuli  in  these 
animals  is  open  to  the  interpretation  that  the  path  by  which  the  impulses 
may  pass  over  to  the  cells  forming  the  primary  centres  intermediate  between 
the  sense-organ  and  the  cortex  is  progressively  diminished.  Thus,  as  the 
pathway  to  the  cortex  becomes  more  permeable,  the  impulses  arriving  at  the 
primary  optic  centres  are  in  a  less  and  less  degree  reflected  toward  the  cord. 
When,  therefore,  the  cortex  has  been  removed,  the  reactions  taking  place  by 
way  of  it  are  disturbed  in  proportion  to  their  normal  importance. 

In  the  first  instance,  when  the  reflection  occurs  in  the  primary  centres  the 
incoming  impulses  are  distributed  toward  the  cord  by  paths  not  known,  while 
in  the  second  they  pass  from  the  cortex  along  the  pyramidal  tracts. 

In  the  cortex  of  the  dog  subdivisions  of  the  visual  area  have  been  made 
by  Munk.1  He  found  that  the  more  anterior  portions  of  the  visual  area  were 
associated  with  the  superior  parts  of  the  retina,  and  the  more  posterior  por- 
tions with  the  inferior,  while  the  area  in  one  hemisphere  corresponded  with 
the  nasal  portion  of  the  retina  of  the  opposite  eye,  and  to  a  less  degree  with  the 
temporal  portion  of  the  retina  of  the  same  side.  The  determination  of  these 
relations  was  made  by  the  removal  of  parts  of  the  visual  area  (dogs)  and  the 
subsequent  examination  of  the  field  of  vision.  It  appears,  therefore,  that  the 
incoming  impulses  from  certain  portions  of  the  retina  are  delivered  at  definite 
parts  of  the  cortex,  and  that  when  the  parts  are  injured  in  the  dog  or  higher 
mammals  these  impulses  are  blocked.  By  stimulation,  it  will  be  remem- 
bered, Schafer  determined  similar  relations  in  the  monkey. 

Before  leaving  the  cerebral  hemispheres,  mention  of  the  fact  should  be 
made  that  still  other  functions,  control  of  the  sphincter  ani  (Fig.  103),  secre- 
tion of  saliva,  and  micturition,  can  be  roused  by  the  stimulation  of  the  cortex 
in  the  appropriate  region — namely,  in  the  region  where  the  muscles  and  glands 
concerned  might  be  expected  to  have  representation  if  they  followed  the  gen- 
eral law  of  arrangement.  Changes  in  the  production  a:id  elimination  of  heat 
from  the  body  follow  interference  with  the  motor  region  of  the  cerebrum,  and 
the  removal  of  portions  of  the  cortex  in  this  region  is  followed  by  a  rise  in 
the  temperature  of  the  muscles  affected  and  an  increased  blood-supply  to  them. 

In  the  encephalon,  the  cerebrum,  and  especially  its  outer  surface,  is  the 
portion  the  functions  of  which  have  been  studied.  The  significance  of  the 
other  portions  of  the  encephalon  can  be  far  less  well  determined.  The  dis- 
turbances caused  by  the  section  and  stimulation  of  the  callosum  have  been 
studied  by  Koranyi 2  and  by  Schafer.3  It  was  found  that  complete  section  of 
the  corpus  callosum  was  not  followed  by  any  perceptible  loss  of  function. 
On  the  other  hand,  stimulation  of  the  uninjured  callosum  from  above  gave 
symmetrical  bilateral  movements,  while  if  the  cortex  on  one  side  was  removed 
stimulation  of  the  callosum  gave  unilateral  movements  on  the  side  controlled 

1  Ueber  die  Functionen  der  Grosshirnrinde,  Berlin,  1881. 

2  Archiv  fur  Anatomie  und  Physiologie,  Bd.  xlvii. 

3  Brain,  1890.     ' 


CENTRAL    NERVOUS  SYSTEM.  271 

by  the  uninjured  hemisphere.  These  results  seem  to  corroborate  the  conclu- 
sion derived  from  liistological  work  to  the  effect  that  the  system  of  the  callo- 
stim  is  composed  only  of  commissural  fil> res,  and  that  it  sends  no  fibres  directly 
into  the  internal  capsule  of  either  side.  Concerning  the  corpora  striata  and 
the  optic  thalami,  very  little  is  known.  In  the  case  of  the  corpora  striata 
injury  causes  in  man  no  permanent  defect  of  sensation  or  motion,  although 
both  forms  of  disturbance  may  at  the  outset  be  present  in  the  case  of  acute 
lesions.  Lesions  of  the  corpora  striata  cause  a  rise  in  body-temperature.1 
Following  a  puncture  of  one  corpus  striatum  there  occurs  in  rabbits  a  rise 
amounting  to  some  3°  C.;  it  begins  a  few  minutes  after  the  operation  and 
may  last  a  week,  but  the  temperature  tends  to  return  to  the  normal.  The 
most  striking  feature  in  these  experiments  is  the  very  wide  effects  produced 
by  an  extremely  small  wound,  like  the  puncture  of  a  probe. 

In  the  cases  where  lesion  of  the  striatum  on  one  side  causes  in  man  a  rise 
of  temperature,  it  appears  mainly  on  the  side  of  the  body  opposite  the  lesion.2 
A  vaso-motor  dilatation  occurs  over  the  parts  of  the  body  where  the  temper- 
ature is  high. 

In  less  degree  a  rise  of  temperature  follows  injury  of  the  optic  thalamus — 
at  least  such  is  the  result  of  experiments  on  rabbits ;  but  the  effect  of  the 
lesion  is  never  so  marked  as  in  the  case  of  the  striatum.  Owing  to  the  dis- 
proportion between  the  area  of  the  lesion  and  the  extent  of  the  effects,  it  is 
difficult  to  conceive  of  the  anatomical  relations  which  permit  the  reaction.  It 
is  of  interest  to  note,  however,  that  similar  relations  hold  for  the  vaso-motor 
centre  in  the  bulb,  in  which  case  the  vessels  supplying  a  great  area  are  con- 
trolled by  a  small  group  of  cells. 

The  difficulty  of  an  anatomical  explanation  is  increased  by  the  fact  that  Ott 3 
enumerates  in  animals  six  heat-centres:  1.  The  cruciate,  about  the  Rolandic 
fissure  ;  2.  The  Sylvian,  at  the  junction  of  the  supra-  and  post-Syl  vian  fissures ; 
3.  The  caudate  nucleus ;  4.  The  tissues  about  the  striatum ;  5.  A  point  be- 
tween the  striatum  and  the  thalamus,  near  the  median  line ;  6.  The  anterior 
mesial  end  of  the  thalamus. 

Thalamus. — In  considering  the  thalamus,  we  find  that  the  various  cell- 
groups  forming  it  are  connected  with  distinct  portions  of  the  cerebral  cortex 
by  double  pathways — one  set  of  axones  having  their  origin  in  cell-bodies 
located  in  the  cortex,  and  the  other  in  cell-bodies  in  the  subdivisions  of  the 
thalamus.  The  relations  between  these  two  divisions  have  been  specially 
studied  by  v.  Monakow,4  who  finds  by  experiment  that  lesion  of  one  part, 
either  cortex  or  the  thalamic  nuclei,  is  followed  by  degeneration  in  the  other 
part,  and  that  the  location  of  the  degeneration  depends  on  that  of  the  lesion. 

Further,  it  has  been  observed  by  Mellus5  that  the  axones  passing  from  the 

1  Aronsohn  und  Snchs:  Archiv   fiir  die  gesammte  Physioloyie,   1885,   Bd.  xxxvii.;  Richet: 
•Comptes  rendus  de  V  Acad.  des  Sciences,  1884;  Ott:  Brain,  1889,  vol.   xi. 

2  Kaiser:  Neurologisches  Centralblatt,  1895,  No.  10.  3Ott:  Loc.  tit. 
*  Archiv  fiir  Psychiatric  und  Nervenkrankheiten,  1893,  Bd.  xxvii. 

5  Proceedings  of  the  Royal  Society,  London,  189-4  and  1895. 


272  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

motor  cortex  of  the  monkey  toward  the  thalamus  are  fibres  of  smaller  calibre 
than  those  destined  for  the  pyramidal  tracts. 

Moreover,  the  studies  of  Tschermak1  on  the  termination  of  the  tracts 
which  continue  the  dorsal  columns  of  the  spinal  cord  in  the  interbrain,  show 
an  abundant  connection  of  the  fibres,  especially  with  the  ventral  cell-groups 
of  the  thalamus.  The  connection  may  be  either  an  actual  ending  of  the  fibre 
or  a  termination  by  means  of  collaterals. 

When  these  anatomical  observations  are  considered  in  connection  with  the 
differences  in  the  reactions  of  the  frog  with  and  without  its  thalami,  it  appears 
that  cell-groups  which  increase  the  responsiveness  of  the  central  system 
must  be  located  here.  On  the  other  hand,  in  the  case  of  Goltz's  dog  without 
its  fore-brain,  the  thalami  (interbrain)  were  so  largely  damaged  that  it  hardly 
seems  possible  that  they  could  have  been  much  utilized  in  the  reactions  which 
were  made  by  that  animal. 

Human  pathology  throws  little  light  on  the  functions  of  the  thalami — 
though  lesion  of  it  is  often  accompanied  by  loss  of  power  to  express  the 
emotions  through  the  muscles  of  the  face — a  symptom  to  which  attention  has 
been  repeatedly  drawn. 

The  Cerebellum. — The  only  other  division  of  the  encephalon,  the  func- 
tions of  which  can  properly  be  described  apart,  is  the  cerebellum.  This 
portion  is  among  vertebrates  almost  as  variable  in  its  development  as  the 
mantle  of  the  cerebral  hemispheres,  and  in  many  fish  and  mammals  is  asym- 
metrical in  its  gross  structure. 

Observation  on  this  subdivision  has  been  carried  out  in  the  first  instance 
by  Luciani,2  and  later  by  Russell 3  and  by  Ferrier.4 

The  cerebellum  is  not  concerned  with  psychical  functions.  The  removal 
of  it  does  not  cause  permanently  either  paralysis  or  anaesthesia,  but  the  im- 
mediate effects  of  an  extensive  injury  are  (in  dogs  and  monkeys)  a  paresis 
and  analgesia  as  well  as  anesthesia  mainly  in  the  hind-legs,  and  in  conse- 
quence a  high  degree  of  inco-ordination  in  locomotion.  A  distinct  series  of 
symptoms,  however,  follows  injury  to  this  organ,  and  these  are  modified 
according  to  the  locality  and  nature  of  the  lesion.  Removal  of  one-half 
(cerebellar  hemisphere  plus  half  the  vermis)  of  the  cerebellum  in  the  dog 
causes  a  deviation  outward  and  downward  of  the  optic  bulb  on  the  opposite 
side,  a  proptosis  of  the  bulbs  on  both  sides,  nystagmus  and  contracture  of 
the  muscles  of  the  neck  on  the  side  of  the  lesion,  and  an  increase  of  the 
tendon-reflexes  in  the  limbs.  In  walking  the  dog  wheels  toward  the  side 
opposite  to  the  lesion,  and  tends  to  fall  toward  the  side  of  the  lesion. 

The  symptoms  are  chiefly  unilateral,  and,  caudad  from  the  cerebellum^ 
are  on  the  side  of  the  lesion.  The  symptoms  are  less  severe  when  only  one 
hemisphere,  instead  of  an  entire  half  of  the  cerebellum,  has  been  removed. 

1  "Notiz  betreffs  des  Kindenfeldes  der  Hinterstrangsbahnen,"  Neurologisches  Centralblatt,  1898, 
No.  4. 

8  Archives  italiennes  de  Biologie,  1891-92,  xvi. 

3  Philosophical  Transactions  of  the  Royal  Society,  1894.  *  Brain,  1893,  vol.  xvi. 


CENTRAL    NERVOUS  SYSTEM.  273 

The  existing  symptoms  are  not  intensified  by  the  removal  of  the  remaining 
half.  The  permanent  condition  of  the  muscles  after  operation  is  expressed 
l>v  an  atonia,  or  lack  of  tonus,  in  the  resting  muscle-  ;  an  asthenia,  or  loss  of 
Strength,  which  was  measured  by  Lueiaui,aud  was  most  marked  in  the  hind- 
leg;  an  astasia,  or  a  lack  of  steadiness  in  the  muscles  during  action;  and 
finally  an  ataxia,  or  a  want  of  orderly  sequence,  in  the  contractions  of  the 
muscle-groups.  The  general  expression  of  these  symptom  is  a  twist  of  the 
trunk,  the  concavity  being  toward  the  operated  side,  combined  with  a  dis- 
orderly gait.  At  the  same  time  there  is  no  demonstrable  permanent  dis- 
turbance of  tactile  or  muscular  sensibility. 

Though  the  two  halves  of  the  cerebellum  are  united  by  strong  commis- 
snral  fibres,  the  complete  division  of  the  organ  in  the  middle  line  is  followed 
by  a  disturbance  of  the  gait  which  is  only  transitory.  Hence  it  is  inferred 
that  the  connections  of  the  cerebellum  are  mainly  with  the  same  side  of  the 
bulb  and  spinal  cord.  Cephalad  of  the  cerebellum  the  connection,  however, 
is  a  crossed  one,  each  cerebellar  hemisphere  being  associated  with  the  contra- 
lateral  cerebral  hemisphere.  Throughout  these  connections,  both  cephalad 
and  candad  to  the  cerebellum  itself,  it  appears  that  there  is  always  a  double 
pathway,  and  the  cerebellum  not  only  sends  impulses  to,  but  receives  them 
from,  the  regions  with  which  it  is  associated. 

One  effect  of  removal  of  one-half  of  the  cerebellum  is  to  increase  the  re- 
sponsiveness of  the  cortex  of  the  contra-lateral  cerebral  hemisphere  to  electri- 
cal stimulation,  thereby  making  it  possible  with  a  weaker  stimulus  to  obtain 
a  reaction  which  could  be  obtained  from  the  other  hemisphere  only  with  a 
stronger  one.  When  an  irritative  lesion  is  made,  instead  of  a  merely  de- 
structive one,  the  rotation  and  falling  are  away  from  the  side  of  the  lesion, 
instead  of  toward  it. 

The  experiments  altogether  show  the  cerebellum  to  be  closely  associated 
with  the  proper  contraction  of  the  muscles,  and  this  is  so  directly  connected 
with  the  maintenance  of  equilibrium  1  that  it  is  not  surprising  to  find  that 
stimulation  or  removal  of  the  cerebellar  cortex,  besides  producing  nystagmus, 
may  give  rise  to  deviations  of  the  eyes  similar  to  those  found  on  injury  of  the 
semicircular  canals  or  stimulation  of  their  nerves  in  fishes.2 

PART  III.— PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM  TAKEN 

AS  A  WHOLE. 

A.  WEIGHT  OF  THE  BRAIN  AND  SPINAL  CORD. 

In  attributing  a  value  to  the  mass  of  the  nervous  system  we  assume  that 
the  elements  which  compose  it  possess  potential  energy.  This  energy  varies 
for  any  given  element  in  accordance  with  a  number  of  conditions,  but  for  the 
moment  it  will  be  sufficient  to  point  out  that  if  the  mass  of  the  entire  system 
is  significant  the  masses  of  its  respective  subdivisions  are  also  significant,  as 

1  A.  Thomas:  "  Le  Cervelet,"  Etn<]<>  mminmique,  dinique  et  physiologique,  Paris,  1897. 

2  Lee :  Journal  of  Physiology,  1893,  vol.  xv. ;  1894,  vol.  xvii. 

VOL.  II.— 18 


274  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

showing  in  some  measure  the  relative  physiological  importance  of  the  several 
parts. 

Weight  of  the  Encephalon  and  Spinal  Cord. — When  the  weight  of  any 
portion  of  the  nervous  system  is  taken,  the  final  record  represents,  in  addi- 
tion to  the  weight  of  the  nerve-tissues  proper,  that  of  the  supporting  and 
nutritive  tissues  normally  associated  with  them,  together  with  the  enclosed 
blood  and  lymph.  It  is,  however,  assumed  that  under  normal  conditions  the 
relation  between  the  nervous  and  non-nervous  tissues  is  nearly  a  constant  one, 
and  that  the  results  of  different  weighings  are  therefore  comparable  among 
themselves. 

Outside  of  the  nervous  tissue  proper  are  the  pia  and  the  fluid  contained  in 
the  vessels  and  ventricular  cavities.  Sometimes  the  encephalon  is  freed  from 


FIG.  117.— Showing  the  principal  divisions  of  the  encephalon  made  for  the  study  of  its  weight:  1, 
hemisphere  seen  from  the  side,  fissuration  according  to  Eberstaller ;  2,  mid-brain,  region  of  the  quad- 
rigemina  :  3,  pons ;  4,  cerebellum,  or  hind-brain  ;  5,  bulb,  or  after-brain.  Divisions  2,  3,  and  5,  taken 
together,  form  what  is  designated  the  "  stem  "  in  the  tables  of  Boyd  (modified  from  Quain's  Anatomy). 

the  pia  and  fluid,  and  at  others  they  are  weighed  all  together.  According  to 
Broca,1  the  weight  of  the  pia  covering  the  encephalon  is,  in  normal  males,  as 
follows : 

20  to  30  years 45  gms. 

31  to  40  "  "       50     " 

60  "       60     " 

The  cast  of  the  ventricles  as  made  by  Welcker  displaces  26  c.c.  of  water, 
which  gives  an  idea  of  the  normal  capacity  of  these  cavities.  In  man,  the 
gray  matter  of  the  cerebrum  has,  on  the  average,  81  per  cent,  of  water ;  while 
the  white  matter  from  various  parts  of  the  central  system  has  70  per  cent.2 

1  Broca,  quoted  by  Topinard  :  Elements  tf  Anthropologie  generate,  1885. 

2  Halliburton  :  Journal  of  Physiology,  1894. 


CENTRAL    NERVOUS  SYSTEM. 


275 


The  specific  gravity  of  the  entire  encephalon  is  for  the  male,  1036.3,  and 
for  the  female,  1036. 

Weight  of  the  Encephalon. — The  encephalon  is  that  portion  of  the 
central  nervous  system  contained  within  the  skull.  The  accompanying  dia- 
gram (Fig.  117)  shows  the  encephalon,  together  with  one  manner  of  sub- 
dividing it.  Its  weight  has  usually  been  taken  while  it  was  still  covered  by 
the  pia,  but  after  allowing  the  fluids  to  drain  away  for  five  minutes  or  more. 
Sometimes  drainage  has  been  facilitated  by  cutting  into  the  brain ;  hence, 
when  the  brain-weight  records  by  any  observer  are  to  be  discussed,  the  first 
question  concerns  the  method  according  to  which  the  brains  were  examined, 
for  the  weights  may  be  either  with  or  without  the  pia  and  with  or  without 
drainage. 

The  anthropologists  classify  encephala  according  to  weight  in  the  follow- 
ing manner : 

TJie  Nomenclature  of  the  Encephalon  according  to  Weight.      Weight  in  Grams. 

( Topinard.) 

Classes.                                                                          Males.  Females. 

Macrocephalic From  1925  to  1701  From  1743  to  1501 

Large "  1700  to  1451  "       1500  to  1351 

Medium "  1450  to  1251  "       1350  to  1151 

Small "  1250  to  1001  "       1150  to    901 

Microcephalic "  1000  to    300  "        900  to    283 

From  the  observations  of  Dr.  Boyd,  in  England,  on  the  weight  of  the  brain 
the  following  table  has  been  compiled  : 

Table  showing  the  Weight  of  the  Encephalon  and  its  Subdivisions  in  Sane 
Persons,  the  Records  being  arranged  according  to  Sex,  Age,  and  Stature 
(from  Marshall's  tables  based  on  Boyd's  records).1 


MALES. 


FEMALES. 


S3 

o 

a 

| 

1 

a' 

3 

s 

H 

S 

1 

1 

1 

a 

d 

I 

JQ 

2 

1 

1 

1 

S 

5 

a 

1 

i 

8 

8 

i 

H 
«< 

Stature  175  cm.  and  upward. 

Stature  163  cm.  and  upward. 

20-40 

1409 

1232 

149 

28 

23 

134 

1108 

1265 

20-40 

41-70 

1363 

1192 

144 

27 

23 

131 

1055 

1209 

41-70 

71-90 

1330 

1167 

137 

26 

24  a 

130 

1012 

1166 

71-90 

Stature  172-167  cm. 

Stature  160-155  cm. 

20-40 

1360 

1188 

144 

28 

26s 

137s 

1055 

1218 

20-40 

41-70 

1335 

1164 

144 

27 

26s 

131 

1055 

1212s 

41-70 

71-90 

1305 

1135 

142s 

28as 

24 

128 

969s 

1121 

71-90 

Stature  164  cm.  and  under. 

Stature  152  cm.  and  under. 

20-40 

1331 

1168 

138 

25 

24s 

130 

1045 

1199 

20-40 

41-70 

1297 

1123 

139a 

25 

25as 

129 

1051  a 

1205  a 

41-70 

71-90 

1251 

1095 

131 

25 

25  as 

123 

974 

1122 

71-90 

1  a  indicates  that  a  record  considered  according  to  age  is  too  large ;  s  indicates  that  a  record 
considered  according  to  stature  is  too  large. 


276  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

The  method  of  weighing  the  brain  used  by  Dr.  Boyd *  was  as  follows : 
The  skull-cap  being  removed  and  the  pia  intact,  the  hemispheres  were  sliced 
away  by  horizontal  sections  as  far  down  as  the  tentorium.  The  parts  of  the 
hemispheres  still  remaining  were  then  removed  by  a  section  passing  in  front 
of  the  quadrigemina.  The  cerebellum  was  next  separated  from  the  stem,  this 
latter  being  represented  by  the  quadrigemina,  the  pons,  and  the  bulb.  Each 
hemisphere,  the  cerebellum,  and  the  stem  was  then  weighed  separately. 

If  groups  of  similar  ages  and  corresponding  statures,  as  entered  in  the 
Table,  are  compared  according  to  sex,  it  is  at  once  seen  that  the  male  pos- 
sesses the  heavier  encephalon,  and  that  all  the  subdivisions  of  it  are  likewise 
heavier. 

When  individuals  of  the  same  sex  and  falling  within  the  same  age-limits 
are  compared  according  to  stature,  those  having  the  greater  stature  are  found 
to  have  the  greater  brain- weight,  though  in  the  case  of  the  subdivisions  of  the 
encephalon,  and  especially  among  the  females,  there  are  some  irregularities, 
but  these  would  probably  disappear  could  the  number  of  observations  be 
increased.  Finally,  within  the  groups  of  those  having  the  same  stature,  but 
different  ages,  the  weight  decreases  with  advancing  age.  The  middle  group, 
forty-one  to  seventy  years  of  age,  is  in  one  way  unfortunate,  because,  while 
the  brain  is  probably  still  growing  (see  curve  of  growth,  Fig.  118)  during  the 
first  third  of  that  period,  and  is  nearly  stationary  (males  especially)  during 
the  second,  it  begins  to  diminish  so  rapidly  during  the  last  third  that  the 
average  weight  is  lower  for  the  cases  between  sixty-one  and  seventy  years 
than  for  the  twenty  years  between  forty-one  and  sixty  years.  Between 
seventy -one  and  ninety  years  the  involutionary  changes  in  the  central  system 
are  most  marked,  and  the  decrease  in  weight  during  this  period  is  clearly 
indicated. 

Before  suggesting  an  explanation  of  these  variations  according  to  age,  sex, 
and  stature,  it  is  to  be  noted  that  they  occur  in  other  mammals  as  well  as  in 
man.  As  regards  the  difference  in  the  weight  of  the  encephalon  due  to  sex, 
it  has  been  shown  to  obtain  among  the  apes,2  the  male  having  the  heavier 
brain  ;  and  from  the  general  relation  of  size  according  to  sex  among  the  mam- 
malia, where  the  male  as  a  rule  has  the  greater  body- weight  and  larger  skull, 
it  is  to  be  anticipated  that  a  similar  difference  in  the  weight  of  the  brain  will 
be  shown  in  other  genera. 

Among  individuals  of  the  same  species,  but  of  different  races  or  of  different 
statures  and  weights,  the  law  holds  good  that  the  larger  races  have  the  heavier 
brains,  as  do  the  larger  and  heavier  individuals.3  Here,  as  in  the  case  of 
man,  it  is  always  assumed  that  the  differences  in  body-weights  are  mainly 
correlated  with  the  active  tissues  like  muscle,  and  not  with  fat.  As  to  the 

1  Philosophical  Transactions  of  the  Royal  Society,  London,  1860  ;    see  also  Marshall:  Journal 
of  Anatomy  and  Physiology,  1892. 

2  Keith  :  Journal  of  Anatomy  and  Physiology,  1895. 

3  Du  Bois,  in  the  Arch,  fur  Anthropol.,  Bd.  xxv.,  maintains  that  among  forms  which  may  be 
fairly  compared,  the  formula  E  —  S°'56,  will  give  the  weight  of  the  encephalon, — E  being  the 
encephalic  weight  and  S  the  body-weight. 


CENTRAL    NERVOUS  SYSTEM.  277 

loss  of  the  brain  in  weight  after  maturity,  observations  on  animals  are  scanty, 
but  point  to  decrease  in  weight  toward  the  natural  close  of  life. 

Interpretations  of  Weight. — Assuming  as  the  simplest  case  that  the 
number  of  the  nerve-elements  composing  a  given  portion  of  the  central  system 
is  constant  within  the  limits  of  the  same  species,  then  differences  in  the  weight 
of  these  portions  in  different  individuals  imply  variations  in  the  size  of  the 
component  cells.  The  significance  of  variations  in  the  size  of  the  nerve-ele- 
ments must  be,  primarily,  that  the  larger  the  cells,  and  especially  the  larger 
the  cell-bodies,  the  greater  the  mass  of  cell-substance  ready  at  any  moment  to 
undergo  chemical  change  leading  to  the  release  of  energy,  and  the  more  nu- 
merous the  probable  connections.  On  the  other  hand,  if  the  number  of  ele- 
ments is  variable,  an  increase  in  the  number  must,  in  view  of  the  law  of 
isolated  conduction,  also  provide  a  larger  number  of  conducting  pathways. 
Whether  this  increase  in  the  number  of  pathways  shall  further  add  to  the 
complication  of  the  system  depends  on  the  localities  at  which  it  occurs. 

In  the  absence  of  fuller  data,  the  explanation  of  the  series  of  differences 
shown  in  Boyd's  table  is  in  a  very  high  degree  tentative.  The  loss  of  weight 
in  advanced  years  appears  to  be  due  to  a  general  atrophy  of  the  nerve-ele- 
ments. The  greater  brain-weight  associated  with  greater  stature  appears  to 
depend  on  the  variations  in  the  size  of  the  elements  rather  than  in  their  num- 
ber, and,  so  far  as  can  be  seen,  the  distinction  according  to  sex  is  also  sus- 
ceptible of  the  latter  explanation. 

Weights  of  Different  Portions. — A  study  of  the  proportional  weights  of 
the  several  subdivisions  of  the  encephalon  according  to  the  sex,  stature,  and 
age,  shows  that  there  is  very  little  difference  caused  by  variations  in  these 
conditions.  This,  too,  so  far  as  it  goes,  suggests  that  the  absolute  weight  is 
dependent  rather  on  variations  in  the  size  than  in  the  number  of  the  elements, 
since  an  harmonious  variation  in  number  would  be  less  probable  than  an  har- 
monious variation  in  size. 

Social  Environment. — It  is  not  to  be  expected  that  the  weight  of  the 
brain  among  the  least-favored  classes  in  any  community  will  be  the  same  as 
that  of  those  who,  during  the  years  of  growth,  are  under  favorable  conditions. 
All  extensive  series  of  observations  which  we  possess  relate  to  the  least- 
favored  social  classes,  and  hence  it  is  not  improbable  that  the  figures  in  the 
foregoing  tables,  which  are  based  on  data  obtained  mainly  at  the  Marylebone 
Workhouse  in  London,  are  decidedly  below  those  which  would  be  obtained 
from  the  more  fortunate  classes  in  the  same  community.  We  have  a  list  of 
brain-weights  which  contains  the  records  for  a  number  of  men  of  acknowl- 
edged eminence,  and  also  for  others  who  attained  recognition  as  able  persons 
without  being  exceptionable  remarkable.  This  list  shows  the  persons  thus 
selected  to  have  brains  on  the  average  heavier  than  the  usual  hospital  sub- 
ject.1 

Brain-weight  of  Criminals. — The  observations  of  Manouvrier  have  shown 
that  among  French  murderers  the  brain-weight  is  similar  to  that  of  the  incli- 
1  Donaldson  :  The  Growth  of  the  Brain,  1895. 


278  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

viduals  usually  examined  in  the  Parisian  hospitals.  In  the  same  manner,  the 
observations  on  the  brain-weight  among  the  insane  indicate,  according  to  the 
records  of  Boyd  and  others,  that  the  insane  as  a  class  (the  microcephalies 
being  excluded)  are  not  characterized  by  a  special  brain-weight.  When, 
however,  the  insane  are  grouped  according  to  the  form  of  disease  from  which 
they  have  suffered,  it  is  evident  that  those  in  which  the  brain  was  congested 
at  death  exhibit  the  higher  weight,  while  those  in  which  the  pathological 
processes  caused  destructive  changes,  exhibit  a  low  weight.  The  differences 
in  these  cases  are  rather  the  results  of  disease  than  the  cause  of  it. 

Brain-weights  of  Different  Races. — Concerning  the  weights  of  the  brain 
in  different  races  there  are  no  extensive  observations  which  have  been  made 
directly  on  the  brain  itself.  Davis,1  however,  has  determined  the  cranial 
capacities  of  a  series  of  skulls  belonging  to  different  races,  and  the  brain- 
weights  have  been  calculated  from  these.2  This  calculation  gives  the  largest 
brain-weights  to  the  western  Europeans,  but  for  a  proper  interpretation  of 
the  results  there  are  needed  at  least  the  data  concerning  stature  and  age  of 
the  cases  studied,  both  of  which  are  here  lacking. 

"Weight  of  the  Spinal  Cord. — Comparatively  few  observations  are  avail- 
able for  the  spinal  cord  :  Mies 3  found  that  in  adults  it  weighed  24  to  33.3 
grams,  with  an  average  weight  of  26.27  grams ;  this  for  the  cord  deprived  of 
the  nerve-roots  but  covered  by  the  pia.  The  variations  due  to  sex  and  stat- 
ure have  not  been  determined.  It  seems  probable,  however,  that  the  cord, 
like  the  brain,  will  be  found  lighter  in  females  and  in  short  persons :  Mies 
states  that  its  decrease  in  old  age  is  proportionately  less  than  that  of  the  brain. 

B.  GROWTH-CHANGES. 

The  characters  of  the  brain  and  cord  thus  far  described  have  been  those 
found  in  the  adult.  Between  birth  and  the  natural  end  of  life,  however, 
great  changes  take  place,  and,  as  it  is  necessary  to  consider  the  functions  of 
the  central  system  at  all  times  in  its  history,  the  importance  of  knowing  the 
direction  in  which  the  growth-changes  are  probably  occurring  is  obvious. 

Growth  of  the  Brain. — The  weight  of  the  brain  from  birth  to  the 
twenty-fifth  year  is  shown  in  Fig.  118.  The  curve  is  based  on  the 
table  of  Vierordt.4 

The  curve  beyond  the  twenty-fifth  year  is  continued  on  the  basis  of  the 
observations  by  Bischoff,5  and  for  comparison  the  curve  representing  the  en- 
cephalic weights  of  a  series  of  eminent  men,  forty-five  in  number,  is  drawn 
in  a  dotted  line,  the  averages  for  decennial  periods  being  alone  dotted. 

These  records  exhibit  the  fact  that  at  birth  the  weight  of  the  brain  is  about 
one-third  of  that  which  it  will  attain  at  maturity.  The  increase  is  very  rapid 

1  Journal  of  the  Academy  of  Natural  Sciences,  Philadelphia,  1869. 

3  Donaldson :  Growth  of  the  Brain,  1895,  p.  115. 

3  Neurologisches  Centralblatt,  1893. 

*  Archiv  fur  Anatomic  und  Physiologic,  1890. 

5  Hirngenncht  des  Menschen,  Bonn,  1880. 


CENTRAL    NERVOUS  SYSTEM. 


279 


during  the   first  year,  and  vigorous   for  the  first  seven  or  eight  years,  after 
which  it  becomes  comparatively  slow.     The  maximum  weight  is  indicated 


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in  the  fifth  decade  (males),  fourth  (females),  although  there  is  a  "  premaxi- 
mum  "  in  the  middle  of  the  second  decade  (at  thirteen  and  fifteen  years  for 
males  and  fourteen  years  for  females),  in  which  the  too  early  and  too  vigorous 


280  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

growth  of  the  encephalon  appears  to  be  an  important  factor  in  the  cause  of 
death  ;  hence  the  larger  brain-weight  found  at  autopsies  during  these  years. 
While,  in  general,  the  individual  may  be  supposed  to  follow,  in  the  develop- 
ment of  his  encephalon,  the  course  here  indicated  by  the  curve,  this  premax- 
imal  increase  must  be  excepted  for  the  reasons  given. 

Relation  between  Growth  of  Body  and  that  of  Encephalon. — On  com- 
paring the  growth  of  the  entire  body  with  that  of  the  encephalon,  it  is  evi- 
dent that  the  growth  is  more  rapid  in  the  central  nervous  system  than  in  the 
body  at  large,  and  that  it  is  almost  completed  in  the  former  at  the  end  of  the 
eighth  year,  whereas  the  body  has  at  that  time  reached  but  one-third  of  the 
weight  which  it  will  attain  at  maturity. 

A  causal  relation  between  a  well-developed  central  system  and  the  subse- 
quent growth  of  the  entire  body  is  thus  suggested,  and  also  it  is  evident  that 
conditions  which  influence  growth  will  at  any  time  find  the  body  on  the  one 
hand,  and  the  central  system  on  the  other,  at  quite  different  phases  in  their 
development. 

The  long-continued  growth  of  the  body  brings  it  about  that  the  central 
system,  which  at  birth  may  form  12  per  cent,  of  the  total  weight  of  the  indi- 
vidual, is  at  maturity  about  2  per  cent,  or  less.  For  this  change  in  propor- 
tion the  increase  of  the  muscular  system  is  mainly  responsible. 

Further,  the  much  smaller  mass  of  the  muscular  system  in  the  female  is  the 
chief  cause  of  the  higher  percentage  value  of  the  central  system  in  the  female 
— a  relation  which  has  been  much  emphasized,  but  which  is  really  not  signifi- 
cant, since  in  both  sexes  this  high  percentage  value  of  the  central  system  is 
most  developed  at  birth,  and  becomes  steadily  less  marked  as  maturity  is 
approached. 

Increase  in  the  Number  of  Functional  Nerve-elements. — Having  thus 
briefly  indicated  the  facts  of  growth  so  far  as  they  can  be  detected  by  the 
balances,  it  still  remains  to  mention  the  series  of  changes  which  may  be 
studied  by  other  means,  such  as  micrometric  measurements  or  enumeration. 
The  results  obtained  by  these  methods  are  somewhat  complex,  and  must  be 
treated  with  great  care.  Human  embryology  indicates  that  after  the  third 
month  of  foetal  life  the  number  of  cells  in  the  central  system  is  not  increased. 
With  the  cessation  in  the  production  of  new  cells  the  only  remaining  means 
of  increase  in  size  is  by  enlargement  of  those  cells  already  present. 

How  this  occurs  is  well  indicated  by  the  accompanying  table  (p.  281), 
which  shows  the  change  in  the  size  of  cell  -bodies  in  a  given  locality  in 
man. 

All  vertebrates  are  not  similar  in  respect  to  the  manner  of  this  change. 
Birge  l  has  shown  that  in  frogs  there  is  a  gradual  increase  in  the  number  of  the 
fibres  forming  the  ventral  and  dorsal  spinal  roots,  and  that  this  goes  on  at  the 
rate  of  about  fifty  additional  fibres  in  the  ventral  roots  and  seventy  in  the 
dorsal,  for  each  gram  added  to  the  total  weight  of  the  frog.  The  increase 
was  still  apparent  in  a  frog  weighing  112  grams.  In  the  case  of  the  ventral 

1  Birge  :  Archivfur  Anatomie  und  Physiologic,  suppl.,  1882. 


CENTRAL    NERVOUS  SYSTEM. 


281 


root-fibres  it  was  also  determined  that  the  cells  of  origin  in  the  ventral  horns 
of  the  spinal  cord  increased  at  a  corresponding  rate.  Here  is  exemplified  an 
instance  of  long-continued  enlargement  of  the  nervous  system  by  the  regular 
development  of  immature  cells,  a  method  of  growth  most  marked  probably 
in  those  animals  which  increase  in  size  so  long  as  they  live. 

Volumes  of  the  Largest  Cell-bodies  in  the  Ventral  Horn  of  the  Cervical  Cord  of 
Han  (based  on  Kaiser's  records  of  the  mean  diameters). 

The  volume  700//3,  in  the  fetus  of  four  weeks,  is  taken  from  His,  and  the  figures  represent 
multiples  of  that  volume. 


Subject. 

Age. 

Proportional 
volume  of 
the  cell-bodies 
l=7(XVs. 

Time  interval. 

Fetus 

4  weeks 

11 

ii 

20      " 

17 

u 

24      " 

31  !• 

36  weeks 

(i 

28      " 

67  1 

<» 

36      " 

81 

Child  at  birth         .                            .... 

124  ) 

Boy  at  fifteen  years  .        

124  ) 

15  years. 

]VIiin  adult  .            

160 

15      " 

It  is  believed  that  in  this  case  the  new  cells  and  new  fibres  are  not, 
strictly  speaking,  new  morphological  elements,  but  are  the  result  of  devel- 
opmental changes  taking  place  in  the  cells  present  in  the  system  from  an 
early  period. 

A  distinction  is  thus  to  be  made  between  cell-elements  which,  because 
they  are  not  developed,  are  therefore  not  a  part  of  the  system  already  physio- 
logically active,  and  those  cells  already  organized  together  and  which  are 
fully  functional.  When,  therefore,  it  is  said  that  the  cells  of  origin  for  the 
ventral  root-fibres  increase  in  number,  the  increase  refers  to  the  latter  group, 
and  not  to  the  total  number  of  elements  of  both  kinds  present  in  the  cord. 
In  other  words,  the  number  of  cells  appears  to  increase  because  the  number 
of  developed  cells  becomes  greater. 

In  support  of  this  suggestion  the  observations  on  the  growth  of  the  fibres 
in  the  roots  of  the  frog's  spinal  nerves  may  be  cited.1  Hardesty  found  the 
greatest  number  of  medullated  fibres  in  the  ventral  roots,  nearest  the  cord, 
and  in  the  dorsal  roots,  nearest  the  spinal  ganglion.  Thus  in  each  the 
greatest  number  was  nearest  the  cells  of  origin,  an  arrangement  which  is 
most  readily  explained  by  assuming  that  some  of  the  fibres  had  grown  but  a 
short  distance  from  their  cells  of  origin  at  the  time  the  frog  was  killed. 

On  the  other  hand,  Schiller2  counted  the  number  of  nerve-fibres  in  the 
oculo-motor  nerves  of  cats,  and  found  but  a  very  slight  difference  in  this 
number  between  birth  and  maturity.  So  far,  then,  as  this  nerve  is  concerned, 
it  is  found  in  the  cat  to  be  nearly  complete  at  the  time  of  birth. 

In  man  there  are  very  few  observations  on  the  increase  in  the  number  of 

1  Hardesty  :  Journal  of  Comparative  Neurology,  1899,  vol.  ix. 

2 Schiller:   Comptes  rendus  de  I' Academic  des  Sciences,  Paris,  1889. 


282 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


functional  nerve-cells  with  age.  Kaiser,1  as  is  shown  in  the  accompanying 
table,  found  in  man  increasing  numbers  of  large  nerve-cells  in  the  ventral 
horns  of  the  spinal  cord  at  the  ages  named  : 

Number  of  Developed  Cells  in  the  Cervical  Enlargement  of  Man  at  Different 

Ages  (Kaiser): 

Age.  Number  of  Nerve-cells. 

Fetus,  16  weeks 50,500 

"      32      "        118,330 

New-born  child 104,270 

Boy,  15  years 211,800 

Male,  adult 221,200 

Here,  as  in  the  frog,  the  apparent  increase  must  be  looked  upon  as  due  to 
the  gradual  development  of  elements  present  from  an  early  date.  And  it 
must  be  further  remembered  that  in  this  case  the  cells  thus  maturing  after 
birth  probably  belong  in  a  large  measure  to  the  group  of  "  central  cells," 
the  function  of  which  is  to  increase  the  complexity  of  the  pathways  within 
the  cord. 


FIG.  119. — Diagram  illustrating  the  extent  of  the  cerebral  cortex.  The  outer  square  v-  >  shows  a  sur- 
face approximately  one-fiftieth  of  2352  sq.  cm.  in  extent ;  the  inner  square  (A)  has  two-thirds  of  this  area, 
and  is  the  proportion  of  the  cortex  sunken  in  the  fissures.  2352  sq.  cm.  are  approximately  the  area  of  the 
entire  cortex  in  a  male  brain  weighing  1360  grams. 

Increase  in  the  Fibres  of  the  Cortex. — The  area  of  the  cerebral  cortex 
(see  Fig.  119)  varies  according  to  several  conditions,  but  in  general  the  more 
voluminous  the  cerebral  hemispheres  the  greater  its  extent.  That  which 
covers  the  walls  of  the  sulci, — the  sunken  cortex — has  in  man  about  twice 
the  extent  of  that  directly  exposed  on  the  surface  of  the  hemispheres. 

In  the  cortex  of  the  human  cerebral  hemispheres  it  has  been  shown  by 
Vulpius 2  that  the  number  of  fibres  in  the  different  layers  is  greater  in  the 

1  Die  Fanctionen  der  Ganglienzelien  des  Halsmarkes,  Haag,  1891. 
3  Vulpius:  Archivfiir  Psychiatric  und  Nervenlcrankheiten,  1892. 


CENTRAL   NERVOUS  SYSTEM.  283 

thirty-third  year  than  ai  earlier  periods,  hut  in  old  age  this  number  is 
decreased.  At  exactly  what  age  decrease  sets  in,  is  not  to  be  determined 
from  these  observations.  They  show,  simply,  that  in  general  the  number  of 
fibres  was  less  at  seventy-nine  years  than  at  thirty-three  year-. 

In  a  similar  way  Kaes  has  shown l  that  the  association  fibres  of  the  human 
cerebral  cortex  form  three  parallel  systems.  In  general  it  is  the  deepest 
layer — i.  e.,  that  farthest  from  the  surface  of  each  system — which  first  becomes 
medullated.  The  first  fibres  appear  at  about  the  fourth  month  of  life  in  the 
deepest  portion  of  the  deepest  layer.  The  middle  system  is  the  last  to  be 
completely  medullated,  this  process  continuing  in  it  up  to  the  forty-fifth 
year  of  life. 

Passow 2  has  shown  that  at  maturity  the  cortex  of  the  central  gyri  exhibits 
association  fibres  which  increase  in  abundance  as  we  pass  from  the  great 
longitudinal  fissure  (leg  area)  toward  the  Sylvian  fissure,  these  fibres  being 
most  abundant  in  the  areas  for  the  hand  and  fingers.  On  the  other  hand,  in 
the  central  gyri  of  a  child  fifteen  months  of  age,  these  fibres  are  equally 
abundant  in  these  two  localities.  From  this  it  appears  that  the  differentia- 
tion takes  place  after  the  first  year  of  life. 

Significance  of  Medullation. — Two  sorts  of  nerve-fibres  are  described — 
those  with  and  those  without  a  medullary  sheath.  Both  have  the  power  of 
isolated  conduction,  but  in  the  peripheral  system  the  non-medullated  fibres 
are  found  in  connection  with  the  sympathetic  system,  where  less  specialized 
functions  are  carried  on,  and  also  in  a  large  but  varying  degree  in  the  central 
system.  The  wider  significance  of  this  difference  in  medullation  is  at  the 
moment  quite  obscure. 

The  first  suggestion,  that  absence  of  the  medullary  sheath  is  an  immature  con- 
dition which  persists  in  various  parts  of  the  nervous  system,  brings  us  at  once 
to  the  question  of  the  physiological  difference  thus  implied,  but  not  explained. 

It  is  known  that  the  central  system  is  at  birth  very  imperfectly  medul- 
lated, and  the  growth  of  these  medullary  sheaths  must  form  a  large  part  of 
the  total  increase  in  its  bulk.  In  the  mature  nerve-fibre  the  axis-cylinder 
and  the  medullary  sheath  have  nearly  equal  volumes,  and  therefore  approxi- 
mately equal  weights.  The  medullated  fibres  form  probably  not  less  than 
97  per  cent.3  of  the  total  weight  of  the  nerve-tissues  composing  the  encepha- 
lon,  and  of  this  nearly  one-half  would  be  medullary  substance. 

Increase  in  the  Mass  of  the  Neurones. — The  amount  of  this  increase 
under  various  conditions  has  already  been  discussed,  and  been  found  to  range 
between  zero. and  fifty-thousand-fold  (p.  176). 

Number  of  Cells. — Any  attempt  to  determine  the  entire  number  of 
nerve-cells,  the  bodies  of  which  lie  within  the  walls  of  the  neural  tube,  must 
be  open  to  many  sources  of  error. 

AKaes:  Wiener  med.  Wochenschrift,  1895,  Nos.  41,  42;  Kaes:  Jahrbuchern  der  Hamburg, 
Stoats  Krankenanstalten,  Jahrgang,  1893-94,  Bd.  iv. 

2  Passow  :  Archivfiir  Psychiatric,  Bd.  31,  S.  859,  860. 

9  Thompson  :  Journal  of  Comparative  Anatomy,  1899,  vol.  ix.  ;   Donaldson:  Ibid.,  No.  2. 


284  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

A  careful  study  of  the  cortex l  based  on  Hammarberg's  records,  gives 
9200  million  cell-bodies  in  this  region  alone.  Considering  the  amount  of 
gray  matter  present  in  the  rest  of  the  central  system,  an  estimate  of  13,000 
million  for  the  total  number  in  the  entire  central  nervous  system  is  probably 
a  conservative  calculation. 

From  the  foregoing  facts,  together  with  those  bearing  on  the  cell-elements, 
it  is  possible  to  get  some  conception  of  the  growth-processes  in  the  central 
system,  and  to  see  how  they  are  due  to  an  enlargement  of  the  nerve-elements 
which  have  been  formed  at  a  very  early  stage  in  the  life-history  of  the  indi- 
vidual. In  such  enlargements  the  chief  increase  is  due  to  the  formation  of 
the  axones,  and  in  them,  in  turn,  about  half  the  substance  is  represented  by 
the  medullary  sheaths. 

In  all  probability  these  sheaths  are  no  exception  to  the  rule  according  to 
which  all  parts  of  the  body  are  variable,  not  only  in  their  absolute,  but  also  in 
their  relative  size,  and  therefore  it  is  possible  that  the  quantitative  variation 
in  this  constituent  is  a  very  important  factor  in  modifying  the  weight  of  the 
central  system,  perhaps  accounting  in  some  cases  for  the  very  heavy  brains 
occasionally  reported. 

Change  in  Specific  Gravity  with  Age. — During  fetal  life  and  at  birth 
the  percentage  of  water  in  the  nerve-tissues  is  high,  but  becomes  less  at 
maturity.  In  white  rats  at  birth  the  percentage  of  water  for  the  encephalon 
is  89  per  cent,  and  for  the  spinal  cord  85.3  per  cent.  At  maturity  it  is  about 
78  per  cent,  for  the  encephalon  and  70.1  per  cent,  for  the  cord.  This  change 
is  correlated  in  some  measure  with  the  development  of  the  medullary  sub- 
stance. 

For  the  gross  physical  changes  which  have  thus  been  indicated  as  occur- 
ring during  growth,  an  explanation  is  to  be  found  in  the  changes  affecting  the 
constituent  elements,  and  these  have  been  set  forth  when  describing  the 
growth  of  the  individual  cells. 

O.  ORGANIZATION  AND  NUTRITION  OF  THE  CENTRAL  NERVOUS  SYSTEM. 

What  is  here  meant  by  organization  may  be  easily  illustrated.  When,  for 
example,  by  later  growth  new  tissue  is  added  to  the  liver,  or  the  skin  is  in- 
creased in  area  or  a  muscle  enlarged,  there  is  caused  by  the  addition  of  new 
substance  a  change  in  the  powrers  of  these  tissues,  which  is  mainly  quantita- 
tive. The  larger  organ  exhibits  the  same  capabilities  that  the  smaller  organ 
exhibited,  but  does  so  in  a  greater  degree. 

In  the  central  nervous  system,  on  the  other  hand,  it  appears  that  with 
growth  the  system  becomes  capable  of  new  reactions  in  the  sense  that  its 
various  responses  are  controlled  and  directed  by  a  larger  number  of  incoming 
impulses,  and  thus  the  number,  complexity,  and  refinement  of  the  reactions 
are  increased,  and  in  this  sense  it  really  attains  new  powers. 

With  the  change  in  the  age  of  the  central  system  there  occurs  from  birth 
up  to  the  prime  of  life,  if  we  may  judge  from  general  reactions,  an  increase 
1  Thompson :  Journal  of  Comparative  Neurology,  1899,,  vol.  ix. 


CENTRAL    NERVOUS  SYSTEM.  285 

in  this  organization.  This  is  maintained  for  a  time,  and  then  in  old  age  it 
breaks  down,  at  first  gradually,  and  later  rapidly.  It  becomes  important, 
therefore,  to  examine  the  manner  in  which  this  organ i/at ion  is  accomplished. 

Organization  in  the  Central  System. — When  first  formed  the  cells  com- 
posing the  central  system  are  completely  separated  from  one  another.  In  the 
mature  nervous  system  the  impulses,  as  has  been  pointed  out,  probably  travel 
for  the  most  part  from  the  axones  of  one  unit  to  the  dendrites  of  another. 

For  organization  the  most  important  changes,  however,  are  those  affecting 
the  cell  outgrowths,  both  dendrites  and  axones.  During  growth  both  of  these 
increase  in  the  length  of  their  main  stems  and  of  their  respective  branches. 
In  picturing  the  approach  of  two  elements  within  the  central  system  the  pro- 
cess is  usually  described  as  that  of  the  outgrowth  of  the  axone  toward  the 
dendrites  or  bodies  of  those  cells  which  are  destined  to  receive  the  impulse, 
but  it  must  not  be  forgotten  that  the  dendrites  are  also  growing,  and  the 
question  of  the  approximation  of  the  branches  of  these  latter  to  those  of  the 
axone  depends  in  part  on  their  own  activities. 

The  conditions  modifying  this  process  are,  however,  obscure.  It  is  evi- 
dent that  medullation  outside  of  the  central  system  is  not  necessary  to  the 
functional  activity  of  a  fibre,  and  therefore  probably  in  the  central  system 
unmedullated  fibres  are  also  in  many  cases  functional.  Whatever  may  be 
the  relation  of  the  establishment  of  new  pathways  to  the  acquisition  of  medul- 
larv  sheaths  by  the  axone  and  its  branches,  it  is  also  found  that  all  fibres 
which  when  mature  are  medullated  begin  as  unmedullated  fibres,  and  that 
the  increase  in  medullation  throughout  the  central  system  is  an  index  of  the 
increase  in  organization.  A  consideration  of  the  facts  of  growth  in  the  layers 
of  the  cortex,  for  instance,  will  show  them  to  be  open  to  this  interpretation. 

Applying  these  ideas  concerning  organization  to  the  three  classes  of  cells, 
afferent,  central,  and  efferent,  which  compose  the  nervous  system,  we  find  the 
following :  In  the  central  system  the  afferent  cells  contribute  to  organization 
by  the  multiplication  of  the  collaterals.  At  the  periphery  the  division  of  the 
branches  of  the  axone  increases  the  number  of  opportunities  for  excitation 
which  such  an  element  offers.  These  cells  are,  for  the  most  part,  without 
dendrites.  Among  the  central  cells  all  possible  modes  of  growth  are  con- 
tributory ;  that  is,  the  branches  of  both  kinds  add  directly  to  the  complexity 
of  the  central  pathways.  On  the  other  hand,  the  efferent  group  contributes 
to  this  complexity  almost  solely  by  the  formation  of  dendrites,  the  collaterals 
which  come  from  the  axones  of  these  cells  forming  but  an  insignificant  con- 
tribution. Not  only,  therefore,  is  organization  in  large  part  dependent  on 
changes  in  the  central  cells  by  reason  of  their  numerical  preponderance,  but 
also  by  reason  of  the  fact  that  to  them  a  multiplication  of  pathways  both  by 
elaboration  of  the  axones  and  the  dendrites  is  alone  possible. 

Defective  Development. — In  view  of  these  facts,  defective  development 
in  the  nervous  system  may  depend  on  failure  in  one  or  more  of  these  several 
processes  by  which  the  system  is  organized,  and  it  should  be  possible  to  corre- 
late defective  development  involving  mainly  one  set  of  elements  with  a  dis- 


286  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

tinct  clinical  picture.  The  results  of  defective  development  are  not  merely 
an  absence  of  certain  powers,  but  in  some  measure  a  diminution  in  the  strength 
and  range  of  those  that  remain  (Hammarberg T). 

Laboratory  Animals. — The  bearing  of  these  facts  on  the  conception 
which  we  form  of  the  nervous  systems  of  those  animals  commonly  employed 
for  laboratory  .experiments  may  be  here  mentioned.  The  frog,  pigeon,  rabbit, 
cat,  and  dog  form  a  series  in  which  the  total  mass  of  the  central  system  in- 
creases from  the  beginning  to  the  end  of  the  series. 

The  number  of  cells  in  the  largest  system,  that  of  the  dog,  is  many  times 
greater  than  that  in  the  smallest,  the  frog ;  and  it  is  probable  that  the  others 
are  in  this  respect  intermediate.  Organization  is  apparently  more  rapidly 
completed  and  more  nearly  simultaneous  throughout  the  entire  system  in 
forms  like  the  frog  and  pigeon,  and  also  in  these  latter  the  organization  is 
least  elaborate.  While  the  educability  of  the  nervous  system  of  the  dog  may 
depend  on  several  conditions,  the  comparative  slowness  of  organization  is 
undoubtedly  one  of  them,  and  a  very  important  one.  Where  the  organiza- 
tion is  early  established  it  is  also  simple,  and  thus  portions  of  the  system 
retain  through  life  a  greater  capacity  for  acting  alone.  In  selecting  an  ani- 
mal, therefore,  on  which  to  make  a  series  of  experiments,  these  several  facts 
must  be  kept  in  view,  for  the  choice  is  by  no  means  a  matter  of  indifference. 

Blood-supply.- — For  the  general  distribution  of  the  blood-vessels  in  rela- 
tion to  the  gross  subdivision  of  the  brain  the  student  is  referred  to  the  works 
on  anatomy.  The  finest  network  of  vessels  is,  however,  to  be  found  where 
the  cell-bodies  are  most  densely  congregated,  and  indeed  the  distinction 
between  the  masses  of  gray  and  white  matter  in  the  central  system  is  as 
clearly  marked  by  the  relative  closeness  of  the  capillary  network  as  in  any 
other  way  (see  p.  191).  One  result  of  this  relation  between  the  blood-sup- 
ply and  the  cell-bodies  which  form  the  gray  matter  is  a  general  arrangement 
of  the  vessels  along  the  radii  of  the  larger  subdivisions  of  the  brain,  as  the 
cerebral  hemispheres  and  the  cerebellum. 

The  conditions  which  control  the  circulation  within  the  cranium  and 
spinal  canal  are  not  exactly  the  same  at  all  periods  of  life,  but  the  variations 
occur  in  minor  points  only. 

The  studies  of  Huber 2  show  that  in  the  cat,  dog,  and  rabbit  at  least,  the 
vessels  in  the  pia  of  the  cerebral  hemispheres  are  supplied  with  both  medul- 
lated  and  unmedullated  nerves.  The  former  are  probably  sensory  in  func- 
tion ;  the  latter,  possibly,  vaso-motor.  These  latter  nerves  have  been  fol- 
lowed to  arteries  so  small  as  to  possess  but  two  layers  of  muscle-cells,  but 
were  not  traced  by  Huber  to  vessels  actually  penetrating  the  nervous  sub- 
stance of  the  hemispheres,  von  Kolliker,  however,  claims  to  have  followed 
them  even  there. 

These  observations  make   the   existence  of  a  corresponding  vaso-rnotor 

1  Hammarberg :  Studien  ueber  Klinik  und  Pathologic  der  Idiotic  nebst    Untersuchungen  ueber 
die  normale  Anatomic  der  Hirnrinde,  Upsala,  1895. 

2  Journal  of  Comparative  Neurology,  1899,  vol.  ix.  No.  1. 


CENTRAL    NERVOUS  SYSTEM.  287 

.supply  to  the  pial  vessels  in  man  probable.  Nevertheless  the  efficiency  of 
this  vaso-motor  mechanism  does  not  appear  to  be  great,  since  various  authors 
fail  to  find  physiological  evidence  for  a  local  control  of  the  arterioles. 

The  reactions  of  the  central  vessels  are  broadly  those  of  a  system  of  elastic 
tubes  in  a  closed  cavity.  As  a  result,  it  is  found  that  the  quantity  of  blood 
in  the  central  system  is  subject  to  very  slight  variations  only.  A  rise  in  the 
arterial  pressure  causes  a  more  rapid  flow  of  the  blood  through  the  encepha- 
lon.  It  also  causes  a  rise  in  the  venous  pressure,  and  with  this  a  correspond- 
ing rise  in  the  intracranial  pressure,  the  last  two  varying  in  the  same  sense 
and  to  the  same  extent.1 

The  flow  through  the  central  system  is  subject  to  the  influence  of  gravity, 
and  takes  place  the  more  readily  the  more  the  resistance  is  diminished.2 
The  principal  controlling  mechanism  is  in  the  splanchnic  area.  According 
to  the  condition  of  the  vessels  in  this  area,  the  intracranial  blood-pressure 
varies. 

It  is  to  be  noted  in  passing  that  when  a  person  lying  on  a  table  is  bal- 
anced on  a  transverse  axis,  this  axis  is  about  8.77  cm.  to  the  cephalic  side  of 
the  line  which  joins  the  heads  of  the  femurs.3  This  leaves,  of  course,  the 
splanchnic  area  mainly  on  the  cephalic  side  of  this  axis,  and  hence  any  inflow 
of  blood  from  the  extremities  would  tend  to  make  the  head  end  of  the  person 
thus  balanced  dip  down.  This  dip  will  occur  even  when  the  splanchnic  area 
alone  is  filled,  and  hence  the  dipping  as  such  would  not  necessarily  indicate 
an  increase  in  the  quantity  of  blood  in  the  encephalon. 

In  the  adult  the  cranial  cavity  is  almost  rigidly  closed.  There  is  an 
opportunity  for  the  escape  of  a  small  quantity  of  cerebro-spinal  fluid  through 
the  foramen  magnum  into  the  vertebral  canal.  When,  as  the  result  of  in- 
creased arterial  pressure,  the  brain  has  distended  so  as  to  drive  out  the  sub- 
dural  fluid,  the  brain  is  forced  against  the  walls  of  the  cranium  and  blocks 
the  outflow  into  the  spinal  canal.  On  the  other  hand,  it  has  been  found 
that  if  a  mass  displacing  from  2  to  3  c.c.  be  introduced  into  the  subdural 
space  of  a  dog,  the  brain  will  adjust  itself  without  rise  of  intracranial  pres- 
sure. If  in  this  case  the  volume  of  the  mass  introduced  is  increased,  there 
follows  a  rise  of  intracranial  pressure,  and  this  rise  in  every  instance  tends  to 
impede  the  circulation  through  the  brain.  While  the  fontanelles  are  open 
the  brain  normally  pulsates,  and  we  recognize  in  its  variations  in  volume  all 
the  different  variations  in  blood -pressure  with  which  we  are  familiar.  The 
pulsation  of  the  brain  is  doubtless  an  important  aid  to  the  movements  of  the 
fluids  within,  and  hence  tends  to  facilitate  nutrition  during  the  earlier  stages 
of  growth. 

In  pathological  cases  where  the  cranial  wall  has  been  destroyed  there  is 
a  similiar  variation  in  volume  to  be  observed  in  the  adult,  and  it  is  possible 
that  the  beneficial  effects  which  in  so  many  instances  follow  trephining  of 

1  Ho  well :  American  Journal  of  Physiology,  1898,  i.  No.  1. 

2  Hill  :  Journal  of  Physiology,  189o,  vol.  xviii. 

3  W.  und  Ed.  Weber:  Mechanik  der  menschlichen  Gehwerkzeuye,  1836. 


288  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

the  skull  may  depend  upon  this  mechanical  release.  Of  course,  in  cases  with 
a  defective  skull-wall  an  increase  in  arterial  pressure  causes  a  more  decided 
increase  in  the  volume  of  blood  in  the  brain  ;  this,  however,  is  much  more 
marked  than  it  would  be  under  ordinary  conditions,  and  is  not  to  be  regarded 
as  the  main  eifect,  which  is  an  increase  in  the  quantity  of  the  blood  passed 
through  the  central  system  in  a  unit  of  time.  Mosso *  has  found  the  tempera- 
ture of  the  blood  coming  from  the  brain  (dog's)  slightly  higher  than  that  of 
the  rectum  and  of  the  arterial  blood.  The  differences  are  very  small,  but  he 
draws  the  conclusion  that  the  metabolic  processes  in  the  brain  are  sufficiently 
intense  to  raise  the  temperature  of  the  blood  passing  through  it. 

As  against  the  intensity  of  the  metabolism  in  the'central  system,  it  has 
been  observed  that  blood  taken  from  the  torcular  Herophili  of  the  dog  was 
intermediate  in  gaseous  content  between  arterial  blood  and  that  taken  from 
the  femoral  vein,  thus  indicating  that  the  arterial  exchange  was  less  intense 
in  the  brain  than  in  the  muscles  of  the  leg.  The  following  is  a  condensed 
statement  of  the  figures  : 

Percentages  of  Oxygen  and  Carbonic  Acid  in  various  Samples  of  Dogs'  Blood 

(Hill):1 

f  CO,  .    .  37.64  per  cent. 
Average  of  52  arterial  samples 1  O  18  25        ' 

f  CO2  .    .  41.65 
Average  of  42  torcular  samples 1  O  13  49 

Average  of  28  femoral  vein  samples {  O  2          6  34 

The  absolute  quantity  of  the  blood  in  the  brain  and  cord  is  certainly 
small  ;  if  we  may  judge  from  the  observations  on  animals,  it  is  not  more 
than  1  per  cent,  of  the  entire  blood  in  the  body.  It  is  to  remembered,  how- 
ever, that  the  cell-bodies,  which  alone  are  well  supplied  with  blood,  probably 
represent  less  than  2  per  cent,  of  the  entire  encephalic  mass. 

With  general  rise  and  fall  of  pressure  elsewhere,  there  is  a  rise  and  a  fall 
of  pressure  within  the  central  system.  During  the  first  phases  of  mental 
activity  blood  is  withdrawn  from  the  limbs ;  the  blood  thus  withdrawn  can 
be  shown  to  pass  toward  the  trunk  and  head,  for  when  a  person  lying  on  a 
horizontal  table  supported  at  the  centre  on  a  transverse  knife-edge  is  just 
balanced,  then  increased  activity  of  the  cerebral  centres  causes  the  head  to 
dip  down  (Mosso),  and  if  the  skull-wall  is  defective  the  brain  is  seen  to  swell. 

In  the  later  stages  of  fatigue  the  blood-supply  to  the  nerve-centres  dimin- 
ishes owing  to  a  decrease  in  force  of  the  heart-beat  and  the  tonicity  of  the 
splanchnic  vessels,  so  that  the  brain  in  birds  exhausted  by  a  long  flight 
has  been  found  by  Mosso  to  be  in  a  high  degree  anaemic.  There  is  much 
reason  to  think  that  in  man  a  similar  reaction  occurs. 

The  study  of  the  cerebral  circulation  in  the  case  of  those  in  whom  the 
skull-wall  is  at  some  point  deficient  shows  a  bulging  of  the  skin  over  the 


1  Die  Temperatur  des  Gehirns,  Leipzig,  1894. 

2  Journal  of  Physiology,  1895,  vol.  xviii. 


CENTRAL    NERVOUS  SYSTEM.  289 

opening  into  the  cranial  cavity  as  a  result  of  mental  effort  or  emotion.  In 
the  normal  adult  this  bulging  cannot,  of  course,  occur  to  anything  like  such  an 
extent,  and  the  space  for  the  arterial  blood  must  be  gained  both  by  driving 
out  the  blood  from  the  cerebral  veins  within  the  cranium  and  through  the 
expulsion  of  the  subdural  fluid. 

Influence  of  Glands. — In  the  growth  of  the  nervous  system  it  is  not  only 
the  quantity,  but  the  peculiar  qualities  of  the  blood  that  are  important,  and 
among  the  various  glands  the  activity  of  which  is  so  necessary  for  the  growth 
of  the  nervous,  as  well  as  the  other  systems,  and  is  also  needed  for  its  full 
maintenance,  the  thyroid  appears  as  very  important.  In  sporadic  cretinism, 
associated  as  it  is  with  atrophy  of  the  thyroid,  the  feeding  of  sheep's  thyroids 
has  produced  remarkable  growth-changes  in  all  parts  of  the  body — the  nerv- 
ous system  included. 

At  the  same  time,  experimental  extirpation  of  the  thyroid  is  followed  by 
destructive  changes  in  the  central  system,  caused  by  disturbances  in  its  nutri- 
tion. The  future  will  doubtless  reveal  other  forms  of  internal  secretion  which 
also  have  a  significance  for  the  activity  of  the  central  system. 

Starvation. — In  starving  animals  the  nervous  system  loses  but  very  little 
in  weight.1  This  small  loss  is  most  striking  in  view  of  the  fact  that  exten- 
sive histological  changes  occur  in  the  cell-bodies.  However,  if  we  consider 
the  cell-bodies  as  the  part  mainly  affected  during  starvation,  then  the  small 
mass  of  the  cell-bodies  would  go  far  toward  explaining  the  result,  but  it  does 
not  explain  why  the  myeline  is  so  resistant. 

Fatigue. — The  histological  basis  of  fatigue  as  expressed  by  the  changes  in 
the  individual  cells,  has  already  been  discussed.  The  fatigue  of  the  system  as 
a  whole  is  but  the  expression  of  fatigue  in  large  numbers  of  its  elements,  but 
the  manner  in  which  the  changes  show  themselves  is  somewhat  complicated. 

When  the  attempt  is  made  to  raise  a  weight  by  the  voluntary  contractions 
of  the  muscles  of  the  index  finger  at  regular  intervals,  say  once  a  second,  it 
is  found  that  if  the  weight  be  heavy  the  power  of  the  finger  decreases,  and 
the  weight  soon  ceases  to  be  lifted  as  high  as  at  first.  Finally  a  point  is 
reached  when  the  voluntary  effort  produces  little  or  no  elevation  of  the  weight. 
If,  however,  despite  this  failure,  the  effort  is  still  made  at  regular  intervals, 
it  happens,  in  some  persons,  that  this  power  returns  gradually,  and  a  few  sec- 
onds later  the  contractions  are  very  nearly  as  high  as  at  the  beginning  of  the 
experiment  (Mosso).  This  phenomenon  may  repeat  itself  many  times,  giving 
a  record  formed  by  groups  of  contractions  most  extensive  near  the  centre  of 
each  group,  these  latter  being  separated  by  portions  of  the  curve  in  which 
the  contractions  are  very  small  or  wanting  (see  Fig.  120).  (See  General 
Physiology  of  Nerve  and  Muscle,  p.  135.) 

Daily  Rhythms. — Within  the  cycle  of  the  astronomical  day  the  progress 
of  events  leading  to  fatigue  is  not  a  steady  one.  Lombard2  found  that  if  the 
capacity  for  voluntary  effort  was  measured  by  the  amount  of  work  which 

1  Voit:  Zeitsclmft  far  Biologic,  1894,  P.d.  xxx. 

2  Journal  of  Physiology,  1892,  vol.  xiii. 
VOL.  IT.— 19 


290 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


could  be  done  by  voluntarily  contracting  the  flexor  muscles  of  the  index 
finger  before  the  first  failure  to  respond  to  a  voluntary  stimulus  appeared,  then 


FIG.  120.— A  record  of  the  extent  of  the  flexions  of  the  forefinger  lifting  a  weight  at  regular  intervals. 
The  light  lines  are  those  for  the  voluntary  contraction  ;  the  heavy  lines,  those  for  contractions  following 
the  direct  stimulation  of  the  flexor  muscles  by  electricity.  In  the  former  there  are  periods,  in  the  latter 
none.  The  arrow  shows  the  direction  in  which  the  record  is  to  be  read  (Lombard). 

the  curve  expressing  this  capacity  for  voluntary  work  throughout  the  day 
was  represented  as  in  Fig.  121.  Briefly,  the  curve  shows  two  maxima,  at  10 
P.  M.,  and  10  A.  M.,  with  two  minima  midway  between  them.  In  general 


80 


40 

FIG.  121.— Showing  at  each  hour  of  the  day  and  night  how  many  centimeters  a  weight  of  3000  grams 
could  be  raised  by  repeated  voluntary  contractions  of  the  forefinger  before  fatigue  sets  in.  The  curve  is 
highest  at  10  to  11  A.  M  ,  and  10  to  11  p.  M.;  lowest,  3  to  4  p.  M.,  and  3  to  4  A.  M.  Circle  with  dots,  observa- 
tion made  just  after  taking  food ;  square  with  dot,  smoking ;  *,  work  done  8  minutes  after  drinking  15 
c.c.  of  whisky  (Lombard). 

the  immediate  effect  of  taking  food  is  to  increase  the  work  done  by  the  sub- 
ject.    Alcohol  has  the  same  effect,  while  smoking  produces  a  decrease. 

Further,  from  day  to  day  this  capacity  for  work  is  influenced  by  a  num- 
ber of  external  conditions — temperature,  barometric  pressure,  etc. 


CENTRAL   NERVOUS  SYSTEM.  291 

Time  Taken  in  Central  Processes. — All  processes  in  the  nervous  system 
take  time,  and  are  for  the  most  part  easy  to  measure.  The  rate  of  the  nerve- 
impulse  has  already  been  given.  When,  however,  it  passes  from  one  element 
to  another,  the  delay  is  even  more  marked,  and  it  is  plausible  to  assume  that 
this  detention  occurs  at  the  juncture  of  the  elements.  Thus  in  those  parts  of 
the  central  system  where  the  cell-elements  and  also  the  cell-junctions  are  most 
numerous,  the  time  taken  is  longest. 

Fig.  122  shows  this  very  well.  Between  the  middle  of  the  cerebral  hem- 
isphere and  the  optic  lobes,  although  the  distance  is  short,  the  impulse  takes 
twice  as  long  to  travel  as  between  the  bulb  and  the  lumbar  enlargement. 
When  this  time  is  measured  in  the  conscious  individual  it  is,  of  course,  open 
to  a  long  series  of  modifying  conditions,  and  these  appear  to  be  in  part  the 
same  conditions  which  modify  the  muscular  endurance  of  the  individual  at 
different  portions  of  the  day.  Thus  it  has  been  determined  that  the  speed 
with  which  reactions  can  be  made  as  indicated  by  the  reaction  time,  is  subject 
to  variations,  and  does  not  steadily  decrease  from  the  morning  to  the  evening. 

0.5  Sec. 


FIG.  122.— To  show  the  rate  at  which  impulses  pass  through  the  nervous  system  of  a  frog.  At  the 
extreme  left  the  vertical  has  the  value  of  0.5  second  and  the  other  verticals  are  compared  with  it;  thus 
between  the  cerebrum  and  the  optic  lobe  requires  about  0.25  second ;  between  the  bulb  and  the  lumbar 
enlargement  a  greater  distance— only  about  half  the  time ;  and  for  the  still  greater  distance  represented 
by  the  length  of  the  sciatic  nerve  even  less  time  is  needed  (Exner). 

It  has  been  the  purpose  of  the  paragraphs  just  preceding  to  indicate  that 
through  the  day  it  is  not  possible  to  demonstrate  a  steady  decline  of  power  in 
the  nervous  system.  We  begin  the  morning,  to  be  sure,  feeling  fresh,  and 
are  fagged  in  the  evening,  but  the  course  by  which  this  condition  has  been 
attained  is  not  a  simple  or  direct  one. 

D.  SLEEP. 

Conditions  Favoring  Sleep. — To  recover  from  fatigue  sleep  is  required. 
The  prime  condition  favoring  sleep  is  the  diminution  of  nerve-impulses  pass- 
ing through  the  central  system.  This  is  accomplished  in  two  ways.  In  the 
first  instance  it  is  usual  to  reduce  all  incoming  stimuli  to  a  minimum.  This 
is  most  directly  under  our  own  control.  On  the  other  hand,  the  permeability  of 
the  nervous  system  and  the  intensity  with  which  it  responds  are  decreased  as 
the  result  of  the  beginning  fatigue.  How  these  conditions  are  brought  about 
has  been  a  matter  of  much  speculation  and  some  experiment. 

The  parts  played  by  the  sensory  and  that  by  the  central  cells  vary  some- 
what at  different  times  of  life,  for  impulses  are  much  less  widely  diffused  in 
early  years  than  at  maturity.  Moreover,  in  childhood  the  amount  of  stored 
material  is  small,  large  at  maturity,  and  small  again  in  old  age,  and  this  holds 


292  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

true  for  all  the  groups  of  cells.  Hence  the  cells  would,  by  reason  of  this  fact, 
have  the  greatest  capability  for  work  in  the  middle  period.  Between  child- 
hood and  old  age  there  is,  however,  this  difference — that  while  in  the  former 
the  non-available  substances  in  the  cell  are  developing,  not  yet  having  ma- 
tured, those  in  the  latter  have  in  some  way  become  permanently  useless.  The 
degree  to  which  the  blood-supply  can  be  controlled  varies  with  age,  and  the 
amounts  of  substance  capable  of  yielding  energy  at  various  periods  of  life  are 
different ;  so  that,  considering  these  factors  alone,  though  there  are  probably 
others,  it  may  be  easily  appreciated  that  the  sleep  of  childhood,  maturity,  and 
old  age  should  be  quite  distinguishable. 

Cause  of  Sleep. — -It  is  recognized  that  local  exercise  is  capable  of  pro- 
ducing general  fatigue,  and  the  fatigued  portions  give  rise  to  afferent  impulses 
which,  reaching  the  central  system,  cause  some  of  the  sensations  of  fatigue; 
moreover,  the  active  tissues  (nerve-cells  and  muscles)  yield  as  the  result  of 
their  activity  some  by-product  which  is  carried  by  the  blood  through  the  cen- 
tral system  and  becomes  the  chief  cause  of  sleep.  It  has  been  shown  by 
Mosso  that  if  a  dog  be  thoroughly  fatigued,  giving  all  the  signs  of  exhaustion, 
and  the  blood  from  this  dog  be  transfused  to  one  that  has  been  at  rest,  then 
after  the  transfusion,  the  dog  which  has  received  the  blood  from  the  exhausted 
animal  will  exhibit  the  symptoms  of  fatigue  in  full  force.  The  inference  is 
that  from  the  tired  animal  certain  by-products  have  thus  been  transferred, 
and  that  these  are  responsible  for  the  reactions.  We  know,  further,  that  we 
can  distinguish  in  ourselves  different  forms  of  the  feeling  of  fatigue,  and  that 
the  sensations  which  follow  the  prolonged  exercise  of  the  muscular  system 
differ  from  those  following  the  exercise  of  the  higher  nerve-centres. 

Two  things  appear  as  highly  probable :  First,  that  there  is  a  wide  individual 
variation  in  the  condition  designated  as  normal  sleep.  Second,  that  normal 
sleep  is  the  result  of  several  sets  of  influences  which  need  not  necessarily  be 
active  to  the  same  degree  during  each  period  of  sleep.  Excluding  the  factor 
represented  by  diminution  of  the  external  stimuli,  sleep  has  been  attributed 
more  or  less  exclusively  to  one  of  the  three  following  influences  : 

1.  Chemical  Influences. — The  theories    emphasizing  the  chemical  factor 
point  out  that  during  the  normal  activity  of  the  body  there  are  formed  and 
taken  up  by  the  blood  substances  which  may  directly  diminish  the  activity 
of  the  nerve-cells  and   directly  or  reflexly  affect  the  circulation  so  as  to 
diminish  the  supply  of  blood  to  the  brain,  and  especially  to  the  cerebral 
cortex. 

2.  Circulatory  Influences. — The  vaso-motor  theories  look  upon  the  changes 
in  the  blood-supply  as  a  prime  cause  of  sleep ;  these  changes  to  be  referred 
in  the  last  instance  to  the  fatigue  of  the  vaso-motor  centre  in  the  bulb. 

3.  Histological  Influences. — These  are  made  dependent  on  the  shrinkage 
of  nerve-cells  during  fatigue,  the  retraction  of  the  dendrites  of  the  cortical 
cells  interrupting  the  nerve-pathways,  or  the  mechanical  separation  of  the 
nerve-elements  through  the  intrusion  of   the  neuroglia-cells  between  them 
(Cajal).     The  vaso-motor  and  chemical  theories  combined  are  at  present  most 


CENTRAL    NERVOUS  SYSTEM.  293 

worthy  of  attention,  and  Howell,1  after  carefully  reviewing  the  several 
theories  of  sleep,  emphasizes  the  fatigue  of  the  vaso-motor  centre  in  the 
bulb  as  the  important  cause  of  the  diminished  blood-supply  to  the  brain, 
this  fatigue  in  turn  being  caused  by  the  continuous  activity  of  this  centre 
during  the  waking  hours. - 

Cessation  of  stimuli,  decreased  responsiveness  of  the  active  tissues,  a 
change  in  the  composition  of  the  blood,  and  a  diminution  of  the  blood-supply 
to  the  brain  are  the  preliminaries  to  sleep. 

A  condition  superficially  resembling  sleep  can  be  induced  in  various  ways. 
Removal  of  all  external  stimuli,  extreme  cold,  anesthetics,  hypnotic  sugges- 
tion, compression  of  the  carotids,  a  blow  on  the  head,  loss  of  blood,  all  pro- 
duce a  state  of  unconsciousness  which,  in  so  far,  has  the  similitude  of  sleep. 
These  conditions  produce  this  state,  however,  by  mechanically  decreasing  the 
blood-supply  or  cutting  off  the  peripheral  stimuli. 

±  3 

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I'l'U.  .il.ll"1' 


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'In,,.        ,„  .Hi'1'1  mm1 


FIG.  123. — Plethysmographic  record  taken  from  the  arm  of  a  person  sleeping  in  the  laboratory.  A  fail 
in  the  curve  indicates  a  decrease  in  the  volume  of  the  arm.  The  curve  is  to  be  read  in  the  direction  of 
the  arrow.  1.  The  night  watchman  entering  the  laboratory,  waking  the  subject,  who  shortly  fell  asleep 
again;  2,  the  watchman  spoke  ;  3,  watchman  went  out;  these  changes  (2  and  3)  occurred  without  awak- 
ening the  subject  (from  experiments  made  by  Messrs.  Bardeen  and  Nichols,  Johns  Hopkins  Medical 
School). 

Normal  sleep  is  tested  by  the  fact  that  during  its  progress  the  changes 
that  occur  in  the  central  system  are  recuperative,  whereas  this  feature  may 
be  almost  absent  in  the  states  which  nearly  resemble  it. 

Condition  of  the  System  During-  Sleep. — It  appears  that  during  sleep 
the  capacity  of  the  central  system  to  react  is  never  lost.  Were  such  the  case 
it  would  not  be  possible  to  awaken  the  sleeper.  The  reactions  most  depressed 
during  sleep  are  those  which  require  the  full  activity  of  the  cerebral  cortex 
for  their  occurrence.  Conversely,  it  is  the  spinal  cord  which  is  least  affected. 
Moreover,  the  sleeping  person  is  far  more  responsive  to  stimuli  from  without 
than  at  first  might  be  thought.  The  close  relations  between  dreams  and  ex- 
ternal stimuli  have  been  recognized,  and  plethysmographic  studies  show  still 
more  clearly  how  the  matter  stands. 

It  was  found  that  when  a  subject  fell  asleep  with  the  arm  in  a  plethys- 
mograph,  various  stimuli  which  did  not  waken  the  sleeper  still  served  to 
cause  a  diminution  in  the  volume  of  the  arm  which  was  certainly  due  to  the 

1  Howell :  Journal  of  Erperimental  Medicine,  1897. 

2  De  Manaceine  :   "  Sleep  :    Its  Physiology,  Pathology,  Hygiene,  and  Psychology,"  Contem- 
porary Science  Series,  London,  1897. 


294 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


withdrawal  of  blood  from  it,  the  blood  supplied  to  the  brain  being  probably 
at  the  same  time  increased  (see  Fig.  123). 

This  experiment  shows  that  during  sleep  the  nervous  system  is  capable 
of  reactions  which  are  not  remembered  in  any  way,  but  which  naturally 
form  a  feature  of  the  condition  intermediate  between  full  consciousness  and 
deep  slumber. 

The  depth  of  sleep  as  determined  by  the  strength  of  the  stimulus  necessary 
to  elicit  an  efficient  response  has  been  measured.  The  stimulus  in  these  ex- 
periments was  the  sound  caused  by  the  fall  of  a  ball  upon  a  plate,  and  the 
measure  was  the  height  from  which  the  ball  must  fall  in  order  to  produce  a 
sound  loud  enough  to  awaken  a  sleeping  person.  The  results  of  the  observa- 
tions are  shown  in  Fig.  124. 


Strength  of  Stimulus. 
800 


700 


Hours     QJ5     1.0      1.5     ZQ     2.5    3.0    3.5     4.0    4.5     5.0    5.5    6.0     6-5    7.0     7.5    7.8 
FIG.  124.— Curve  illustrating  the  strength  of  an  auditory  stimulus  (a  ball  falling  from  a  height)  neces- 
sary to  waken  a  sleeping  person.    The  hours  marked  below.    The  tests  were  made  at  half-hour  intervals. 
The  curve  indicates  that  the  distance  through  which  the  ball  required  to  be  dropped  increased  during 
the  first  hour,  and  then  diminished,  at  first  very  rapidly,  then  slowly  (Kolschutter). 

It  is  seen  from  this  that  the  period  of  deep  slumber  is  short,  less  than  two 
hours  ;  and  is  followed  by  a  long  period,  that  of  an  average  night's  rest,  dur- 
ing which  a  comparatively  slight  stimulus  is  sufficient  to  awaken.  A  some- 
what different  curve  has  been  more  recently  obtained  by  Monninghoff  and 
Piesbergen.1 

It  is  evident  that  the  effectiveness  of  such  a  stimulus  is,  however,  no 
measure  of  the  recuperative  processes  in  the  central  system.  Repair  is  by  no 
means  accomplished  during  the  interval  of  deep  sleep,  and  experience  has 
shown,  as  in  the  case  of  persons  undertaking  to  walk  a  thousand  miles  in  one 
thousand  hours  that  although  such  an  arrangement  left  the  subject  with  two- 

1  Zeitschrift  fur  Biologie,  1893,  Bd.  xix. 


CENTRAL    NERVOUS  SYSTEM.  295 

thirds  of  the  total  time  for  rest  and  refreshment,  yet  the  feat  was  most 
difficult  to  accomplish  by  reason  of  the  discontinuity  in  the  sleep. 

The  changes  leading  to  recuperation  needed  longer  periods  than  those 
permitted  by  the  conditions  of  the  experiment. 

Loss  of  Sleep. — Loss  of  sleep  is  more  damaging  to  the  organism  as  a 
whole  than  is  starvation.  It  has  been  found  (Manaceine)  that  in  young  dogs 
which  can  recover  from  starvation  extending  over  twenty  days,  loss  of  sleep 
for  five  days  or  more  was  fatal.  Toward  the  end  of  such  a  period  the  body- 
temperature  may  fall  as  much  as  8°  C.  below  the  normal  and  the  reflexes 
disappear.  The  red  blood-corpuscles  are  first  diminished  in  number ;  to  be 
finally  increased  during  the  last  two  days,  when  the  animal  refuses  food. 
The  most  widespread  change  in  the  tissues  is  a  fatty  degeneration,  and  in  the 
nervous  system  there  were  found  capillary  hemorrhages  in  the  cerebral  hemi- 
spheres, the  spinal  cord  appearing  abnormally  dry  and  anaemic. 

Patrick  and  Gilbert1  have  studied  the  effects  of  loss  of  sleep  in  man 
(three  subjects,  young  men,  observed  during  ninety  hours  without  sleep). 
All  the  subjects  gained  slightly  in  weight  during  the  period,  but  lost  this 
excess  in  the  course  of  the  first  sleep  following  the  experiment.  The  excre- 
tion of  nitrogen  and  phosphoric  acid  was  increased  during  the  period,  the 
increase  being  relatively  greater  in  the  case  of  the  phosphoric  acid.  There 
was  a  marked  tendency  to  a  decrease  in  the  pulse-rate,  and  some  tendency 
for  the  body-temperature  to  fall.  During  these  ninety  hours  the  subjects  were 
tested  at  intervals  of  six  hours  (the  tests  required  some  two  hours  on  each 
occasion),  to  determine  variations  in  the  muscular  and  mental  powers. 

In  brief,  it  may  be  said  that  most  tests  revealed  a  loss,  which  early 
appeared  in  the  reactions  of  the  muscular  system,  and  later  in  those  of  the 
nervous  system.  In  the  test  for  the  acuteness  of  vision  (measured  by  the 
distance  at  which  the  subject  could  read  a  printed  page  illuminated  by  the 
light  of  one  standard  candle  at  a  distance  of  25  cm.)  there  was,  however, 
an  increase  in  capability  in  all  the  subjects.  At  the  end  of  the  experiment  a 
small  number  of  hours  of  sleep  in  excess  of  that  customarily  taken  appeared 
to  bring  about  a  complete  restoration  of  the  subject.  The  disproportion 
between  this  amount  of  extra  sleep  and  the  amount  lost  during  the  period 
of  experiment  is  noted  by  the  authors,  though  it  still  lacks  satisfactory 

explanation. 

E.     OLD  AGE  OF  THE  CENTRAL  SYSTEM. 

Metabolism  in  the  Nerve-cells. — Connected  closely  with  fatigue  are 
those  alterations  both  of  the  constituent  nerve-cells  and  of  the  entire  system 
found  in  old  age.  The  picture  of  the  changes  in  the  living  cells  is  that  of 
anabolic  and  catabolic  processes  always  going  on,  but  varying  in  their  absolute 
and  relative  intensity  according  to  several  conditions.  Of  these  conditions 
one  of  the  most  important  is  the  age  of  the  individual.  In  youth  and  during 
the  growing  period  of  life  the  anabolic  changes  appear  within  the  daily  cycle 
of  activity  and  repose  to  overbalance  the  katabolic,  the  total  expenditure  of 
1  Patrick  and  Gilbert  :  Psychological  Review,  1896,  vol.  iii.  No.  5. 


296  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

energy  increasing  toward  maturity.  During  middle  life  the  two  processes 
are  more  nearly  in  equilibrium,  though  the  total  expenditure  of  energy  is 
probably  greatest  then  ;  and  finally  in  old  age  the  total  expenditure  of  energy 
diminishes,  while  at  the  same  time  the  anabolic  processes  become  less  and 
less  competent  to  repair  the  waste.  The  question  why  in  the  nervous  system 
the  energies  wane  with  advanced  age  is  but  the  obverse  of  the  question  why 
they  wax  during  the  growing  period.  The  essential  nature  of  these  changes 
is  in  both  instances  equally  obscure. 

Decrease  in  Weight  of  the  Brain. — Between  the  fiftieth  and  sixtieth  years 
of  life  there  is  a  decrease  in  the  bulk  of  the  encephalon  in  those  persons 
belonging  to  the  classes  from  which  the  greater  number  of  the  records  have 
been  obtained.  So  far  as  can  be  seen  from  the  present  records,  there  is  no 
marked  change  in  the  proportional  development  of  the  encephalon  in  old 
age,  though  the  loss  appears  to  be  slightly  greater  in  the  cerebral  hemi- 
spheres than  in  the  other  portions. 

Changes  in  the  Encephalon. — The  thickness  of  the  cerebral  cortex 
diminishes  in  harmony  with  the  shrinkage  of  the  entire  system.  In  large 
measure  this  must  depend  on  the  loss  of  volume  in  the  various  fibre-systems, 
which,  according  to  the  observations  of  Vulpius,  show  a  senile  decrease  in 
the  number  of  fibres  composing  them.  This  decrease  is  more  marked  in  the 
motor  than  in  the  sensory  areas.  The  time  at  which  it  commences  cannot, 
however,  be  accurately  stated,  owing  to  the  small  number  of  records  after  the 
thirty-third  year.  Where  records  have  been  made  between  this  and  the 
seventy-ninth  year  it  appears  that  there  is  no  decided  diminution  until 
after  the  fiftieth  year,  though  at  the  seventy-ninth  year  the  decrease  is 
clearly  shown.  Engel  has  shown  that  the  branches  of  the  arbor  vitse  of 
the  human  cerebellum  decrease  in  size  and  number  in  old  age.1 

Changes  in  the  Cerebellum. — In  the  case  of  a  man  dying  of  old  age 
(Hodge)  some  cells  in  the  cerebellum  were  found  shrunken  and  others  (cells 
of  Purkinje)  had  completely  disappeared.  In  the  antennary  ganglion  of 
bees  a  very  striking  difference  appears  between  those  dying  of  old  age  and 
the  adult  just  emerged  from  its  larval  skin.  These  changes  are  comparable 
with  those  described  in  mammals,  and  it  further  appears  that  in  passing  from 
the  youngest  to  the  oldest  forms  cells  have  disappeared  from  the  ganglia^ 
and  that  in  the  young  form  of  the  bee  there  are  some  twenty-nine  cells 
present  for  each  one  found  at  a  later  period. 

To  the  anatomy  of  the  human  nervous  system  in  old  age  contributions 
have  been  made  by  studies  on  the  pathological  anatomy  of  paralysis  agitans.2 

In  subjects  suffering  from  this  affection  the  bodies  of  the  nerve-cells  are 
shrunken,  pigmented,  and  showr  in  some  cases  a  granular  degeneration  ;  the 
fibres  in  part  are  atrophied  and  degenerated ;  the  supporting  tissues  increase, 
and  the  walls  of  the  small  blood-vessels  are  thickened.  These  changes  have 
been  found  principally  in  the  spinal  cord,  being  most  marked  in  the  lumbar 

1  Engel:    Wiener  medicinische  Wochenschrift,  1863. 

2  Ketcher  :  Zeitschrift  fiir  Heilkunde,  1892  ;  Redlich  :  Jahrbuchfur  Psychiatrie,  1893. 


CENTRAL    NERVOUS  SYSTEM.  297 

region.  But  the  cords  of  aged  persons  who  do  not  exhibit  the  symptoms 
of  paralysis  agitans  show  similar  changes,  though  usually  they  are  not  so 
evident,  and  hence  the  pathological  anatomy  of  this  disease  resolves  itself  into 
a  somewhat  premature  and  excessive  senility  of  the  central  system. 

Shrinkage,  decay,  and  destruction  mark  the  progress  of  senescence,  and 
the  nervous  system  as  a  whole  becomes  less  vigorous  in  its  responses,  less 
capable  of  repair  or  extra  strain,  and  less  permeable  to  the  nervous  impulses 
that  fall  upon  it ;  and  it  thus  breaks  down,  not  into  the  disconnected  elements 
of  the  fetus,  but  into  groups  of  elements,  so  that  its  capacities  are  lost  in  a 
fragmentary  and  uneven  way. 


III.  THE  SPECIAL  SENSES. 


A.   VISION. 

The  Physiology  of  Vision. — The  eye  is  the  organ  by  means  of  which 
certain  vibrations  of  the  luminiferous  ether  are  enabled  to  affect  our  conscious- 
ness, producing  the  sensation  which  we  call  "  light."  Hence  the  essential  part 
of  an  organ  of  vision  is  a  substance  or  an  apparatus  which,  on  the  one  hand, 
is  of  a  nature  to  be  stimulated  by  waves  of  light,  and,  on  the  other,  is  so  con- 
nected with  a  nerve  that  its  activity  causes  'nerve-impulses  to  be  transmitted  to 
the  nerve-centres.  Any  animal  in  which  a  portion  of  the  ectoderm  is  thus 
differentiated  and  connected  may  be  said  to  possess  an  eye — i.  e.  an  organ 
through  which  the  animal  may  consciously  or  unconsciously  react  to  the  exist- 
ence of  light  around  it.1  But  the  human  eye,  as  well  as  that  of  all  the  higher 
animals,  not  only  informs  us  of  the  existence  of  light,  but  enables  us  to  form 
correct  ideas  of  the  direction  from  which  the  light  comes  and  of  the  form,  color, 
and  distance  of  the  luminous  body.  To  accomplish  this  result  the  substance 
sensitive  to,  light  must  form  a  part  of  a  complicated  piece  of  apparatus  capable 
of  very  varied  adjustments.  The  eye  is,  in  other  words,  an  optical  instrument, 
and  its  description,  like  that  of  all  optical  instruments,  includes  a  consideration 
of  its  mechanical  adjustments  and  of  its  refracting  media. 

Mechanical  Movements. — The  first  point  to  be  observed  in  studying  the 
movements  of  the  eye  is  that  they  are  essentially  those  of  a  ball-and-socket 
joint,  the  globe  of  the  eye  revolving  freely  in  the  socket  formed  by  the  capsule 
of  Tenon  through  a  horizontal  angle  of  almost  88°  and  a  vertical  angle  of  about 
80°.  The  centre  of  rotation  of  the  eye  (which  is  not,  however,  an  absolutely 
fixed  point)  does  not  coincide  with  the  centre  of  the  eyeball,  but  lies  a  little 
behind  it.  It  is  rather  farther  forward  in  hypermetropic  than  in  myopic  eyes. 
The  movements  of  the  eye,  especially  those  in  a  horizontal  direction,  are  sup- 
plemented by  the  movements  of  the  head  upon  the  shoulders.  The  combined 
eye  and  head  movements  are  in  most  persons  sufficiently  extensive  to  enable 
the  individual,  without  any  movement  of  the  body,  to  receive  upon  the  lateral 
portion  of  the  retina  the  image  of  an  object  directly  behind  his  back.  The 
rotation  of  the  eye  in  the  socket  is  of  course  easiest  and  most  extensive  when 
the  eyeball  has  an  approximately  spherical  shape,  as  in  the  normal  or  emme- 
tropic  eye.  When  the  antero-posterior  diameter  is  very  much  longer  than  those 

1  In  certain  of  the  lower  orders  of  animals  no  local  differentiations  seem  to  have  occurred, 
and  the  whole  surface  of  the  body  appears  to  be  obscurely  sensitive  to  light.     See  Nagel :  Der 
Lichtisinn  augenloser  Thiere,  Jena,  1896. 
298 


THE  SENSE    OF    VISION.  299 

at  right  angles  to  it,  as  in  extremely  myopic  or  short-sighted  eyes,  the  rotation 
of  the  eyeball  may  be  considerably  limited  in  its  extent.  In  addition  to  the 
movements  of  rotation  round  a  centre  situated  in  the  axis  of  vision,  the  eye- 
ball may  be  moved  forward  and  backward  in  the  socket  to  the  extent  of  about 
one  millimeter.  This  movement  may  be  observed  whenever  the  eyelids  are 
widely  opened,  and  is  supposed  to  be  effected  by  the  simultaneous  contraction  of 
both  the  oblique  muscles.  A  slight  lateral  movement  has  also  been  described. 

The  movements  of  the  eye  will  be  best  understood  when  considered  as 
referred  to  three  axes  at  right  angles  to  each  other  and  passing  through  the 
centre  of  rotation  of  the  eye.  The  first  of  these  axes,  which  may  be  called 
the  longitudinal  axis,  is  best  described  as  coinciding  with  the  axis  of  vision 
when,  with  head  erect,  we  look  straight  forward  to  the  distant  horizon ;  the 
second,  or  transverse,  axis  is  defined  as  a  line  passing  through  the  centres  of 
rotation  of  the  two  eyes ;  and  the  third,  or  vertical,  axis  is  a  vertical  line  nec- 
essarily perpendicular  to  the  other  two  and  also  passing  through  the  centre  of 
rotation.  When  the  axis  of  vision  coincides  with  the  longitudinal  axis,  the  eye 
is  said  to  be  in  the  primary  position.  When  it  moves  (from  the  primary  posi- 
tion by  revolving  around  either  the  transverse  or  the  vertical  axis,  it  is  said  to 
assume  secondary  positions.  All  other  positions  are  called  tertiary  positions, 
and  are  reached  from  the  primary  position  by  rotation  round  an  axis  which 
lies  in  the  same  plane  as  the  vertical  and  horizontal  axis — i.  e.  in  the  "  equato- 
rial plane "  of  the  eye.  When  the  eye  passes  from  a  secondary  to  a  tertiary 
position,  or  from  one  tertiary  position  to  another,  the  position  assumed  by  the 
eye  is  identical  with  that  which  it  would  have  had  if  it  had  reached  it  from 
the  primary  position  by  rotation  round  an  axis  in  the  equatorial  plane.  In 
other  words,  every  direction  of  the  axis  of  vision  is  associated  with  a  fixed 
position  of  the  whole  eye — a  condition  of  the  greatest  importance  for  the  easy 
and  correct  use  of  the  eyes.  A  rotation  of  the  eye  round  its  antero-posterior 
axis  takes  place  in  connection  with  certain  movements,  but  authorities  differ 
with  regard  to  the  direction  and  amount  of  this  rotation. 

Muscles  of  the  Eye. — The  muscles  of  the  eye  are  six  in  number — viz : 
the  superior,  inferior,  internal  and  external  recti,  and  the  superior  and  inferior 
oblique.  This  apparent  superfluity  of  muscles  (for  four  muscles  would  suffice 
to  turn  the  eye  in  any  desired  direction)  is  probably  of  advant^v  in  reducing 
the  amount  of  muscular  exertion  required  to  put  the  eye  ;^to  any  given  posi- 
tion, and  thus  facilitating  the  recognition  of  slight  differences  of  direction,  for, 
according  to  Fechner's  psycho-physic  law  the  smaller  perceptible  difference  in 
a  sensation  is  proportionate  to  the  total  amount  of  tbe  sensation.  Hence  if  the 
eye  can  be  brought  into  a  given  position  by  a  sliglt  muscular  effort,  a  change 
from  that  position  will  be  more  easily  perceived  thui  if  a  powerful  effort  were 
necessary. 

Each  of  the  eye-muscles,  acting  singly,  tends  t>  rotate  the  eye  round  an  axis 
which  may  be  called  the  axis  of  rotation  of  th:-t  muscle.  Now,  none  of  the 
muscles  have  axes  of  rotation  lying  exactly  ii  the  equator  of  the  eye — i.  e. 
in  a  plane  passing  through  the  centre  of  rota  ion  perpendicular  to  the  axis 


^00 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


ofVision.1  But  all  movements  of  the  eye  from  the  primary  position  take  place, 
as  \  ;e  have  seen,  round  an  axis  lying  in  this  plane.  Hence  all  such  movements 
mu&v\be  produced  by  more  than  one  muscle,  and  this  circumstance  also  is  prob- 
ably o£  advantage  in  estimating  the  extent  and  direction  of  the  movement.  In 
this  connection  it  is  interesting  to  note  that  the  eye-muscles  have  an  exception- 
ally abundant  nerve-supply — a  fact  which  it  is  natural  to  associate  with  their 
power  of  extremely  delicate  adjustment.  It  has  been  found  by  actual  count 
that  in  the  muscles  of  the  human  eye  ea^*  nerve-fibre  supplies  only  two  or  three 
muscle-fibres,  while  in  the  muscles  of  the Climbs  the  ratio  is  as  high  as  1  to 
40-1 25.2 

Although  each  eye  has  its  own  supply  of  muscles  and  nerves,  yet  the  two 
eyes  are  not  independent  of  each  other  in  their  movements.  The  nature  of 
their  connections  with  the  nerve-centres  is  such  that  only  those  movements  are, 
as  a  rule,  possible  in  which  both  axes  of  vision  remain  in  the  same  plane.  This 
condition  being  fulfilled,  the  eyes  may  be  together  directed  to  any  desired  point 
above,  below,  or  at  either  side  of  the  observer.  The  axes  may  also  be  con- 
verged, as  is  indeed  necessary  in  looking  at  near  objects,  and  to  facilitate  this 
convergence  the  internal  recti  muscles  are  inserted  nearer  to  the  cornea  than  the 
other  muscles  of  the  eye.  Though  in  the  ordinary  use  of  the  eyes  there  is  never 
any  occasion  to  diverge  the  axes  of  vision,  yet  most  persons  are  able  to  effect  a 
divergence  of  about  four  degrees,  as  shown  by  their  power  to  overcome  the  ten- 
dency to  double  vision  produced  by  holding  a  prism  in  front  of  one  of  the  eyes. 
The  nervous  mechanism  through  which  this  remarkable  co-ordination  of  the 
muscles  of  the  two  eyes  is  effected,  and  their  motions  limited  to  those  which 
are  useful  in  binocular  vision,  is  not  completely  understood,  but  it  is  supposed 
to  have  its  seat  in  part  in  the  tubercula  quadrigemina,  in  connection  with  the 
nuclei  of  origin  of  the  third,  fourth,  and  sixth  cranial  nerves.  Its  disturbance 
by  disease,  alcoholic  intoxication,  etc.  causes  strabismus,  confusion,  dizziness, 
and  double  vision. 

A  nerve  termination  sensitive  to  light,  and  so  arranged  that  it  can  be  turned  ^ 
in  different  directions,  is  sufficient  to  give  information  of  the  direction  from 
which  the  light  comes,  for  the  contraction  of  the  various  eye-muscles  indicates, 
throngl]  the  nerves  of  muscular  sense,  the  position  into  which  the  eye  is  nor- 
mally brougiitj unorder  to  h@st_receive_Lbe  luminous  rays,  or,  in  other  words, 
the  direction  of  the  luminous  body.  The  eye,  however,  informs  us  not  only  of 
the  direction,  but  of  the  form  of  the  object  from  which  the  light  proceeds ;  and 
to  understand  how  this  is  Affected  it  will  be  necessary  to  consider  the  refracting 
media  of  the  eye  by  meant  of  which  an  optical  image  of  the  luminous  object 
is  thrown  upon  the  expanded  termination  of  the  optic  nerve — viz.  the  retina. 

Dioptric  Apparatus  of  the  Eye. — For  the  better  comprehension  of  this 
portion  of  the  subject  a  few  Definitions  in  elementary  optics  may  be  given.  A 

1  The  axes  of  rotation  of  the  internal  and  external  recti,  however,  deviate  but  slightly  from 
the  equatorial  plane. 

?  P.  Tergast :  "  Ueber  das  Verhainiss  voa  Nerven  and  Muskeln,"  Archiv  fur  mikr.  Anat.. 
ix.  36-46. 


THE   SENSE    OF    VISION. 


301 


dioptric  system  in  its  simplest  form  consists  of  two  adjacent  media  which  have 
different  indices  of  refraction  and  whose  surface  of  separation  is  the  segment 
of  a  sphere.  A  line  joining  the  middle  of  the  segment  with  the  centre  of  the 
sphere  and  prolonged  in  either  direction  is  called  the  axis  of  the  system.  Let 
the  line  APE  in  Figure  125  represent  in  section  such  a  spherical  surface  the 


FIG.  125.— Diagram  of  simple  optical  system  (after  Foster). 

centre  of  which  is  at  N9  the  rarer  medium  being  to  the  left  and  the  denser  me- 
dium to  the  right  of  the  line.  Any  ray  of  light  which,  in  passing  from  the  rarer  t- 
to  the  denser  medium,  is  perpendicular  to  the  spherical  surface  will  be  unchanged 
iii  its  direction — i.  e.  will  undergo  no  refraction.  Such  rays  are  represented  by 
the  lines  0  P,  M  D,  and  Mf  E.  If  a  pencil  of  rays  having  its  ^origin  in  the  rarer 
medium  at  any  point  in  the  axis  falls  upon^the  spherical  surface,  there  will  be 
one  ray — viz.  the  one  which  coincides  with  the  axis^of  the  system,  which  will 
pass  into  the  second  medium  unchanged  in  its  direction.  This  ray  is  called 
the  principal  ray  (OP),  and  its  point  of  intersection  (P)  with  the  spherical 
surface  is  called  the  principal  point.  The  centre  of  the  sphere  (N)  through 
which  the  principal  ray  necessarily. passes  is  called  the  nodal  point.  All  the 
other  rays  in  the  pencil  are  refracted  toward  the  principal  ray  by  an  amount 


FIG.  126.— Diagram  to  show  method  of  finding  principal  foci  (Neumann). 

which  depends,  for  a  given  radius  of  curvature,  upon  the  difference  in  the 
refractive  power  of  the  media,  or,  in  other  words,  upon  the  retardation  of  light 
in  passing  from  one  medium  to  the  other.  If  the  incident  rays  have  their 
origin  at  a  point  infinitely  distant  on  the  axis — i.  e.  if  they  are  parallel  to  each 
ot^cr--they  will  all  be  refracted  to  a  point  behind  the  spherical  surface  known 


302  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

as  the  principal  focus,  F.  There  is  another  principal  focus  (F')  in  front  of  the 
spherical  surface — viz.  the  point  from  which  diverging  incident  rays  will  be 
refracted  into  parallelism  on  passing  the  spherical  surface,  or,  in  other  words, 
the  point  at  which  parallel  rays  coming  from  the  opposite  direction  will  be 
brought  to  a  focus.  The  position  of  these  two  principal  foci  may  be  deter- 
mined by  the  construction  shown  in  Figure  126.  Let  CA  Cf  represent  a  sec- 
tion of  a  spherical  refracting  surface  with  the  axis  A  N,  the  nodal  point  N,  and 
the  principal  point  A.  The  problem  is  to  find  the  foci  of  rays  parallel  to  the 
axis.  Erect  perpendiculars  at  A  and  N.  Set  off  on  each  perpendicular  dis- 
tances No,  Np,  Aof,  Apf  proportionate  to  the  rapidity  of  light  in  the  two  media 
(e.  g.  2  :  3).  The  points  where  the  lines  pf  o  and  p  o'  prolonged  will  cut  the 
axis  are  the  two  principal  foci  F  and  F' — i.  e.  the  points  at  which  parallel  rays 
coming  from  either  direction  are  brought  to  a  focus  after  passing  the  spherical 
refracting  surface.  If  the  rays  are  not  parallel,  but  diverging — i.  e.  coming 
from  an  object  at  a  finite  distance — the  point  where  the  rays  will  be  brought  to 
a  focus,  or,  in  other  words,  the  point  where  the  optical  image  of  the  luminous 
object  will  be  formed,  may  be  determined  by  a  construction  which  combines 
any  two  of  the  three  rays  whose  course  is  given  in  the  manner  above  described. 
Thus  in  Figure  127  let  AN  be  the  axis,  and  F  and  F'  the  principal  foci  of 


°\ 

FIG.  127. — Diagram  to  show  method  of  finding  conjugate  foci. 

the  spherical  refracting  surface  CA  C',  with  a  nodal  point  at  N.  Let  B  be 
the  origin  of  a  pencil  of  rays  the  focus  of  which  is  to  be  determined.  Draw 
the  line  B  C  representing  the  course  of  an  incident  ray  parallel  to  the  axis. 
This  ray  will  necessarily  be  refracted  through  the  focus  F,  its  course  being 
represented  by  the  line  CF  and  its  prolongation.  Similarly,  the  incident  ray 
passing  through  the  focus  Ff  and  striking  the  spherical  surface  at  C'  will,  after 
refraction,  be  parallel  to  the  axis — i.  e.  it  will  have  the  Direction  Cr  b.  The 
principal  ray  of  the  pencil  will  of  course  pass  through  the  spherical  surface  and 
the  nodal  point  N  without  change  of  direction.  These  three  rays  will  come 
together  at  the  same  point  6,  the  position  of  which  may  be  determined  by  con- 
structing the  course  of  any  two  of  the  three.  The  points  B  and  b  are  called 
conjugate  foci,  and  are  related  to  each  other  in  such  a  way  that  an  optical  image 
is  formed  at  one  point  of  a  luminous  object  situated  at  the  other.  When  the 
rays  of  light  pass  through  several  refracting  surfaces  in  succession  their  course 
may  be  determined  by  separate  calculations  for  each  surface,  a  process  which 
is  much  simplified  when  the  surfaces  are  "  centred  " — i.  e.  have  their  centres 
of  curvature  lying  in  the  same  axis,  as  is  approximately  the  case  in  the  eye. 

Refracting-  Media  of  the  Eye. — Rays  of  light  in  passing  through  the  eye 
penetrate  seven  different  media  and  are  refracted  at  seven  surfaces.    The  media 


THE  SENSE    OF    VISION.  303 

are  as  follows :  layer  of  tears,  cornea,  aqueous  humor,  anterior  capsule  of  lens, 
lens,  posterior  capsule  of  lens,  vitreous  humor.  The  surfaces  are  those  which 
separate  the  successive  media  from  each  other  and  that  which  separates  the  tear 
layer  from  the  air.  For  purposes  of  practical  calculation  the  number  of  sur- 
faces and  media  may  be  reduced  to  three.  In  the  first  place,  the  layer  of  tears 
which  moistens  the  surface  of  the  cornea  has  the  same  index  of  refraction  as 
the  aqueous  humor.  Hence  the  index  of  refraction  of  the  cornea  may  be  left 
out  of  account,  since,  having  practically  parallel  surfaces  and  being  bounded 
on  both  sides  by  substances  having  the  same  index  of  refraction,  it  does  not 
influence  the  direction  of  rays  of  light  passing  through  it.  For  this  same 
reason  objects  seen  obliquely  through  a  window  appear  in  their  true  direction, 
the  refraction  of  the  rays  of  light  on  entering  the  glass  being  equal  in  amount 
and  opposite  in  direction  to  that  which  occurs  in  leaving  it.  For  purposes  of 
optical  calculation  we  may,  therefore,  disregard  the  refraction  of  the  cornea 
(which,  moreover,  does  not  differ  materially  from  that  of  the  aqueous  humor), 
and  imagine  the  aqueous  humor  extending  forward  to  the  anterior  surface  of 
the  layer  of  tears  which  bathes  the  corneal  epithelium.  Furthermore,  the  cap- 
sule of  the  lens  has  the  same  index  of  refraction  as  the  outer  layer  of  the  lens 
itself,  and  for  optical  purposes  may  be  regarded  as  replaced  by  it.  Hence 
the  optical  apparatus  of  the  eye  may  be  regarded  as  consisting  of  the  fol- 
lowing three  refracting  media:  Aqueous  humor,  index  of  refraction  1.33; 
HppSj  average  index  of  refraction  1.45 ;  vitreous  humor,  index  of  refraction 
1.33.  The  surfaces  at  which  refraction  occurs  are  also  three  in  number :  An- 
terior  surface  of  cornea?  radius  of  curvature  8  millimeters;  anterior  surface 
.ojfjens.  radius  of  curvature  10  millimeters ;  posterior  surface  of  lens,  radius  of 
curvature  6  millimeters.  It  will  thus  be  seen  that  the  anterior  surface  of  the 
lens  is  less  and  the  posterior  surface  more  convex  than  the  cornea. 

To  the  values  of  the  optical  constants  of  the  eye  as  above  given  may  be 
added  the  following :  Distance  from  the  anterior  surface  of  the  cornea  to  the 
anterior  surface  of  the  lens,  3.6  millimeters ;  distance  from  the  posterior  sur- 
face of  the  lens  to  the  retina,  15.  millimeters ;  thickness  of  lens,  3.6  millimeters. 

The  methods  usually  employed  for  determining  these  constants  are  the  fol- 
lowing :  The  indices  of  refraction  of  the  aqueous  and  vitreous  humor  are 
determined  by  filling  the  space  between  a  glass  lens  and  a  glass  plate  with  the 
fresh  humor.  The  aqueous  or  vitreous  humor  thus  forms  a  convex  or  concave 
lens,  from  the  form  and  focal  distance  of  which  the  index  can  be  calculated. 
Another  method  consists  in  placing  a  thin  layer  of  the  medium  between  the 
hypothenuse  surfaces  of  two  right-angled  prisms  and  determining  the  angle  at 
which  total  internal  reflection  takes  place.  In  the  case  of  the  crystalline  lens 
the  index  is  foi.nd  by  determining  its  focal  distance  as  for  an  ordinary  lens, 
and  solving  the  equation  which  expresses  the  value  of  the  index  in  terms  of 
radius  of  curvature  and  focal  distance,  thickness,  and  focal  length.  The 
refractive  irdex  of  the  lens  increases  from  the  surface  toward  the  centre,  a 
peculiarity  which  tends  to  correct  the  disturbances  due  to  spherical  aberration, 
as  well  as  to  increase  the  refractive  power  of  the  lens  as  a  whole. 


304  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

The  curvature  of  the  refracting  surfaces  of  the  eye  is  determined  by  an 
instrument  known  as  an  ophthalmometer,  which  measures  the  size  of  the 
reflected  image  of  a  known  object  in  the  various  curved  surfaces.  The 
radius  of  curvature  of  the  surface  is  determined  by  the  following  formula  : 


T 

B  :b  =  A  :-•  or  r  =  —75-,  in  which  B  =  the  size  of  the  object,  b  =  the  size  of 
2  B 

the  image,  A  =  distance  between  the  object  and  the  reflecting  surface,  and 
r  =  the  radius  of  the  reflecting  surface.  The  distances  between  the  various 
surfaces  of  the  eye  are  measured  on  frozen  sections  of  the  organ,  or  can  be 
determined  upon  the  living  eye  by  optical  methods  too  complicated  to  be  here 
described.  It  should  be  borne  in  mind  that  the  above  values  of  the  so-called 
"optical  constants"  of  the  eye  are  subject  to  considerable  individual  variation, 
and  that  the  statements  of  authors  concerning  them  are  not  always  consistent. 
The  refracting  surfaces  of  the  eye  may  be  regarded  as  still  further  sim- 
plified, and  a  so-called  "  reduced  eye  "  constructed  which  is  very  useful  for 
purposes  of  optical  calculation.  This  reduced  eye,  which  for  optical  purposes 
is  the  equivalent  of  the  actual  eye,  is  regarded  as  consisting  of  a  single  refract- 
ing medium  having  an  index  of  1.33,  a  radius  of  curvature  of  5.017  milli- 
meters, its  principal  point  2.148  millimeters  behind  the  anterior  surface  of  the 
cornea,  and  its  nodal  point  0.04  millimeter  in  front  of  the  posterior  surface 
of  the  lens.1  The  principal  foci  of  the  reduced  eye  are  respectively  12.918 
millimeters  in  front  of  and  22.231  millimeters  behind  the  anterior  surface  of 
the  cornea.  Its  optical  power  is  equal  to  50.8  dioptrics.2  The  position  of  thi& 
imaginary  refracting  surface  is  indicated  by  the  dotted  line  p  in  Figure  1  28.  The 


FIG.  128.— Diagram  of  th*1  formation  of  a  retinal  image  (after  Foster). 

nodal  point,  n,  in  this  construction  may  be  regarded  as  the  crossing-point  of  all 
the  principal  rays  which  enter  the  eye,  and,  as  these  rays  are  unchanged  in  their 
direction  by  refraction,  it  is  evident  that  the  image  of  the  point  whence  they 
proceed  will  be  formed  at  the  point  where  they  strike  the  retina.  Hence  to 
determine  the  size  and  position  of  the  retinal  image  of  any  external  object— 
e.  g.  the  arrow  in  Figure  128— it  is  only  necessary  to  draw  Iin3s  from  various 

1  Strictly  speaking,  there  are. in  this  imaginary  refracting  apparatus  which  is  regarded  as 
equivalent  to  the  actual  eye  two  principal  and  two  nodal  points,  each  pair  about  0.4  millimeter 
apart.     The  distance  is  so  small  that  the  two  points  may,  for  all  ordinary  constructions,  be 
regarded  as  coincident. 

2  The  optical  power  of  a  lens  is  the  reciprocal  of  its  focal  length.     The  dioptry  or  unit  of 
optical  power  is  the  power  of  a  lans  with  a  focal  length  of  1  meter. 


THE  SENSE    OF    VISION. 


305 


points  of  the  object  through  the  above-mentioned  nodal  point  and  to  prolong 
them  till  they  strike  the  retina.  It  is  evident  that  the  size  of  the  retinal  image 
\vill  be  as  much  smaller  than  that  of  the  object  as  the  distance  of  the  nodal 
point  from  the  retina  is  smaller  than  its  distance  from  the  object. 

According  to  the  figures  above  given,  the  nodal  point  is  about  7.2  milli- 
meters behind  the  anterior  surface  of  the  cornea  and  about  15.0  millimeters  in 
front  of  the  retina.  Hence  the  size  of  the  retinal  image  of  an  object  of  known 
size  and  distance  can  be  readily  calculated — a  problem  which  has  frequently  to  be 
solved  in  the  study  of  physiological  optics.  The  construction  given  in  Figure 
128  shows  that  from  all  external  objects  inverted  images  are  projected  upon  the 
retina,  and  such  inverted  images  can  actually  be  seen  under  favorable  condi- 
tions. If,  for  instance,  the  eye  of  a  white  rabbit,  which  contains  no  choroidal 
pigment,  be  excised  and  held  with  the  cornea  directed  toward  a  window  or 
other  source  of  light,  an  inverted  image  of  the  luminous  object  will  be  seen 
through  the  transparent  sclerotic  in  the  same  way  that  one  sees  an  inverted 
image  of  a  landscape  on  the  ground-glass  plate  of  a  photographic  camera. 
The  question  is  often  asked,  "  Why,  if  the  images  are  inverted  in  the  retina, 
do  we  not  see  objects  upside  down  ?"  The  only  answer  to  such  a  question  is 
that  it  is  precisely  because  images  are  inverted  on  the  retina  that  we  do  not  see 
objects  upside  down,  for  we  have  learned  through  lifelong  practice  to  asso- 
ciate an  impression  made  upon  any  portion  of  the  retina  with  light  coming 
from  the  opposite  portion  of  the  field  of  vision.  Thus  if  an  image  falls  upon 
the  lower  portion  of  the  retina,  our  experience,  gained  chiefly  through  mus- 
cular movements  and  tactile  sensations,  has  taught  us  that  this  image  must  cor- 
respond to  an  object  in  the  upper  portion  of  our  field  of  vision.  In  whatever 
way  the  retina  is  stimulated  the  same  effect  is  produced.  If,  for  instance,^ 
gentle  pressure  is  made  with  the  finger  on  the  lateral  portion  of  the  eyeball 
through  the  closed  lids  a  circle  of  light  known  as  a  phosphene  immediately 
appears  on  the  opposite  side  of  the  eye.  Another  good  illustration  of  the 
same  general  rule  is  found  in  the  effect  of  throwing  a  shadow  upon  the  retina 
from  an  object  as  close  as  possible  to  the  eye.  For  this  purpose  place  a  card 


B  p 

FIG.  129.— Diagram  illustrating  the  projection  of  a  shadow  on  the  retina. 

with  a  small  pin-hole  in  it  in  front  of  a  source  of  light,  and  three  or  four 
centimeters  distant  from  the  eye — i.  e.  within  the  near  point  of  distinct 
vision.  Then  hold  some  object  smaller  than  the  pupil — e.g.  the  head 
of  a  pin — as  close  as  possible  to  the  cornea.  Under  these  conditions 
neither  the  pin-hole  nor  the  pin-head  can  be  really  seen — i.  e.  they  are 

VOL.  II.— 20 


306  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

both  too  near  to  have  their  images  focussed  upon  the  retina.  The  pin-hole 
becomes  itself  a  source  of  light,  and  appears  as  a  luminous  circle  bounded  by 
the  shadow  thrown  by  the  edge  of  the  iris.  Within  this  circle  of  light  is  seen 
the  shadow  of  the  pin-head,  but  the  pin-head  appears  inverted,  for  the  obvious 
reason  that  the  eye,  being  accustomed  to  interpret  all  retinal  impressions  as 
corresponding  to  objects  in  the  opposite  portion  of  the  field  of  vision,  regards 
the  upright  shadow  of  the  pin-head  as  the  representation  of  an  inverted  object. 
The  course  of  the  rays  in  this  experiment  is  shown  in  Figure  129,  in  which 
A  B  represents  the  card  with  a  pin-hole  in  it,  P  the  pin,  and  P'  its  upright 
shadow  thrown  ,pn  the  retina. 

Accommodation. — From  what  has  been  said  of  conjugate  foci  and  their 
relation  to  each  other  it  is  evident  that  any  change  in  the  distance  of  the  object 
from  the  refracting  media  will  involve  a  corresponding  change  in  the  position 
of  the  image,  or,  in  other  words,  only  objects  at  a  given  distance  can  be 
focussed  upon  a  plane  which  has  a  fixed  position  with  regard  to  the  refracting 
surface  or  surfaces.  Hence  all  optical  instruments  in  which  the  principle  of 
conjugate  foci  finds  its  application  have  adjustments  for  distance.  In  the 
telescope  and  opera-glass  the  adjustment  is  eifected  by  changes  in  the  distance 
between  the  lenses,  and  in  the  photographic  camera  by  a  change  in  the  posi- 
tion of  the  ground-glass  plate  representing  the  focal  plane.  In  the  microscope 
the  adjustment  is  effected  by  changing  the  distance  of  the  object  to  suit  the 
lenses,  the  higher  powers  having  a  shorter  "  working  distance." 

We  must  now  consider  in  what  way  the  eye  adapts  itself  to  see  objects  dis- 
tinctly at  different  distances.  That  this  power  of  adaptation,  or  "  accommo- 
dation," really  exists  we  can  easily  convince  ourselves  by  looking  at  different 
objects  through  a  network  of  fine  wire  held  near  the  eyes.  When  with  normal 
vision  the  eyes  are  directed  to  the  distant  objects  the  network  nearly  disappears, 
and  if  we  attempt  to  see  the  network  distinctly  the  outlines  of  the  distant 
objects  become  obscure.  In  other  words,  it  is  impossible  to  see  both  the 
network  and  the  distant  objects  distinctly  at  the  same  time.  It  is  also  evident 
that  in  accommodation  for  distant  objects  the  eyes  are  at  rest,  for  when  they 
are  suddenly  opened  after  having  been  closed  for  a  short  time  they  are  found 
to  be  accommodated  for  distant  objects,  and  we  are  conscious  of  a  distinct 
effort  in  directing  them-  to  any  near  object.1 

From  the  optical  principles  above  described  it  is  clear  that  the  accommo- 
dation of  the  eye  for  near  objects  may  be  conceived  of  as  taking  place  in  three 
different  ways :  1st,  By  an  increase  of  the  distance  between  the  refracting  sur- 
faces of  the  eye  and  the  retina ;  2d,  By  an  increase  of  the  index  of  refraction 
of  one  or  more  of  the  media ;  3d,  By  a  diminution  of  the  radius  of  curvature 
of  one  or  more  of  the  surfaces/  Thejirst  of  these  methods  was  formerly  sup- 
posed to  be  the  one  actually  in  use,  a  lengthening  of  the  eyeball  under  a  pres- 

1  It  has  been  shown  by  Beer  (Archivfilr  die  gesammte  Physiolngie,  Iviii.  523)  that  in  fishes 
the  eyes  when  at  rest  are  accommodated  for  near  objects,  and  that  accommodation  for  distant 
objects  is  effected  by  the  contraction  of  a  muscle  for  which  the  name  "retractor  lentis"  is  pro- 
posed. 


THE  SENSE    OF    VISION.  307 

sure  produced  by  the  eye-muscles  being  assumed  to  occur.  This  lengthening 
would,  in  the  case  of  a  normal  eye  accommodating  itself  for  an  object  at  a 
distance  of  15  centimeters,  amount  to  not  less  than  2  millimeters — a  change 
which  could  hardly  be  brought  about  by  the  action  of  any  muscles  connected 
with  the  eye.  Moreover,  accommodation  changes  can  be  observed  upon  elec- 
trical stimulation  of  the  excised  eye.  Its  mechanism  must,  therefore,  lie  within 
the  eye  itself.  As  for  the  second  of  these  methods,  there  is  no  conceivable  way 
by  which  a  change  in  the  index  of  refraction  of  the  media  can  be  effected ,  and 
we  are  thus  forced  to  the  conclusion  that  accommodation  is  brought  about  by 
a  change  in  the  curvature  of  the  refracting  surfaces — i.  e.  by  a  method  quite 
different  from  any  which  is  employed  in  optical  instruments  of  human  con- 
struction. Now,  by  measuring  the  curvature  of  the  cornea  of  a  person  who*. 
looks  alternately  at  near  and  distant  objects  it  has  been  shown  that  the  cornea 
undergoes  no  change  of  form  in  the  act  of  accommodation.  By  a  process  of 
exclusion,  therefore,  the  lens  is  indicated  as  the  essential  organ  in  this  function 
of  the  eye,  and,  in  fact,  the  complicated  structure  and  connections  of  the  lens 
at  once  suggest  the  thought  that  it  is  in  the  surfaces  of  this  portion  of  the  eye 
that  the  necessary  changes  take  place.  Indeed,  from  a  teleological  point  of 
view  the  lens  would  seem  somewhat  superfluous  if  it  were  not  important  to  ^ 
have  a  transparent  refracting  body  of  variable  form  in  the  eye,  for  the  amount 
of  refraction  which  takes  place  in  the  lens  could  be  produced  by  a  slightly 
increased  curvature  of  the  cornea.  Now,  the  changes  of  curvature  which  occur  • 
in  the  surfaces  of  the  lens  when  the  eye  is  directed  to  distant  and  near  objects 
alternately  can  be  actually  observed  and  measured  with  considerable  accuracy. 
For  this  purpose  the  changes  in  the  form,  size,  and  position  of  the  images  of 
brilliant  objects  reflected  in  these  two  surfaces  are  studied.  If  a  candle  is  held 
in  a  dark  room  on  a  level  with  and  about  50  centimeters  away  from  the  eye  in 
which  the  accommodation  is  to  be  studied,  an  observer,  so  placed  that  his  own 
axis  of  vision  makes  about  the  same  angle  (15°-20°)  with  that  of  the  ob- 
served eye  that  is  made  by  a  line  joining  the  observed  eye  and  the  candle,  will 
readily  see  a  small  upright  image  of  the  candle  reflected  in  the  cornea  of  the 
observed  eye.  Near  this  and  within  the  outline  of  the  pupil  are  two  other 
images  of  the  candle,  which,  though  much  less  easily  seen  than  the  corneal 
image,  can  usually  be  made  out  by  a  proper  adjustment  of  the  light.  The 
first  of  these  is  a  large  faint  upright  image  reflected  from  the  anterior  surface 
of  the  lens,  and  the  second  is  a  small  inverted  image  reflected  from  the  pos- 
terior surface  of  the  lens.  It  will  be  observed  that  the  size  of  these  images 
varies  with  the  radius  of  curvature  of  the  three  reflecting  surfaces  as  given  on 
p.  303.  The  relative  size  and  position  of  these  images  having  been  recog- 
nized while  the  eye  is  at  rest — i.  e.  is  accommodated  for  distance — let  the 
person  who  is  under  observation  be  now  requested  to  direct  his  eye  to  a  near 
object  lying  in  the  same  direction.  When  this  is  done  the  corneal  image  and 
that  reflected  from  the  posterior  surface  of  the  lens  will  remain  unchanged,1 

1  A  very  slight  diminution  in  size  may  sometimes  be  observed  in  the  image  reflected  from 
the  posterior  surface  of  the  lens. 


308 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


while  that  reflected  from  the  anterior  surface  of  the  lens  will  become  smaller 
and  move  toward  the  corneal  image.  This  change  in  the  size  and  position  of 
the  reflected  image  can  only  mean  that  the  surface  from  which  the  reflection 
takes  place  has  become  more  convex  and  has  moved  forward.  Coincident 
with  this  change  a  contraction  of  the  pupil  will  be  observed. 

An  apparatus  for  making  observations  of  this  sort  is  known  as  the  phako- 
scope  of  Helmholtz  (Fig.  130).  The  eye  in  which  the  changes  due  to  accom- 
modation are  to  be  observed  is  placed  at  an  opening 
in  the  back  of  the  instrument  at  C,  and  directed  al- 
ternately to  a  needle  placed  in  the  opening  D  and 
to  a  distant  object  lying  in  the  same  direction.  Two 
prisms  at  B  and  Br  serve  to  throw  the  light  of  a 
candle  on  to  the  observed  eye,  and  the  eye  of  an 
observer  at  A  sees  the  three  reflected  images,  each 
as  two  small  square  spots  of  light.  The  movement 
and  the  change  of  size  of  the  image  reflected  from 
the  anterior  surface  of  the  lens  can  be  thus  much 
better  observed  than  when  a  candle-flame  is  used. 
The*  course  of  the  rays  of  light  in  this  experi- 
ment is  shown  diagrammatically  in  Figure  131. 
The  observed  eye  is  directed  to  the  point  A,  while 
the  candle  and  the  eye  of  the  observer  are  placed 
symmetrically  on  either  side.  The  images  of  the  candle  reflected  from  the  various 
surfaces  of  the  eye  will  be  seen  projected  on  the  dark  background  of  the  pupil 


FIG.   130.— Phakoscope  of 
Helmholtz. 


FIG.  131.— Diagram  explaining  the  change  in  the  position  of  the  image  reflected  from  the  anterior  surface 
of  the  crystalline  lens  (Williams,  after  Bonders). 

in  the  directions  indicated  by  the  dotted  lines  ending  at  a,  6,  and  c.  When  the 
eye  is  accommodated  for  a  near  object  the  middle  one  of  the  three  images  moves 
nearer  the  corneal  image — i.  e.  it  changes  in  its  direction  from  b  to  6',  showing 
that  the  anterior  surface  of  the  lens  has  bulged  forward  into  the  position  indi- 


THE  SENSE    OF    VISION. 


309 


cated  by  the  dotted  line.  The  change  in  the  appearance  of  the  images  is 
represented  diagrammatically  in  Figure  132.  On  the  left  is  shown  the  appear- 
ance of  the  images  as  seen  when  the  eye  is  at  rest,  a  representing  the  corneal 
image,  b  that  reflected  from  the  anterior,  and  c  that  from  the  posterior  surface 
of  the  lens  when  the  observing  eye  and  the  candle  are  in  the  position  repre- 


FIG.  132. — Reflected  images  of  a  candle-flame  as  seen  in  the  pupil  of  an  eye  at  rest  and  accommodated 

for  near  objects  (Williams). 

sented  in  Figure  131.  The  images  are  represented  as  they  appear  in  the  dark 
background  of  the  pupil,  though  of  course  the  corneal  image  may,  in  certain 
positions  of  the  light,  appear  outside  of  the  pupillary  region.  When  the  eye 
is  accommodated  for  near  objects  the  images  appear  as  shown  in  the  circle  on 
the  right,  the  image  6  becoming  smaller  and  brighter  and  moving  toward  the 
corneal  image,  while  the  pupil  contracts  as  indicated  by  the  circle  drawn  round 
the  images. 

The  changes  produced  in  the  eye  by  an  effort  of  accommodation  are  indi- 
cated in  Figure  133,  the  left-hand  side  of  the  diagram  showing  the  condition 


FIG.  133.— Showing  changes  in  the  eye  produced  by  the  act  of  accommodation  (Helmholtz). 

of  the  eye  at  rest,  and  the  right-hand  side  that  in  extreme  accommodation  for 
near  objects. 

It  will  be  observed  that  the  iris  is  pushed  forward  by  the  bulging  lens  and 
that  its  free  border  approaches  the  median  line.  In  other  words,  the  pupil  is 
contracted  in  accommodation  for  near  objects.  The  following  explanation  of 
the  mechanism  by  which  this  change  in  the  shape  of  the  lens  is  effected  has 
been  proposed  by  Helmholtz,  and  is  still  generally  accepted.  The  structure 
of  the  lens  is  such  that  by  its  own  elasticity  it  tends  constantly  to  assume  a 
more  convex  form  than  the  pressure  of  the  capsule  and  the  tension  of  the  sus- 
pensory ligaments  (s,  «,  Fig.  133)  allow.  This  pressure  and  tension  are  dimin- 
ished when  the  eye  is  accommodated  for  near  vision  by  the  contraction  of  the 
ciliary  muscles  (c,  c,  Fig.  133),  most  of  whose  fibres,  having  their  origin  at  the 


310  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

point  of  union  of  the  cornea  and  sclerotic,  extend  radially  outward  in  every 
direction  and  are  attached  to  the  front  part  of  the  choroid.  The  contrac- 
tion of  the  ciliary  muscle,  drawing  forward  the  membranes  of  the  eye,  will 
relax  the  tension  of  the  suspensory  ligament  and  allow  the  lens  to  take 
the  form  determined  by  its  own  elastic  structure.  According  to  another 
theory  of  accommodation  proposed  by  Tscherning,1  the  suspensory  liga- 
ment is  stretched  and  not  relaxed  by  the  contraction  of  the  ciliary  muscle. 
In  consequence  of  the  pressure  thus  produced  upon  the 
lens,  the  soft  external  portions  are  moulded  upon  the 
harder  nuclear  portion  in  such  a  way  as  to  give  to  the 
anterior  (and  to  some  extent  to  the  posterior)  surface  a 
hyperboloid  instead  of  a  spherical  form.  A  similar  theory 
has  been  recently  brought  forward  by  Schoen,*  who  com- 
pares the  action  of  the  ciliary  muscle  upon  the  lens  to  that 
of  the  fingers  compressing  a  rubber  ball,  as  shown  in  Fig- 
ure 134.  These  theories  have  an  advantage  over  that 
oifered  by  Helmholtz,  inasmuch  as  they  afford  a  better 
explanation  of  the  presence  of  circular  fibres  in  the  ciliary 
muscle.  They  also  make  the  fact  of  so-called  "astig- 
matic accommodation"  comprehensible.  This  term  is 
FIG.  134— TO  illustrate  applied  to  the  power  said  to  be  sometimes  gradually 

Schoen's  theory  of  ac-  -,   -,  .,,          ,.  ,.     •>  c 

commodation.  acquired  by  persons  with  astigmatic 3  eyes  of  correcting 

this  defect  of  vision  by  accommodating  the  eye  more 
strongly  in  one  meridian  than  another.  The  theory  of  Tscherning  is  sup- 
ported by  Crzellitzer4  as  the  result  of  investigations  into  the  hyperboloid 
form  of  the  lens  in  accommodation.  On  the  other  hand,  it  is  maintained  by 
Priestley  Smith5  that  this  form  of  the  lens  is  not  inconsistent  with  the  Helm- 
holtz theory.  Moreover,  it  has  been  shown  by  Hess 6  and  Heine 7  that  in 
extreme  accommodation  the  lens  drops  slightly  toward  the  lower  part  of  the 
eye,  a  movement  which  seems  to  indicate  a  relaxation  of  the  suspensory  liga- 
ment. The  weight  of  evidence  seems,  therefore,  on  the  whole,  to  be  on  the 
side  of  the  theory  of  Helmholtz. 

Whatever  views  may  be  entertained  as  to  the  exact  mechanism  by  which  its 
change  of  shape  is  brought  about,  there  can  be  no  doubt  that  the  lens  is  the 
portion  of  the  eye  chiefly  or  wholly  concerned  in  accommodation,  and  it  is 
accordingly  found  that  the  removal  of  the  lens  in  the  operation  for  cataract 
destroys  the  power  of  accommodation,  and  the  patient  is  compelled  to  use 
convex  lenses  for  distant  and  still  stronger  ones  for  near  objects. 

It  is  interesting  to  notice  that  the  act  of  accommodation,  though  distinctly 
voluntary,  is  performed  by  the  agency  of  the  unstriped  fibres  of  the  ciliary 
muscles.  It  is  evident,  therefore,  that  the  term  "  involuntary "  sometimes 

1  Archives  de  Physiologic,  1894,  p.  40.  2  Archivfilr  die  gesammte  Physiologic,  lix.  427. 

3  See  p.  317.  *  Archivfilr  Ophthcdmologie,  xlii.  (4)  S.  36. 

5  Ophthalmic  Review,  xvii.  p.  341. 

6  Archivfilr  Ophthalmologie,  xlii.  S.  288,  and  xliii.  S.  477. 

7  Ibid.,  xliv.  (2)  S.  299,  and  xlvii.  (2)  S.  662. 


THE  SENSE    OF    VISION.  311 

applied  to  muscular  fibres  of  this  sort  may  be  misleading.  The  voluntary 
character  of  the  act  of  accommodation  is  not  affected  by  the  circumstance  that 
the  will  needs,  as  a  rule,  to  be  assisted  by  visual  sensations.  The  fact  that 
most  persons  cannot  affect  the  necessary  change  in  the  eye  unless  they  direct 
their  attention  to  some  near  or  far  object  is  only  an  instance  of  the  close  rela- 
tion between  sensory  impressions  and  motor  impulses,  which  is  further  exem- 
plified by  such  phenomena  as  the  paralysis  of  the  lip  of  a  horse  caused  by 
division  of  the  fifth  nerve.  It  is  found,  moreover,  that  by  practice  the 
power  of  accommodating  the  eye  without  directing  it  to  near  and  distant 
objects  can  be  acquired.  The  nerve-channels  through  which  accommodation 
is  affected  are  the  anterior  part  of  the  nucleus  of  the  third  pair  of  nerves 
lying  in  the  extreme  hind  part  of  the  floor  of  the  third  ventricle,  the  most 
anterior  bundle  of  the  nerve-root,  the  third  nerve  itself,  the  lenticular  ganglion, 
and  the  short  ciliary  nerves  (see  diagram  p.  323). 

The  mechanism  of  accommodation  is  affected  in  a  remarkable  way  by  drugs, 
the  most  important  of  which  are  atropia  and  physostigmin,  the  former  para- 
lyzing and  the  latter  stimulating  the  ciliary  muscle.  As  these  drugs  exert  a 
corresponding  effect  upon  the  iris,  it  will  be  convenient  to  discuss  their  action 
in  connection  with  the  physiology  of  that  organ. 

The  changes  occurring  in  the  eye  during  the  act  of  accommodation  are 
indicated  in  the  following  table,  which  shows,  both  for  the  actual  and  the 
reduced  eye,  the  extent  to  which  the  refracting  media  change  their  form  and 
position,  and  the  consequent  changes  in  the  position  of  the  foci  : 

Accommodation  for 
Actual  Eye.  distant  objects.          near  objects. 

Kadius  of  cornea 8  mm.  8  mm. 

Radius  of  anterior  surface  of  lens 10  "  6 

Radius  of  posterior  surface  of  lens 6  5.5  " 

Distance  from  cornea  to  anterior  surface  of  lens    .    .    3.6        "  3.2  " 

Distance  from  cornea  to  posterior  surface  of  lens      .    7.2        "  7.2  " 


Reduced  Eye. 

Radius  of  curvature 5.02 

Distance  from  cornea  to  principal  point 2.15 

Distance  from  cornea  to  nodal  point 7.16 

Distance  from  cornea  to  anterior  focus 12.918 


4.48 

2.26 

6.74 

11.241 

20.248 


Distance  from  cornea  to  posterior  focus 22.231 

It  will  be  noticed  that  no  change  occurs  in  the  curvature  of  the  cornea,  and 
next  to  none  in  the  posterior  surface  of  the  lens,  while  the  anterior  surface  of 
the  lens  undergoes  material  alterations  both  in  its  shape  and  position. 

Associated  with  the  accommodative  movements  above  described,  two  other 
changes  take  place  in  the  eyes  to  adapt  them  for  near  vision.  In  the  first 
place,  the  axes  of  the  eyes  are  converged  upon  the  near  object,  so  that  the 
images  formed  in  the  two  eyes  shall  fall  upon  corresponding  points  of  the 
retinas,  as  will  be  more  fully  explained  in  connection  with  the  subject  of 
binocular  vision.  In  the  second  place,  the  pupil  becomes  contracted,  thus 
reducing  the  size  of  the  pencil  of  rays  that  enters  the  eye.  The  importance 
of  this  movement  of  the  pupil  will  be  better  understood  after  the  subject  of 


312  ^1^  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

spherical  aberration  of  light  has  been  explained.  These  three  adjustments, 
focal,  axial,  and  pupillary,  are  so  habitually  associated  in  looking  at  near  objects 
that  the  axial  can  only  by  an  effort  be  dissociated  from  the  other  two,  while 
these  two  are  quite  inseparable  from  one  another.  This  may  be  illustrated 
by  a  simple  experiment.  On  a  sheet  of  paper  about  40  centimeters  distant 
from  the  eyes  draw  two  letters  or  figures  precisely  alike  and  about  3  centimeters 
apart.  (Two  letters  cut  from  a  newspaper  and  fastened  to  the  sheet  will  answer 
the  same  purpose.)  Hold  a  small  object  like  the  head  of  a  pin  between  the 
eyes  and  the  paper  at  the  point  of  intersection  of  a  line  joining  the  right  eye 
and  the  left  letter  with  a  line  joining  the  left  eye  and  the  right  letter.  If  the 
axes  of  vision  are  converged  upon  the  pin-head,  that  object  will  be  seen  dis- 
tinctly, and  beyond  it  will  be  seen  indistinctly  three  images  of  the  letter,  the 
central  one  being  formed  by  the  blending  of  the  inner  one  of  each  pair  of 
images  formed  on  the  two  retinas.  If  now  the  attention  be  directed  to  the 
middle  image,  it  will  gradually  become  perfectly  distinct  as  the  eye  accommo- 
dates itself  for  that  distance.  We  have  thus  an  axial  adjustment  for  a  very 
near  object  and  a  focal  adjustment  for  a  more  distant  one.  If  the  pupil  of  the 
individual  making  this  observation  be  watched  by  another  person,  it  will  b£ 
found  that  at  the  moment  when  the  middle  image  of  the  letter  becomes  distin^r 
the  pupil,  which  had  been  contracted  in  viewing  the  pin-head,  suddenly  dilates. 
It  is  thus  seen  that  when  the  axial  and  focal  adjustments  are  dissociated  from 
each  other  the  pupillary  adjustment  allies  itself  with  the  latter. 

The  opposite  form  of  dissociation — viz.  the  axial  adjustment  for  distance 
and  the  focal  adjustment  for  near  vision — is  less  easy  to  bring  about.  It  may 
perhaps  be  best  accomplished  by  holding  a  pair  of  stereoscopic  pictures  before 
the  eyes  and  endeavoring  to  direct  the  right  eye  to  the  right  and  the  left  eye  to 
the  left  picture — i.  e.  to  keep  the  axes  of  vision  parallel  while  the  eyes  are 
accommodated  for  near  objects.  One  who  is  successful  in  this  species  of  ocular 
gymnastics  sees  the  two  pictures  blend  into  one  having  all  the  appearance  of 
a  solid  object.  The  power  of  thus  studying  stereoscopic  pictures  without  a 
stereoscope  is  often  a  great  convenience  to  the  possessor,  but  individuals  differ 
very  much  in  their  ability  to  acquire  it. 

Range  of  Accommodation. — By  means  of  the  mechanism  above  described 
it  is  possible  for  the  eye  to  produce  a  distinct  image  upon  the  retina  of  objects 
lying  at  various  distances  from  the  cornea.  The  point  farthest  from  the  eye 
at  which  an  object  can  be  distinctly  seen  is  called  the  far-point,  and  the  nearest 
point  of  distinct  vision  is  called  the  near-point  of  the  eye,  and  the  distance 
between  the  near-point  and  the  far-point  is  called  the  range  of  distinct  vision 
or  the  range  of  accommodation.  As  the  normal  emmetropic  eye  is  adapted, 
when  at  rest,  to  bring  parallel  rays  of  light  to  a  focus  upon  the  retina,  its  far- 
point  may  be  regarded  as  at  an  infinite  distance.  Its  near-point  varies  with  age, 
as  will  be  described  under  Presbyopia.  In  early  adult  life  it  is  from  10  to 
13  centimeters  from  the  eye.  For  every  point  within  this  range  there  will  be 
theoretically  a  corresponding  condition  of  the  lens  adapted  to  bring  rays  pro- 
ceeding from  that  point  to  a  focus  on  the  retina,  but  as  rays  reaching  the  eye 
from  a  point  175  to  200  centimeters  distant  do  not,  owing  to  the  small  size  of 


THE   SENSE    OF    VISION. 


313 


the  pupil,  differ  sensibly  from  parallel  rays,  there  is  no  appreciable  change  in 
the  lens  unless  the  object  looked  at  lies  within  that  distance.  It  is  also  evi- 
dent that  as  an  object  approaches  the  eye  a  given  change  of  distance  will 
cause  a  constantly  increasing  amount  of  divergence  of  the  rays  proceeding  from 
it,  and  will  therefore  necessitate  a  constantly  increasing  amount  of  change  in 
the  lens  to  enable  it  to  focus  the  rays  on  the  retina.  We  find,  accordingly,  that 
all  objects  more  than  two  meters  distant  from  the  eye  can  be  seen  distinctly  at 
the  same  time — i.  e.  without  any  change  in  the  accommodative  mechanism — 
but  for  objects  within  that  distance  we  are  conscious  of  a  special  eifort  of 
accommodation  which  becomes  more  and  more  distinct  the  shorter  the  distance 
between  the  eye  and  the  object. 

Myopia  and  Hypermetropia. — There  are  two  conditions  of  the  eye  in 
which  the  range  of  accommodation  may  differ  from  that  which  has  just  been 
described  as  normal.  These  conditions,  which  are  too  frequent  to  be  regarded 
(except  in  extreme  cases)  as  pathological,  are  generally  dependent  upon  the 
eyeball  being  unduly  lengthened  or 
shortened.  In  Fig.  135  are  shown 
diagram matically  the  three  conditions 
known  as  emmetropia,  myopia,  and 
hypermetropia.  In  the  normal  or 
emmetropic  eye,  A,  parallel  rays  are 
represented  as  brought  to  a  focus  on 
the  retina;  in  the  short-sighted,  or 
myopic,  eye,  B,  similar  rays  are 
focussed  in  front  of  the  retina,  since 
the  latter  is  abnormally  distant;  while 
in  the  over-sighted,  or  hypermetropic, 
eye,  C,  they  are  focussed  behind  the 
retina,  since  it  is  abnormally  near. 

It  is  evident  that  when  the  eye  is 
at  rest  both  the  myopic  and  the  hy- 
permetropic eye  will  see  distant  ob- 
jects indistinctly,  but  there  is  this 
important  difference :  that  in  hyper- 
metropia the  difficulty  can  be  cor- 
rected by  an  effort  of  accommodation, 
while  in  myopia  this  is  impossible, 
since  there  is  no  mechanism  by  which 
the  radius  of  the  lenticular  surfaces  can  be  increased.  Hence  an  individual 
affected  with  myopia  is  always  aware  of  the  infirmity,  while  a  person  with 
hypermetropic  eyes  often  goes  through  life  unconscious  of  the  defect.  In  this 
case  the  accomodation  is  constantly  called  into  play  even  for  distant  objects,  and 
if  the  hypermetropia  is  excessive,  any  prolonged  use  of  the  eyes  is  apt  to  be 
attended  by  a  feeling  of  fatigue,  headache,  and  a  train  of  nervous  symptoms 
familiar  to  the  ophthalmic  surgeon.  Hence  it  is  important  to  discover  this  defect 
where  it  exists  and  to  apply  the  appropriate  remedy — viz.  convex  lenses  placed 


FIG.  135.— Diagram  showing  the  difference  betweer 
normal,  myopic,  and  hypermetropic  eyes. 


314  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

in  front  of  the  eyes  in  order  to  make  the  rays  slightly  convergent  when  they 
enter  the  eye.  Thus  aided,  the  refractive  power  of  the  eye  at  rest  is  sufficient 
to  bring  the  rays  to  a  focus  upon  the  retina  and  thus  relieve  the  accommoda- 
tion. This  action  of  a  convex  lens  in  hypermetropia  is  indicated  by  the  dotted 
lines  in  Fig.  135,  (?,  and  the  corresponding  use  of  a  concave  lens  in  myopia  is 
shown  in  Fig.  135,  B. 

The  detection  and  quantitative  determination  of  hypermetropia  are  best 
made  after  the  accommodation  has  been  paralyzed  by  the  use  of  atropia,  by 
ascertaining  how  strong  a  convex  lens  must  be  placed  before  the  eye  to  pro- 
duce distinct  vision  of  distant  objects. 

The  range  of  accommodation  varies  very  much  from  the  normal  in  myopic 
and  hypermetropic  eyes.  In  myopia  the  near-point  is  often  5  or  6  centimeters 
from  the  cornea,  while  the  far-point,  instead  of  being  infinitely  far  off,  is  at  a 
variable  but  no  very  great  distance  from  the  eye.  The  range  of  accommoda- 
tion is  therefore  very  limited.  In  hypermetropia  the  near-point  is  slightly 
farther  than  normal  from  the  eye,  and  the  far-point  cannot  be  said  to  exist, 
for  the  eye  at  rest  is  adapted  to  bring  converging  rays  to  a  focus  on  the  retina, 
and  such  pencils  of  rays  do  not  exist  in  nature.  Mathematically,  the  far-point 
may  be  said  to  be  at  more  than  an  infinite  distance  from  the  eye.  The  range 
of  effective  accommodation  is  therefore  reduced,  for  a  portion  of  the  accommo- 
dative power  is  used  up  in  adapting  the  eye  to  receive  parallel  rays. 

Presbyopia. — The  power  of  accommodation  diminishes  with  age,  owing 
apparently  to  a  loss  of  elasticity  of  the  lens.  The  change  is  regularly  pro- 
gressive, and  can  be  detected  as  early  as  the  fifteenth  year,  though  in  normal 
eyes  it  does  not  usually  attract  attention  until  the  individual  is  between  forty 
and  forty-five  years  of  age.  At  this  period  of  life  a  difficulty  is  commonly 
experienced  in  reading  ordinary  type  held  at  a  convenient  distance  from  the 
eye,  and  the  individual  becomes  old-sighted  or  presbyopic — a  condition  which 
can,  of  course,  be  remedied  by  the  use  of  convex  glasses.  According  to 
Helmholtz,  the  far-point  also  recedes  somewhat  after  fifty  years  of  age. 
Hence  emmetropic  eyes  may  become  hypermetropic  and  slightly  myopic 
eyes  emmetropic.  Cases  are  occasionally  reported  of  persons  recovering 
their  power  of  near  vision  in  extreme  old  age  and  discontinuing  the  use  of 
the  glasses  previously  employed  for  reading.  In  these  cases  there  is  apparently 
not  a  restoration  of  the  power  of  accommodation,  but  an  increase  in  the  refrac- 
tive power  of  the  lens  through  local  changes  in  its  tissue.  A  diminution  in 
the  size  of  the  pupil,  sometimes  noticed  in  old  age,1  may  also  contribute  to 
the  distinctness  of  the  retinal  image,  as  will  be  described  in  connection  with 
spherical  aberration. 

Defects  of  the  Dioptric  Apparatus. — The  above-described  imperfections 
of  the  eye — viz.  myopia  and  hypermetropia — being  generally  (though  not 
invariably)  due  to  an  abnormal  length  of  the  longitudinal  axis,  are  to  be 
regarded  as  defects  of  construction  affecting  only  a  comparatively  small 

1  The  average  diameter  of  the  pupil  is  said  to  be  in  youth  4.1  mm.  and  in  old  age  3  mm. 
Silberkuhl:  Archivfiir  Ophthalmologie,  xlii.  (3)  S.  179.' 


THE  SENSE    OF    VISION. 


315 


number  of  eyes.  There  are,  however,  a  number  of  imperfections  of  the  diop- 
tric apparatus,  many  of  which  affect  all  eyes  alike.  Of  these  imperfections 
some  affect  the  eye  in  common  with  all  optical  instruments,  while  others  are 
peculiar  to  the  eye  and  are  not  found  in  instruments  of  human  construction. 
The  former  class  will  be  first  considered. 

Spherical  Aberration. — It  has  been  stated  that  a  pencil  of  rays  falling 
upon  a  spherical  refracting  surface  will  be  refracted  to  a  common  focus. 
Strictly  speaking,  however,  the  outer  rays  of  the  pencil — i.  e.  those  which  fall 
near  the  periphery  of  the  refracting  surface — will  be  refracted  more  than  those 
which  lie  near  the  axis  and  will  come  to  a  focus  sooner.  This  phenomenon, 
which  is  called  spherical  aberration,  is  more  marked  with  diverging  than  with 
parallel  rays,  and  tends,  of  course,  to  produce  an  indistinctness  of  the  image 
which  will  increase  with  the  extent  of  the  surface  through  which  the  rays 
pass.  %  The  effect  of  a  diaphragm  used  in  many  optical  instruments  to  reduce 
the  amount  of  spherical  aberration  by  cutting  off  the  side  rays  is  shown  dia- 
grammatically  in  Fig.  136. 


FIG.  136.— Diagram  showing  the  effect  of  a  diaphragm  in  reducing  the  amount  of  spherical 

aberration. 

The  r6le  of  the  iris  in  the  vision  of  near  objects  is  now  evident,  for  when 
the  eye  is  directed  to  a  near  object  the  spherical  aberration  is  increased  in  con- 
sequence of  the  rays  becoming  more  divergent,  but  the  contraction  of  the 
pupil  which  accompanies  accommodation  tends,  by  cutting  off  the  side  rays,  to 
prevent  a  blurring  of  the  image  which  otherwise  would  be  produced.  It  must, 
however,  be  remembered  that  the  crystalline  lens,  unlike  any  lens  of  human 
construction,  has  a  greater  index  of  refraction  at  the  centre  than  at  the  periph- 
ery. This,  of  course,  tends  to  correct  spherical  aberration,  and,  in  so  far  as  it 
does  so,  to  render  the  cutting  off  of  the  side  rays  unnecessary.  Indeed,  the 
total  amount  of  possible  spherical  aberration  in  the  eye  is  so  small  that  its 
effect  on  vision  may  be  regarded  as  insignificant  in  comparison  with  that  caused 
by  the  other  optical  imperfections  of  the  eye. 


316  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Chromatic  Aberration. — In  the  above  account  of  the  dioptric  apparatus 
of  the  eye  the  phenomena  have  been  described  as  they  would  occur  with  mono- 
chromatic light — i.  e.  with  light  having  but  one  degree  of  refrangibility.  But 
the  light  of  the  sun  is  composed  of  an  infinite  number  of  rays  of  different 
degrees  of  refrangibility.  Hence  when  an  image  is  formed  by  a  simple  lens 
the  more  refrangible  rays — i.  e.  the  violet  rays  of  the  spectrum — are  brought 
to  a  focus  sooner  than  the  less  refrangible  red  rays.  The  image  therefore 
appears  bordered  by  fringes  of  colored  light.  This  phenomenon  of  chromatic 
aberration  can  be  well  observed  by  looking  at  objects  through  the  lateral  por- 
tion of  a  simple  lens,  or,  still  better,  by  observing  them  through  two  simple 
lenses  held  at  a  distance  apart  equal  to  the  Sum  of  their  focal  distances.  The 
objects  will  appear  inverted  (as  through  afn  astronomical  telescope)  and  sur- 
rounded with  borders  of  colored  light.  N/ow,  the  chromatic  aberration  of  the 
eye  is  so  slight  that  it  is  not  easily  detectecj,  and  the  physicists  of  the  eighteenth 
century,  in  their  efforts  to  produce  an  Achromatic  lens,  seem  to  have  been 
impressed  by  the  fact  that  in  the  eye  a  ^combination  of  media  of  different 
refractive  powers  is  employed,  and  to  havk  sought  in  this  circumstance  an 
explanation  of  the  supposed  achromatism  of  the  eye.  Work  directed  on  this 
line  was  crowned  with  brilliant  success,  for  by  combining  two  sorts  of  glass  of 
different  refractive  and  dispersive  powers  it  was  found  possible  to  refract  a  ray 
of  light  without  dispersing  it  into  its  different  coloredrays,  and  the  achromatic 
lens,  thus  constructed,  became  at  once  an  essential  part Nof  every  first-class  opti- 
cal instrument.  Now,  as  there  is  not  only  no  evidenceHhat  the  principle  of 
the  achromatic  lens  is  employed  in  the  eye,  but  distinct  evidence  that  the  eye 
is  uncorrected  for  chromatic  aberration,  we  have  here  a  remarkable  instance  of 
a  misconception  of  a  physical  fact  leading  to  an  important  discovery  in  physics. 
The  chromatic  aberration  of  the  eye,  though  so  slight  as  not  to  interfere  at  all 
with  ordinary  vision,  can  be  readily  shown  to  exist  by  the  simple  experiment 
of  covering  up  one  half  of  the  pupil  and  looking  at  a  bright  source  of  light 
e.  g.  a  window.  If  the  lower  half  of  the  pupil  be  covered,  the  cross-bars  of 


FIG.  137. — Diagram  to  illustrate  chromatic  aberration. 

the  window  will  appear  bordered  with  a  fringe  of  blue  light  on  the  lower  and 
reddish  light  on  the  upper  side.  The  explanation  usually  given  of  the  way  in 
which  this  result  is  produced  is'illustrated  in  Fig.  137.  Owing  to  the  chromatic 
aberration  of  the  eye  all  the  rays  emanating  from  an  object  at  A  are  not 
focussed  accurately  on  the  retina,  but  if  the  eye  is  accommodated  for  a  ray  of 
medium  refrangibility,  the  violet  rays  will  be  brought  to  a  focus  in  front  of 
the  retina  at  I7,  while  the  red  rays  will  be  focnssed  behind  the  retina  at  R. 


THE   SENSE    OF    VISION. 


317 


On  the  retiiiii  itself  will  be  formed  not  an  accurate  optical  image  of  the  point 
A,  but  a  smull  circle  of  dispersion  in  which  the  various  colored  rays  are  mixed 
together,  the  violet  rays  after  crossing  falling  upon  the  same  part  of  the  retina 
as  the  red  rays  before  crossing.  Thus  by  a  sort  of  compensation,  which,  how- 
ever, cannot  be  equivalent  to  the  synthetic  reproduction  of  white  light  by  the 
union  of  the  spectral  colors,  the  disturbing  effect  of  chromatic  aberration  is 
diminished.  When  the  lower  half  of  the  pupil  is  covered  by  the  edge  of  a 
card  held  in  front  of  the  cornea  at  D,  the  aberration  produced  in  the  upper 
half  of  the  eye  is  not  compensated  by  that  of  the  lower  half.  Hence  the 
image  of  a  point  of  white  light  at  A  will  appear  as  a  row  of  spectral  colors 
on  the  retina,  and  all  objects  will  appear  bordered  by  colored  fringes.  Another 
good  illustration  of  the  chromatic  aberration  of  the  eye  is  obtained  by  cutting 
two  holes  of  any  convenient  shape  in  a  piece  of  black  cardboard  and  placing 
behind  one  of  them  a  piece  of  blue  and  behind  the  other  a  piece  of  red  glass. 
If  the  card  is  placed  in  a  window  some  distance  (10  meters)  from  the  observer, 
in  such  a  position  that  the  white  light  of  the  sky  may  be  seen  through  the  col- 
ored glasses,  it  will  be  found  that  the  outlines  of  the  two  holes  will  generally 
be  seen  with  unequal  distinctness.  To  most  eyes  the  red  outline  will  appear 
quite  distinct,  while  the  blue  figure  will  seem  much  blurred.  To  a  few  indi- 
viduals the  blue  figure  appears  the  more  distinct,  and  these  will  generally  be 
found  to  be  hypermetropic. 


FIG.  138.— Model  to  illustrate  astigmatism. 

Astigmatism. — The  defect  known  as  astigmatism  is  due  to  irregularities 
of  curvature  of  the  refracting  surfaces,  in  consequence  of  which  all  the  rays 
proceeding  from  a  single  point  cannot  be  brought  to  a  single  focus  on  the 
retina. 

Astigmatism  is  said  to  be  regular  when  one  of  the  surfaces,  generally  the 


318  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

cornea,  is  not  spherical,  but  ellipsoidal — i.  e.  having  meridians  of  maximum 
and  minimum  curvature  at  right  angles  to  each  other,  though  in  each  meridian 
the  curvature  is  regular  When  this  is  the  case  the  rays  proceeding  from  a 
single  luminous  point  are  brought  to  a  focus  earliest  when  they  lie  in  the 
meridian  in  which  the  surface  is  most  convex.  Hence  the  pencil  of  rays  will 
have  two  linear  foci,  at  right  angles  to  the  meridians  of  greatest  and  least 
curvature  separated  by  a  space  in  which  a  section  of  the  cone  of  rays  will  be 
first  elliptical,  then  circular,  and  then  again  elliptical.  This  defect  exists  to  a 
certain  extent  in  nearly  all  eyes,  and  is,  in  some  cases,  a  serious  obstacle  to  dis- 
tinct vision.  The  course  of  the  rays  when  thus  refracted  is  illustrated  in  Fig.  138, 
which  represents  the  interior  of  a  box  through  which  black  threads  are  drawn 
to  indicate  the  course  of  the  rays  of  light.  The  threads  start  at  one  end  of  the 
box  from  a  circle  representing  the  cornea,  and  converge  with  different  degrees 
of  rapidity  in  different  meridians,  so  that  a  section  of  the  cone  of  rays  will  be 
successively  an  ellipse,  a  straight  line,  an  ellipse,  a  circle,  etc.,  as  shown  by  the 
model  represented  in  Fig.  139.  It  will  be  noticed  that  this  and  the  preced- 


FIG.  139.— Model  to  illustrate  astigmatism. 

ing  figure  are  drawn  in  duplicate,  but  that  the  lines  are  not  precisely  alike  on 
the  two  sides.  In  fact,  the  lines  on  the  left  represent  the  model  as  it  would 
be  seen  with  the  right  eye,  and  those  on  the  right  as  it  would  appear  to 
the  left  eye,  which  is  just  the  opposite  from  an  ordinary  stereoscopic  slide. 
The  figures  are  drawn  in  this  way  because  they  are  intended  to  produce  a 
"  pseudoscopic  "  effect  in  a  way  which  will  be  explained  in  connection  with 
the  subject  of  binocular  vision.  For  this  purpose  it  is  only  necessary  to  cross 
the  axes  of  vision  in  front  of  the  page,  as  in  the  experiment  described  on  page 
312,  for  studying  the  relation  between  the  focal,  axial,  and  pupillary  adjust- 
ments of  the  eye.  As  soon  as  the  middle  image  becomes  distinct  it  assumes  a 


THE  SENSE    OF    VISION.  319 

stereoscopic  appearance,  and  the  correct  relations  between  the  different  parts  of 
the  model  are  at  once  obvious. 

This  imperfection  of  the  eye  may  be  detected  by  looking  at  lines  such  as  are 
shown  in  Figure  140,  and  testing  each  eye  separately.  If  the  straight  lines 
drawn  in  various  directions  through  a  common  point  cannot  be  seen  with  equal 
distinctness  at  the  same  time,  it  is  evident  that  the  eye  is  better  adapted  to  focus 
rays  in  one  meridian  than  in  another — i.  e.  it  is  astigmatic.  The  concentric 


FIG.  140.— Lines  for  the  detection  of  astigmatism. 

circles  are  a  still  more  delicate  test.  Few  persons  can  look  at  this  figure  attentively 
without  noticing  that  the  lines  are  not  everywhere  equally  distinct,  but  that  in 
certain  sectors  the  circles  present  a  blurred  appearance.  Not  infrequently  it 
will  be  found  that  the  blurred  sectors  do  not  occupy  a  constant  position,  but 
oscillate  rapidly  from  one  part  of  the  series  of  circles  to  another.  This  phe- 
nomenon seems  to  be  due  to  slight  involuntary  contractions  of  the  ciliary 
muscle  causing  changes  in  accommodation. 

The  direction  of  the  meridians  of  greatest  and  least  curvature  of  the  cornea 
of  a  regularly  astigmatic  eye,  and  the  difference  in  the  amount  of  this  curvature, 
can  be  very  accurately  measured  by  means  of  the  ophthalmometer  (see  p.  304). 
These  points  being  determined,  the  defect  of  the  eye  can  be  perfectly  corrected 
by  cylindrical  glasses  adapted  to  compensate  for  the  excessive  or  deficient 
refraction  of  the  eye  in  certain  meridians. 

By  another  method  known  as  "  skiascopy,"  which  consists  in  studying  the 
light  reflected  from  the  fundus  of  the  eye  when  the  ophthalmoscopic  mirror  is 
moved  in  various  directions,  the  amount  and  direction  of  the  astigmatism  of 
the  eye  as  a  whole  (and  not  that  of  the  cornea  alone)  may  be  ascertained. 

Astigmatism  is  said  to  be  irregular  when  in  certain  meridians  the  curvatures 
of  the  refracting  surfaces  are  not  arcs  of  circles  or  ellipses,  or  when  there  is  a 
lack  of  homogeneousness  in  the  refracting  media.  This  imperfection  exists  to 
a  greater  or  less  extent  in  all  eyes,  and,  unlike  regular  astigmatism,  is  incapable 
of  correction.  It  manifests  itself  by  causing  the  outlines  of  all  brilliant  objects 
to  appear  irregular.  It  is  on  this  account  that  the  fixed  stars  do  not  appear  to 
us  like  points  of  light,  but  as  luminous  bodies  with  irregular  "  star  "-shaped 
outlines.  The  phenomenon  can  be  conveniently  studied  by  looking  at  a  pin- 
hole  in  a  large  black  card  held  at  a  convenient  distance  between  the  eye  and  a 
strong  light.  The  hole  will  appear  to  have  an  irregular  outline,  and  to  some 
eyes  will  appear  double  or  treble. 


320  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Intraocular  Images. — Light  entering  the  eye  makes  visible,  under  certain 
circumstances,  a  number  of  objects  which  lie  within  the  eye  itself.  These 
objects  are  usually  opacities  in  the  media  of  the  eye  which  are  ordinarily  invisi- 
ble, because  the  retina  is  illuminated  by  light  coming  from  all  parts  of  the 
pupil,  and  with  such  a  broad  source  of  light  no  object,  unless  it  is  a  very  large 
one  or  one  lying  very  near  the  back  of  the  eye,  can  cast  a  shadow  on  the  retina. 
Such  shadows  can,  however,  be  made  apparent  by  allowing  the  media  of  the 
eye  to  be  traversed  by  parallel  rays  of  light.  This  can  be  accomplished  by 
holding  a  small  polished  sphere — e.  g.  the  steel  head  of  a  shawl-pin  illuminated 
by  sunlight  or  strong  artificial  light — in  the  anterior  focus  of  the  eye — i.  e. 
about  22  millimeters  in  front  of  the  cornea,  or  by  placing  a  dark  screen  with  a 
pin-hole  in  it  in  the  same  position  between  the  eye  and  a  source  of  uniform 
diffused  light,  such  as  the  sky  or  the  porcelain  shade  of  a  student  lamp.  In 
either  case  the  rays  of  light  diverging  from  the  minute  source  will  be  refracted 
into  parallelism  by  the  media  of  the  eye,  and  will  produce  the  sensation  of  a 
circle  of  diffused  light,  the  size  of  which  will  depend  upon  the  amount  of  dila- 
tation of  the  pupil.  Within  this  circle  of  light  will  be  seen  the  shadows  of  any 
opaque  substances  that  may  be  present  in  the  media  of  the  eye.  These  shadows, 
being  cast  by  parallel  rays,  will  be  of  the  same  size  as  the  objects  themselves, 
as  is  shown  diagrammatical ly  in  Figure  141,  in  which  A  represents  a  source 


FIG.  141.— Showing  the  method  of  studying  intraocular  images  (Helmholtz). 

of  light  at  the  anterior  focus  of  the  eye,  and  b  an  opacity  in  the  vitreous  humor 
casting  a  shadow  B  of  the  same  size  as  itself  upon  the  retina.  It  is  evident  that 
if  the  source  of  light  A  is  moved  from  side  to  side  the  various  opacities  will  be 
displaced  relatively  to  the  circle  of  light  surrounding  them  by  an  amount  de- 
pending upon  the  distance  of  the  opacities  from  the  retina.  A  study  of  these 
displacements  will  therefore  afford  a  means  of  determining  the  position  of  the 
opacities  within  the  media  of  the  eye. 

Muscse  Volitantes. — Among  the  objects  to  be  seen  in  thus  examining  the 
eye  the  most  conspicuous  are  those  known  as  the  muscce  volitantes.  These  pre- 
sent 'themselves  in  the  form  of  beads,  either  singly  or  in  groups,  or  of  streaks, 
patches,  and  granules.  They  have  an  almost  constant  floating  motion,  which 
is  increased  by  the  movements  of  the  eye  and  head.  They  usually  avoid  the 
line  of  vision,  floating  away  when  an  attempt  is  made  to  fix  the  sight  upon 
them.  When  the  eye  is  directed  vertically,  however,  they  sometimes  place 
themselves  directly  in  line  with  the  object  looked  at.  If  the  intraocular  object 
is  at  the  same  time  sufficiently  near  the  back  of  the  eye  to  cast  a  shadow  which 


THE  SENSE    OF    VISION.  321 

is  visible  without  the  use  of  the  focal  illumination,  some  inconvenience  may 
thus  be  caused  in  using  a  vertical  microscope. 

A  study  of  the  motions  of  the  musccc  volitantes  makes  it  evident  that  the 
phenomenon  is  due  to  small  bodies  floating  in  a  liquid  medium  of  a  little 
greater  specific  gravity  than  themselves.  Their  movements  are  chiefly  in 
planes  perpendicular  to  the  axis  of  vision,  for  when  the  eye  is  directed  verti- 
cally upward  they  move  as  usual  through  the  field  of  vision  without  increasing 
the  distance  from  the  retina.  They  are  generally  supposed  to  be  the  remains 
of  the  embyronic  structure  of  the  vitreous  body — i.  e.  portions  of  the  cells  and 
fibres  which  have  not  undergone  complete  mucous  transformation. 

In  addition  to  these  floating  opacities  in  the  vitreous  body  various  other 
defects  in  the  transparent  media  of  the  eye  may  be  revealed  by  the  method  of 
focal  illumination.  Among  these  may  be  mentioned  spots  and  stripes  due  to 
irregularities  in  the  lens  or  its  capsule,  and  radiating  lines  indicating  the  stel- 
late structure  of  the  lens. 

Retinal  Vessels. — Owing  to  the  fact  that  the  blood-vessels  ramify  near  the 
anterior  surface  of  the  retina,  while  those  structures  which  are  sensitive  to  light 
constitute  the  posterior  layer  of  that  organ,  it  is  evident  that  light  entering  the 
eye  will  cast  a  shadow  of  the  vessels  on  the  light-perceiving  elements  of  the 
retina.  Since,  however,  the  diameter  of  the  largest  blood-vessels  is  not  more 
than  one-sixth  of  the  thickness  of  the  retina,  and  the  diameter  of  the  pupil  is 
one-fourth  or  one-fifth  of  the  distance  from  the  iris  to  the  retina,  it  is  evident 
that  when  the  eye  is  directed  to  the  sky  or  other  broad  illuminated  surfaces  it 
is  only  the  penumbra  of  the  vessels  that  will  reach  the  rods  and  cones,  the  umbra 
terminating  conically  somewhere  in  the  thickness  of  the  retina.  But  if  light 
is  allowed  to  enter  the  eye  through  a  pin-hole  in  a  card  held  a  short  distance 
from  the  cornea,  as  in  the  above-described  method  of  focal  illumination,  a 
sharply  defined  shadow  of  the  vessels  will  be  thrown  on  the  rods  and  cones. 
Yet  under  these  conditions  the  retinal  vessels  are  not  rendered  visible  unless 
the  perforated  card  is  moved  rapidly  to  and  fro,  so  as  to  throw  the  shadow 
continually  on  to  fresh  portions  of  the  retinal  surface.  When  this  is  done  the 
vessels  appear,  ramifying  usually  as  dark  lines  on  a  lighter  background,  but 
the  dark  lines  are  sometimes  bordered  by  bright  edges.  It  will  be  observed 
that  those  vessels  appear  most  distinctly  the  course  of  which  is  at  right  angles 
to  the  direction  in  which  the  card  is  moved.  Hence  in  order  to  see  all  the 
vessels  with  equal  distinctness  it  is  best  to  move  the  card  rapidly  in  a  circle 
the  diameter  of  which  should  not  exceed  that  of  the  pupil.  In  this  manner 
the  distribution  of  the  vessels  in  one's  own  retina  may  be  accurately  observed, 
and  in  many  cases  the  position  of  the  fovea  centralis  may  be  determined  by  the 
absence  of  vessels  from  that  portion  of  the  macula  lutea. 

The  retinal  vessels  may  also  be  made  visible  in  several  other  ways — e.  g.y 
1.  By  directing  the  eye  toward  a  dark  background  and  moving  a  candle  to  and 
fro  in  front  of  the  eye,  but  below  or  to  one  side  of  the  line  of  vision.  2.  By 
concentrating  a  strong  light  by  means  of  a  lens  of  short  focus  upon  a  point 
of  the  sclerotic  as  distant  as  possible  from  the  cornea.  By  either  of  these 
methods  a  small  image  of  the  external  source  of  light  is  formed  upon  the 


TT.— 9.1 


322  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

lateral  portion  of  the  eye,  and  this  image  is  the  source  of  light  which  throws 
shadows  of  the  retinal  vessels  on  to  the  rods  and  cones. 

Circulation  of  Blood  in  the  Retina. — When  the  eye  is  directed  toward  a 
surface  which  is  uniformly  and  brightly  illuminated — e.  g.  the  sky  or  a  sheet 
of  white  paper  on  which  the  sun  is  shining — the  field  of  vision  is  soon  seen  to 
be  filled  with  small  bright  bodies  moving  with  considerable  rapidity  in  irregu- 
lar curved  lines,  but  with  a  certain  uniformity  which  suggests  that  their 
movements  are  confined  to  definite  channels.  They  are  usually  better  seen 
when  one  or  more  sheets  of  cobalt  glass  are  held  before  the  face,  so  that  the 
eyes  are  bathed  in  blue  light.  That  the  phenomenon  depends  upon  the  circu- 
lation of  the  blood  globules  in  the  retina  is  evident  from  the  fact  that  the 
moving  bodies  follow  paths  which  correspond  with  the  form  of  the  retinal 
capillaries  as  seen  by  the  methods  above  described,  and  also  from  the  corre- 
spondence between  the  rate  of  movement  of  the  intraocular  image  and  the 
rapidity  of  the  capillary  circulation  in  those  organs  in  which  it  can  be  di- 
rectly measured  under  the  microscope.  The  exact  way  in  which  the  moving 
globules  stimulate  the  retina  so  as  to  produce  the  observed  phenomenon  must 
be  regarded  as  an  unsettled  question. 

We  have  thus  seen  that  the  eye,  regarded  from  the  optician's  point  of  view, 
has  not  only  all  the  faults  inherent  in  optical  instruments  generally,  but  many 
others  which  would  not  be  tolerated  in  an  instrument  of  human  construction. 
Yet  with  all  its  imperfections  the  eye  is  perhaps  the  most  wonderful  instance 
in  nature  of  the  development  of  a  highly  specialized  organ  to  fulfil  a  definite 
purpose.  In  the  accomplishment  of  this  object  the  various  parts  of  the  eye 
have  been  perfected  to  a  degree  sufficient  to  enable  it  to  meet  the  requirements 
of  the  nervous  system  with  which  it  is  connected,  and  no  farther.  In  the 
ordinary  use  of  the  eye  we  are  unconscious  of  its  various  irregularities,  shadows, 
opacities,  etc.,  for  these  imperfections  are  all  so  slight  that  the  resulting  inac- 
curacy of  the  image  does  not  much  exceed  the  limit  which  the  size  of  the 
light-perceiving  elements  of  the  retina  imposes  upon  the  delicacy  of  our  visual 
perceptions,  and  it  is  only  by  illuminating  the  eye  in  some  unusual  way  that 
the  existence  of  these  imperfections  can  be  detected.  In  other  words,  the  eye 
is  as  good  an  optical  instrument  as  the  nervous  system  can  appreciate  and 
make  use  of.  Moreover,  when  we  reflect  upon  the  difficulty  of  the  problem 
which  nature  has  solved,  of  constructing  an  optical  instrument  out  of  living 
and  growing  animal  tissue,  we  cannot  fail  to  be  struck  by  the  perfection  of  the 
dioptric  apparatus  of  the  eye  as  well  as  by  its  adaptation  to  the  needs  of  the 
organism  of  which  it  forms  a  part. 

Iris. — The  importance  of  the  iris  as  an  adjustable  diaphragm  for  cutting 
off  side  rays  and  thus  securing  good  definition  in  near  vision  has  been  described 
in  connection  with  the  act  of  accommodation.  Its  other  function  of  protecting 
the  retina  from  an  excess  of  light  is  no  less  important,  and  we  must  now  con- 
sider how  this  pupillary  adjustment  may  be  studied  and  by  what  mechanism 
it  is  effected.  The  changes  in  the  size  of  the  pupil  may  be  conveniently  ob- 
served in  man  and  animals  by  holding  a  millimeter  scale  in  front  of  the  eye 
and  noticing  the  variations  in  the  diameter  of  the  pupil.  It  should  be  borne 


THE  SENSE    OF    VISION. 


323 


Lc 


in  mind  that  the  iris,  seen  in  this  way,  does  not  appear  in  its  natural  size  and 
position,  but  somewhat  enlarged  and  bulged  forward  by  the  magnifying  effect 
of  the  cornea  and  the  aqueous  humor.  The  changes  in  one's  own  pupil  may 
be  readily  observed  by  noticing  the  varying  size  of  the  circle  of  light  thrown 
upon  the  retina  when  the  eye  is  illuminated  by  a  point  of  light  held  at  the 
anterior  focus,  as  in  the  method  above  described  for  the  study  of  intraocular 
images. 

The  muscles  of  the  iris  are,  except  in  birds,  of  the  unstriped  variety,  and 
are  arranged  concentrically  around  the  pupil.  Radiating  fibres  are  also  recog- 
nized by  many  observers,  though  their  existence  has  been  called  in  question 
by  others.  The  circular  or  constricting  muscles  of  the  iris  are  under  the  con- 
trol of  the  third  pair  of  cranial  nerves, 
and  are  normally  brought  into  activity 
in  consequence  of  light  falling  upon 
the  retina.  This  is  a  reflex  phenom- 
enon, the  optic  nerve  being  the  affer- 
ent, and  the  third  pair,  the  ciliary 
ganglion,  and  the  short  ciliary  nerves 
the  efferent,  channel,  as  indicated  in 
Figure  142.  This  reflex  is  in  man 
and  many  of  the  higher  animals  bi- 
lateral— i.  e.  light  falling  upon  one 
retina  will  cause  a  contraction  of  both 
pupils.  This  may  readily  be  observed 
in  one's  own  eye  when  focally  illumi- 
nated in  the  manner  above  described. 
Opening  the  other  eye  will,  under 
these  conditions,  cause  a  diminution, 
and  closing  it  an  increase,  in  the  size 
of  the  circle  of  light.  This  bilateral 
character  is  found  to  be  dependent 
upon  the  nature  of  the  decussation  of 
the  optic  nerves,  for  in  animals  in 
which  the  crossing  is  complete  the 
reflex  is  confined  to  the  illuminated 

••I-M  t»          i  i»  \  1HJIVC,    c.  y,  viiia-i  j    ga-ngnwu,  f     i  .  ut  ito    OJ-iiri  b    i.  WK  J.&VTAU 

eye.     Ine  arrangement  ot  the  nbres   j//)motor-ocuii  nerve  -.gym,  its  sympathetic  root  -,r.i, 

in    the    Optic   Commissure   is  in  general     its  long  root  from  V.ophthalmo-nasal  branch  of  oph- 
\         .  .        .  .  .         °         .        thalmic  division  of  fifth  nerve ;  «.  c.  short  ciliary 

associated    With     the    position    of    the    nerves ;  I.  c,  long  ciliary  nerves. 

eyes   in   the   head.     When    the   eyes 

are  so  placed  that  they  can  both  be  directed  to  the  same  object,  as  in  man 
and  many  of  the  higher  animals,  the  fibres  of  each  optic  nerve  are  usually 
found  to  be  distributed  to  both  optic  tracts,  while  in  animals  whose  eyes 
are  in  opposite  sides  of  the  head  there  is  complete  crossing  of  the  optic  nerves. 
Hence  it  may  be  said  that  animals  having  binocular  vision  have  in  general 
a  bilateral  pupillary  reflex.  The  rule  is,  however,  not  without  exceptions, 
for  owls,  though  their  visual  axes  are  parallel,  have,  like  other  birds,  a  corn- 


Course  of  constrictor  nerve-fibres 

"        dilator  " 

FIG.  142.— Diagrammatic  representation  of  the 
nerves  governing  the  pupil  (after  Foster) :  II,  optic 
nerve;  l.g,  ciliary  -ganglion ;  r.  b,  its  short  root  from 


324  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

plete  crossing  of  the  optic  nerves,  and  consequently  a  unilateral  pupillary 
reflex.1 

A  direct  as  well  as  a  reflex  constriction  of  the  pupil  under  the  influence  of 
light  has  been  observed  in  the  excised  eyes  of  eels,  frogs,  and  some  other  ani- 
mals. As  the  phenomenon  can  be  seen  in  preparations  consisting  of  the  iris 
alone  or  of  the  iris  and  cornea  together,  it  is  evident  that  the  light  exerts  its 
influence  directly  upon  the  tissues  of  the  iris  and  not  through  an  intraocular 
connection  with  the  retina.  The  maximum  effect  is  produced  by  the  yellowish- 
green  portion  of  the  spectrum. 

Antagonizing  the  motor  oculi  nerve  in  its  constricting  influence  on  the 
pupil  is  a  set  of  nerve-fibres  the  function  of  which  is  to  increase  the  size  of 
the  pupil.  Most  of  these  fibres  seem  to  run  their  course  from  a  centre  which 
lies  in  the  floor  of  the  third  ventricle  not  far  from  the  origin  of  the  third  pair, 
through  the  bulb,  the  cervical  cord,  the  anterior  roots  of  the  upper  dorsal 
nerves,  the  upper  thoracic  ganglion,  the  cervical  sympathetic  nerve  as  far  as 
the  upper  cervical  ganglion ;  then  through  a  branch  which  accompanies  the 
internal  carotid  artery,  passes  over  the  Gasserian  ganglion  and  joins  the  oph- 
thalmic branch  of  the  fifth  pair ;  then  through  the  nasal  branch  of  the  latter 
nerve  and  the  long  ciliary  nerves  to  the  eye 2  (see  diagram,  p.  323).  These 
fibres  appear  to  be  in  a  state  of  tonic  activity,  for  section  of  them  in  any  part 
of  their  course  (most  conveniently  in  the  cervical  sympathetic)  causes  a  con- 
traction of  the  pupil  which,  on  stimulation  of  the  peripheral  end  of  the  divided 
nerve,  gives  place  to  a  marked  dilatation.  Their  activity  can  be  increased  in 
various  ways.  Thus  dilatation  of  the  pupil  may  be  caused  by  dyspnea,  vio- 
lent muscular  efforts,  etc.  Stimulation  of  various  sensory  nerves  may  also 
cause  reflex  dilatation  of  the  pupil,  and  since  this  phenomenon  may  be  observed, 
though  greatly  diminished  in  intensity,  after  extirpation  of  the  superior  cervi- 
cal sympathetic  ganglion,  it  is  probable  that  the  dilatation  depends  in  part 
upon  a  reflex  inhibition  of  the  constrictor  nerves. 

Since  the  cervical  sympathetic  nerve  contains  vaso-constrictor  fibres  for  the 
head  and  neck,  it  has  been  thought  that  its  dilating  effect  upon  the  pupil  might 
be  explained  by  its  power  of  causing  changes  in  the  amount  of  blood  in  the 
vessels  of  the  iris.  There  is  no  doubt  that  a  condition  of  vascular  turgescence 
or  depletion  will  tend  to  produce  contraction  or  dilatation  of  the  pupil,  but  it  is 
impossible  to  explain  the  observed  phenomena  in  this  way,  since  the  pupillary 
are  more  prompt  than  the  vascular  changes,  and  may  be  observed  on  a  bloodless 
eye.  Moreover,  the  nerve-fibres  producing  them  are  said  to  have  a  somewhat 
different  course.  Another  explanation  of  the  influence  of  the  sympathetic  on 
the  pupil  is  that  it  acts  by  inhibiting  the  contraction  of  the  sphincter  muscles, 
and  that  the  dilatation  is  simply  an  elastic  reaction.  But  since  it  is  posssible  to 
produce  local  dilatation  of  the  pu<pil  by  circumscribed  stimulation  at  or  near 

1  Steinach  :  Archiv  fur  die  gesammte  Physiologic,  xlvii.  313. 

2  Langley  :   Journal  of  Physiology,  xiii.  p.  575.     For  the  evidence  of  the  existence  of  a 
"cilio-spinal"  centre  in  the  cord,  see  Steil  and  Langendorff:    Archiv  fur  die  gesammte  Phys- 
iologic, Iviii.  S.  155 ;  also  Schenck :  Ibid.,  Ixii.  S.  494. 


THE  SENSE    OF    VISION.  325 

the  outer  border  of  the  iris,  it  seems  more  reasonable  to  conclude  that  the 
dilator  nerves  of  the  pupil  act  upon  radial  muscular  fibres  in  the  substance  of 
the  iris,  in  spite  of  the  fact  that  the  existence  of  such  fibres  has  not  been  uni- 
versally admitted. 

Whatever  view  may  be  taken  of  the  mechanism  by  which  the  sympathetic 
nerves  influence  the  pupil,  there  is  no  doubt  that  the  iris  is  under  the  control 
of  two  antagonistic  sets  of  nerve-fibres,  both  of  which  are,  under  normal  cir- 
cumstances, in  a  state  of  tonic  activity.  Therefore,  when  the  sympathetic 
nerve  is  divided  the  pupil  contracts  under  the  influence  of  the  motor  oculi,  and 
section  of  the  motor  oculi  causes  dilatation  through  the  unopposed  influence  of 
the  sympathetic. 

The  movements  of  the  iris,  though  performed  by  smooth  muscles,  are  more 
rapid  than  those  of  smooth  muscles  found  elsewhere — e.  g.  in  the  intestines 
and  the  arteries.  The  contraction  of  the  pupil  when  the  retina  of  the  oppo- 
site eye  is  illuminated  occupies  about  0.3" ;  the  dilatation  when  the  light  is  cut 
off  from  the  eye,  about  3"  or  4".  The  latter  determination  is,  however,  diffi- 
cult to  make  with  precision,  since  dilatation  of  the  pupil  takes  place  at  first 
rapidly  and  then  more  slowly,  so  that  the  moment  when  the  process  is  at  an 
end  is  not  easily  determined.  After  remaining  a  considerable  time  in  absolute 
darkness  the  pupils  become  enormously  dilated,  as  has  been  shown  by  flash- 
light photographs  taken  under  these  conditions.  In  sleep,  though  the  eyes  are 
protected  from  the  light,  the  pupils  are  strongly  contracted,  but  dilate  on 
stimulation  of  sensory  nerves,  even  though  the  stimulation  may  be  insufficient 
to  rouse  the  sleeper. 

Many  drugs  when  introduced  into  the  system  or  applied  locally  to  the  con- 
junctiva produce  effects  upon  the  pupil.  Those  which  dilate  it  are  known  as 
mydriatica,  those  which  contract  it  as  myotics.  Of  the  former  class  the  most 
important  is  atropin,  the  alkaloid  of  the  Atropa  belladonna,  and  of  the  latter 
physostigmin,  the  alkaloid  of  the  Calabar  bean.  In  addition  to  their  action 
upon  the  pupil,  mydriatics  paralyze  the  accommodation,  thus  focussing  the  eye 
for  distant  objects,  while  myotics,  by  producing  a  cramp  of  the  ciliary  muscle, 
adjust  the  eye  for  near  vision.  The  effect  on  the  accommodation  usually 
begins  later  and  passes  off  sooner  than  the  affection  of  the  pupil.  Atropin 
seems  to  act  by  producing  local  paralysis  of  the  terminations  of  the  third  pair 
of  cranial  nerves  in  the  sphincter  iridis  and  the  ciliary  muscle.  In  large 
doses  it  may  also  paralyze  the  muscle-fibres  of  the  sphincter.  With  this  para- 
lyzing action  there  appears  to  be  combined  a  stimulating  effect  upon  the  dilator 
muscles  of  the  iris.  The  myotic  action  of  physostigmin  seems  to  be  due  to  a 
local  stimulation  of  the  fibres  of  the  sphincter  of  the  iris.1 

Although  in  going  from  a  dark  room  to  a  lighter  one  the  pupil  at  first  con- 
tracts, this  contraction  soon  gives  place  to  a  dilatation,  and  in  about  three  or 
four  minutes  the  pupil  usually  regains  its  former  size.  In  a  similar  manner 
the  primary  dilatation  of  the  pupil  caused  by  entering  a  dark  room  from  a 
lighter  one  is  followed  by  a  contraction  which  usually  restores  the  pupil  to  its 
original  size  within  fifteen  or  twenty  minutes.  It  is  thus  evident  that  the 
1  See  Paul  Schultz  :  Archiv  fur  Physiologic,  1898,  S.  47. 


326 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


amount  of  light  falling  upon  the  retina  is  not  the  only  factor  in  determining 
the  size  of  the  pupil.  In  fact,  if  the  light  acts  for  a  sufficient  length  of  time 
the  pupil  may  have  the  same  size  under  the  influence  of  widely  different 
degrees  of  illumination.1 

This  so-called  "  adaptation  "  of  the  eye  to  various  amounts  of  light  seems 
to  be  connected  with  the  movements  of  the  retinal  pigment-granules  and  with 
the  chemical  changes  of  the  visual  purple,  to  be  more  fully  described  in  con- 
nection with  the  physiology  of  the  retina. 

The  Ophthalmoscope. — Under  normal  conditions  the  pupil  of  the  eye 
appears  as  a  black  spot  in  the  middle  of  the  colored  iris.  The  cause  of  this 
dark  appearance  of  the  pupil  is  to  be  found  in  the  fact  that  a  source  of  light 
and  the  retina  lie  in  the  conjugate  foci  of  the  dioptric  apparatus  of  the  eye. 
Hence  any  light  entering  the  eye  that  escapes  absorption  by  the  retinal  pig- 
ment and  is  reflected  from  the  fundus  must  be  refracted  back  to  the  source 
from  which  it  came.  The  eye  of  an  observer  who  looks  at  the  pupil  from 
another  direction  will  see  no  light  coming  from  it,  and  it  will  therefore  appear 
to  him  black.  It  is  therefore  evident  that  the  essential  condition  for  perceiving 
light  coming  from  the  fundus  of  the  eye  is  that  the  line  of  vision  of  the 
observing  eye  shall  be  in  the  line  of  illumination.  This  condition  is  fulfilled 
by  means  of  instruments  known  as  ophthalmoscopes.  The  principles  involved 
in  the  construction  of  the  most  common  form  of  ophthalmoscope  are  illustrated 
diagrammatically  in  Figure  143. 


M 


FIG.  143.— Diagram  to  illustrate  the  principles  of  a  simple  ophthalmoscope  (after  Foster). 

The  rays  from  a  source  of  light  L,  after  being  brought  to  a  focus  at  a  by 
the  concave  perforated  mirror  M  M,  pass  on  and  are  rendered  parallel  by  the 
lens  I.  Then,  entering  the  observed  eye  B,  they  are  brought  to  a  focus  on  the 
retina  at  af.  Any  rays  which  are  reflected  back  from  the  part  of  the  retina 
thus  illuminated  will  follow  the  course  of  the  entering  rays  and  be  brought  to 
a  focus  at  a.  The  eye  of  an  observer  at  Aj  looking  through  the  hole  in  the 
mirror,  will  therefore  see  at  a  an  inverted  image  of  the  retina,  the  observation 
of  which  may  be  facilitated  by  a  convex  lens  placed  immediately  in  front  of 
the  observer's  eye. 

1  Schirmer  :  Archivfiir  Ophthalmologie,  xi.  5. 


THE  SENSE    OF    VISION.  327 

The  fundus  of  the  eye  thus  observed  presents  a  reddish  background  on 
which  the  retinal  vessels  are  distinctly  visible. 

Retina. — Having  considered  the  mechanism  by  which  optical  images  of 
objects  at  various  distances  from  the  eye  are  formed  upon  the  retina,  we  must 
next  inquire  what  part  of  the  retina  is  affected  by  the  rays  of  light,  and  in 
what  this  affection  consists.  To  the  former  of  these  questions  it  will  be  found 
possible  to  give  a  fairly  satisfactory  answer.  With  regard  to  the  latter  nothing 
positive  is  known. 

The  structure  of  the  retina  is  exceedingly  complicated,  but,  as  very  little 
is  known  of  the  functions  of  the  ganglion  cells  and  of  the  molecular  and 
nuclear  layers,  it  will  suffice  for  the  present  purpose  of  physiological  descrip- 
tion to  regard  the  retina  as  consisting  of  fibres  of  the  optic  nerve  which  are 
connected  through  various  intermediate  structures  with  the  layer  of  rods  and 


FIG.  144.— Diagrammatic  representation  of  the  retina. 

Figure  144  is  intended  to  show,  diagram matically,  the  mutual  relation  of 
these  various  portions  of  the  retina  in  different  parts  of  the  eye,  and  is  not 
drawn  to  scale.  It  will  be  observed  that  the  optic  nerve  0,  where  it  enters  the 
eye,  interrupts  the  continuity  of  the  layer  of  rods  and  cones  R  and  of  the 
intermediate  structures  /.  Its  fibres  spread  themselves  out  in  all  directions, 
forming  the  internal  layer  of  the  retina  N.  The  central  artery  of  the  retina 
A  accompanying  the  optic  nerve  ramifies  in  the  layer  of  nerve-fibres  and  in 
the  immediately  adjacent  layers  of  the  retina,  forming  a  vascular  layer  V.  In 
the  fovea  centralis  F  of  the  macula  lutea  (the  centre  of  distinct  vision)  the 
layer  of  rods  and  cones  becomes  more  highly  developed,  while  the  other  layers 
of  the  retina  are  much  reduced  in  thickness  and  the  blood-vessels  entirely  dis- 
appear. This  histological  observation  points  strongly  to  the  conclusion  that 
the  rods  and  cones  are  the  structures  which  are  essential  to  vision,  and  that  in 
them  are  found  the  conditions  for  the  conversion  of  the  vibrations  of  the 
luminiferous  ether  into  a  stimulus  for  a  nerve-fibre.  This  view  derives  con- 
firmation from  the  observations  on  the  retinal  blood-vessels,  for  it  is  found 
that  the  distance  between  the  vascular  layer  of  the  retina  and  the  layer 
of  rods  and  cones  determined  by  histological  methods  corresponds  with  that 
which  must  exist  between  the  vessels  and  the  light-perceiving  elements  of  the 
retina,  as  calculated  from  the  apparent  displacement  of  the  shadow  caused  by 
given  movements  of  the  source  of  light  used  in  studying  intraocular  images l  as 
1  "  Dimmer:  Verb.  d.  phys.  Clubs  zu  Wien,  24  April,  1894,"  CenlralbL  fur  Physiolofa;  1894, 159. 


328 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


described  on  p.  321.  Another  argument  in  favor  of  this  view  is  found  in  the 
correspondence  between  the  size  of  the  smallest  visible  images  on  the  retina  and 
the  diameter  of  the  rods  and  cones.  '  A  double  star  can  be  recognized  as  double 
by  the  normal  eye  when  the  distance  between  the  components  corresponds  to 
a  visual  angle  of  60".  Two  white  lines  on  a  black  ground  are  seen  to  be  dis- 
tinct when  the  distance  between  them  subtends  a  visual  angle  of  64"-73". 
These  angles  correspond  to  a  retinal  image  of  0.0044,  0.0046,  and  0.0053  mil- 
limeter. Now,  the  diameter  of  the  cones  in  the  macula  lutea,  as  determined 
by  Kolliker,  is  0.0045-0.0055  millimeter,  a  size  which  agrees  well  with  the 
hypothesis  that  each  cone  when  stimulated  can  produce  a  special  sensation  of 
light  distinguishable  from  those  caused  by  the  stimulation  of  the  neighboring 
cones.  The  existence  of  the  so-called  blind  spot  in  the  retina  at  the  point  of 
entrance  of  the  optic  nerve  is  sometimes  regarded  as  evidence  of  the  light- 
perceiving  function  of  the  rods  and  cones,  but  as  the  other  layers  of  the  retina, 
as  well  as  the  rods  and  cones,  are  absent  at  this  point,  and  the  retina  here 
consists  solely  of  nerve-fibres,  it  is  evident  that  the  presence  of  the  blind  spot 


FIG.  145.— To  demonstrate  the  blind  spot. 

only  proves  that  the  optic  nerve-fibres  are  insensible  to  light.  Figure  145  is 
intended  to  demonstrate  this  insensibility.  For  this  purpose  it  should  be  held 
at  a  distance  of  about  23  centimeters  from  the  eyes  (i.  e.  about  3.5  times  the  dis- 
tance between  the  cross  and  the  round  spot).  If  the  left  eye  be  closed  and  the 
right  eye  fixed  upon  the  cross,  the  round  spot  will  disappear  from  view,  though 
it  will  become  visible  if  the  eye  be  directed  either  to  the  right  or  to  the  left  of 
the  cross,  or  if  the  figure  be  held  either  a  greater  or  a  less  distance  from  the 
eye.  The  size  and  shape  of  the  blind  spot  may  readily  be  determined  as 
follows  :  Fix  the  eye  upon  a  definite  point  marked  upon  a  sheet  of  white 
paper.  Bring  the  black  point  of  a  lead  pencil  (which,  except  the  point,  has 
been  painted  white  or  covered  with  white  paper)  into  the  invisible  portion  of 
the  field  of  vision  and  carry  it  outward  in  any  direction  until  it  becomes  vis- 
ible. Mark  upon  the  paper  the  point 
at  which  it  just  begins  to  be  seen,  and 
by  repeating  the  process  in  as  many 
different  directions  as  possible  the  out- 
line of  the  blind  spot  may  be  marked 
out.  Figure  146  shows  the  shape  of 
the  blind  spot  determined  by  Helm- 
holtz  in  his  own  right  eye,  a  being 

FIG.  146,-Porm  of  the  blind  spot  (Helmholts).         the   P°int  °f  fixati°n   °f  thf  *?*>  and 

the  line  A  B  being  one-third  of  the 

distance  between  the  eye  and  the  paper.     The  irregularities  of  outline,  as  at 


THE  SENSE    OF    VISION. 


329 


Rods. 


Cones. 


d,  are  due  to  shadows  of  the  large  retinal  vessels.  During  this  determination 
it  is  of  course  necessary  that  the  head  should  occupy  a  fixed  position  with 
regard  to  the  paper.  This  condition  can  be  secured  by  holding  firmly  between 
the  teeth  a  piece  of  wood  that  is  clamped  in  a  suitable  position  to  the  edge  of 
the  table.  The  diameter  of  the  blind  spot,  as  thus  determined,  has  been  found 
to  correspond  to  a  visual  angle  varying  from  3°  39'  to  9°  47',  the  average 
measurement  being  6°  10'.  This  is  about  the  angle  that  is  subtended  by  the 
human  face  seen  at  a  distance  of  two  meters.  Although  a  considerable  por- 
tion of  the  retina  is  thus  insensible  to  light,  we  are,  in  the  ordinary  use  of  the 
eyes,  conscious  of  no  corresponding  blank  in  the  field  of  vision.  By  what 
psychical  operation  we  "  fill  up "  the  gap  in  our  subjective  field  of  vision 
caused  by  the  blind  spot  of  the  retina  is  a  question  that  has  been  much  dis- 
cussed without  being  definitely  settled. 

The  above-mentioned  reasons  for  regarding  the  rods  and  cones  as  the  light- 
perceiving  elements  of  the  retina  seem  sufficiently  conclusive.  Whether  there 
is  any  difference  between  the  rods  and  the  cones  with  regard  to  their  light- 
perceiving  function  is  a  question  which  may  be  best  considered  in  connection 
with  a  description  of  the  qualitative  modifications  of  light. 

The  histological  relation  between  the  various  layers  of  the  retina  is  still 
under  discussion.  According  to  recent  observations  of  Cajal,1  the  connection 
between  the  rods  and  cones  on  the  one 
side  and  the  fibres  of  the  optic  nerve 
on  the  other  is  established  in  a  man- 
ner which  is  represented  diagram- 
matically  in  Figure  147.  The  pro- 
longations of  the  bipolar  cells  of  the 
internal  nuclear  layer  E  break  up  into 
fine  fibres  in  the  external  molecular 
(or  plexiform)  layer  (7.  Here  they  are 
brought  into  contact,  though  not  into 
anatomical  continuity,  with  the  termi- 
nal fibres  of  the  rods  and  cones.  The 
inner  prolongations  of  the  same  bipolar 
cells  penetrate  into  the  internal  molec- 
ular (or  plexiform)  layer  F,  and  there 
come  into  contact  with  the  dendrites 
coming  from  the  layer  of  ganglion-cells 

G.     These  cells  are,  in  their  turn,  con- 

*  ' 

nected  by  their  axis-cylinder  processes 
with  the  fibres  of  the  optic  nerve.  The 
bipolar  cells  which  serve  as  connective 
links  between  the  rods  and  the  optic 
nerve-fibres  are  anatomically  distin- 
guishable (as  indicated  in  the  diagram) 


FIG.  147.— Diagrammatic  representation  of  the 
structure  of  the  retina  (Cajal) :  A,  layer  of  rods 
and  cones ;  B,  external  nuclear  layer ;  C,  external 
molecular  (or  plexiform)  layer;  E,  internal  nu- 
clear layer ;  F,  internal  molecular  (or  plexiform) 
layer;  6,  layer  of  ganglion-cells;  H,  layer  of 
nerve-fibres. 


Die  Retina  der  Wirbeltkiere,  Wiesbaden,  1894. 


330  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

from  those  which  perform  the  same  function  for  the  cones.  Whatever  be  the 
precise  mode  of  connection  between  the  rods  and  cones  and  the  fibres  of  the 
optic  nerve,  it  is  evident  that  each  retinal  element  cannot  be  connected  with 
the  nerve-centres  by  a  separate  independent  nerve-channel,  since  the  retina 
contains  many  millions  of  rods  and  cones,  while  the  optic  nerve  has  only 
about  438,000  nerve-fibres,1  though  of  course  such  a  connection  may  exist  in 
the  fovea  centralis,  as  Cajal  has  shown  is  probably  the  case  in  reptiles  and  birds. 
Changes  Produced  in  the  Retina  by  Light. — We  must  now  inquire 
what  changes  can  be  supposed  to  occur  in  the  rods  and  cones  under  the  influ- 
ence of  light  by  means  of  which  they  are  able  to  transform  the  energy  of  the 
ether  vibrations  into  a  stimulus  for  the  fibres  of  the  optic  nerve.  Though  in 
the  present  state  of  our  knowledge  no  satisfactory  answer  can  be  given  to  this 
question,  yet  certain  direct  effects  of  light  upon  the  retina  have  been  observed 
which  are  doubtless  associated  in  some  way  with  the  transformation  in 
question. 

The  retina  of  an  eye  which  has  been  protected  from  light  for  a  considerable 
length  of  time  has  a  purplish-red  color,  which  upon  exposure  to  light  changes 
to  yellow  and  then  fades  away.  This  bleaching  occurs  also  in  monochromatic 
light,  the  most  powerful  rays  being  those  of  the  greenish-yellow  portion  of 
the  spectrum — i.  e.  those  rays  which  are  most  completely  absorbed  by  the  pur- 
plish-red coloring  matter.  A  microscopic  examination  of  the  retina  shows 
that  this  coloring  matter,  which  has  been  termed  visual  purple,  is  entirely  con- 
fined to  the  outer  portion  of  the  retinal  rods  and  does  not  occur  at  all  in  the 
cones.  After  being  bleached  by  light  it  is,  during  life,  restored  through  the 
agency  of  the  pigment  epithelium,  the  cells  of  which,  under  the  influence  of 
light,  send  their  prolongations  inward  to  envelop  the  outer  limbs  of  the  rods 
and  cones  with  pigment.  If  an  eye,  either  excised  or  in  its  natural  position, 
is  protected  from  light  for  a  time,  and  then  placed  in  such  a  position  that  the 
image  of  a  lamp  or  a  window  is  thrown  upon  the  retina  for  a  time  which  may 
vary  with  the  amount  of  light  from  seven  seconds  to  ten  minutes,  it  will  be 
found  that  the  retina,  if  removed  and  examined  under  red  light,  will  show  the 
image  of  the  luminous  object  impressed  upon  it  by  the 
bleaching  of  the  visual  purple. 

If  the  retina  be  treated  with  a  4  per  cent,  solution  of 
alum,  the  restoration  of  the  visual  purple  will  be  pre- 
vented, and  the  so-called  "  optogram "  will  be,  as  pho- 
tographers say,  "  fixed." 2 
FIG.  i48.-optogram  in  eye          Figure  148  shows  the  appearance  of  a  rabbit's  retina 

of  rabbit  (Kuhne).  °  «  *  •    j         11 

on  which  the  optogram  of  a  window  has  been  impressed. 

Although  the  chemical  changes  in  the  visual  purple  under  the  influence  of 

light  seem,  at  first  sight,  to  aiford  an  explanation  of  the  transformation  of  the 

vibrations  of  the  luminiferous  ether  into  a  stimulation  for  the  optic  nerve,  yet 

the  fact  that  vision  is  most  distinct  in  the  fovea  centralis  of  the  retina,  which, 

1  Salzer :  Wiener  Sitzungsberichte,  1880,  Bd.  Ixxxi.  S.  3. 

2  Kiihne :  Untersuchungen  a.  d.  phys.  Inst.  d.  Universitdt  Heidelberg,  i.  1. 


THE  SENSE   OF    VISION.  331 

as  it  contains  no  rods,  is  destitute  of  visual  purple,  makes  it  impossible  to 
regard  this  coloring  matter  as  essential  to  vision.  The  most  probable  theory 
of  its  function  is  perhaps  that  which  connects  it  with  the  adaptation  of  the 
eye  to  varying  amounts  of  light,  as  described  on  p.  326. 

In  addition  to  the  above-mentioned  movements  of  the  pigment  epithelium 
cells  under  the  influence  of  light,  certain  changes  in  the  retinal  cones  of  frogs 
and  fishes  have  been  observed.1  The  change  consists  in  a  shortening  and  thick- 
ening of  the  inner  portion  of  the  cones  when  illuminated,  but  the  relation  of 
the  phenomenon  to  vision  has  not  been  explained. 

Like  most  of  the  living  tissues  of  the  body,  the  retina  is  the  seat  of  electri- 
cal currents.  In  repose  the  fibres  of  the  optic  nerve  are  said  to  be  positive  in 
relation  to  the  layer  of  rods  and  cones.  When  light  falls  upon  the  retina  this 
current  is  at  first  increased  and  then  diminished  in  intensity. 

Sensation  of  Light. — Whatever  view  may  be  adopted  with  regard  to  the 
mechanism  by  which  light  is  enabled  to  become  a  stimulus  for  the  optic  nerve, 
the  fundamental  fact  remains  that  the  retina  (and  in  all  probability  the  layer 
of  rods  and  cones  in  the  retina)  alone  supplies  the  conditions  under  which  this 
transformation  of  energy  is  possible.  But  in  accordance  with  the  "  law  of 
specific  energy  "  a  sensation  of  light  may  be  produced  in  whatever  way  the 
optic  nerve  be  stimulated,  for  a  stimulus  reaching  the  visual  centres  through 
the  optic  nerve  is  interpreted  as  a  visual  sensation,  in  the  same  way  that 
pressure  on  a  nerve  caused  by  the  contracting  cicatrix  of  an  amputated  leg 
often  causes  a  painful  sensation  which  is  referred  to  the  lost  toes  to  which  the 
nerve  was  formerly  distributed.  Thus  local  pressure  on  the  eyeball  by  stimu- 
lating the  underlying  retina  causes  luminous  sensations,  already  described  as 
"  phosphenes,"  and  electrical  stimulation  of  the  eye  as  a  whole  or  of  the  stump 
of  the  optic  nerve  after  the  removal  of  the  eye  is  found  to  give  rise  to  sensa- 
tions of  light. 

Vibrations  of  the  luminiferous  ether  constitute,  however,  the  normal  stirn- 
ulusof  the  retina,  and  we  must  now  endeavor  to  analyze  the  sensation  thus 
produced.  In  the  first  place,  it  must  be  borne  in  mind  that  the  so-called  ether 
waves  differ  among  themselves  very  widely  in  regard  to  their  rate  of  oscilla- 
tion. The  slowest  known  vibrations  of  the  ether  molecules  have  a  frequency 
of  about  107,000,000,000,000  in  a  second,  and  the  fastest  a  rate  of  about 
40,000,000,000,000,000  in  a  second — a  range,  expressed  in  musical  terms,  of 
about  eight  and  one-half  octaves.  All  these  ether  waves  are  capable  of  warm- 
ing bodies  upon  which  they  strike  and  of  breaking  up  certain  chemical  com- 
binations, the  slowly  vibrating  waves  being  especially  adapted  to  produce  the 
former  and  the  rapidly  vibrating  ones  the  latter  effect.  Certain  waves  of 
intermediate  rates  of  oscillation — viz.  those  ranging  between  392,000,000,- 
000,000  and  757,000,000,000,000  in  a  second — not  only  produce  thermic  and 
chemical  effects,  but  have  the  power,  when  they  strike  the  retina,  of  causing 
changes  in  the  layer  of  rods  and  cones,  which,  in  their  turn,  act  as  a  stimulus 
to  the  optic  nerve.  The  ether  waves  which  produce  these  various  phenomena 
are  often  spoken  of  as  heat  rays,  light  rays,  and  actinic  or  chemical  rays,  but 
1  Engelmann  :  Archiv  fur  die  yemmmte  Physiologic,  xxxv.  498. 


332  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

it  must  be  remembered  that  the  same  wave  may  produce  all  three  classes  of 
phenomena,  the  effect  depending  upon  the  nature  of  the  substance  upon  which 
it  strikes.  It  will  be  observed  that  the  range  of  vibrations  capable  of  affecting 
the  retina  is  rather  less  than  one  octave,  a  limitation  which  obviously  tends  to 
reduce  the  amount  of  chromatic  aberration. 

In  this  connection  it  is  interesting  to  notice  that  the  highest  audible  note  is 
produced  by  about  40,000  sonorous  impulses  in  a  second.  Between  the  high- 
est audible  note  and  the  lowest  visible  color  there  is  a  gap  of  nearly  thirty-four 
octaves  in  which  neither  the  vibrations  of  the  air  nor  those  of  the  luminifer- 
ous  ether  affect  our  senses.  Even  if  the  slowly  vibrating  heat-rays  which 
affect  our  cutaneous  nerves  are  taken  into  account,  there  still  remain  over 
thirty-one  octaves  of  vibrations,  either  of  the  air  or  of  the  luminiferous  ether, 
which  may  be,  and  very  likely  are,  filling  the  universe  around  us  without  in 
any  way  impressing  themselves  upon  our  consciousness.1 

Qualitative  Modifications  of  Light. — All  the  ethereal  vibrations  which 
are  capable  of  affecting  the  retina  are  transmitted  with  very  nearly  the  same 
rapidity  through  air,  but  when  they  enter  a  denser  medium  the  waves  having 
a  rapid  vibration  are  retarded  more  than  those  vibrating  more  slowly.  Hence 
when  a  ray  of  sunlight  composed  of  all  the  visible  ether  waves  strikes  upon  a 

plane  surface  of  glass,  the  greater 
retardation  of  the  waves  of  rapid 
vibration  causes  them  to  be  more 
refracted  than  those  of  slower  vibra- 
tion, and  if  the  glass  has  the  form 
of  a  prism,  as  shown  in  Figure  149, 
this  so-called  "dispersion"  of  the 
rays  is  still  further  increased  when 
the  rays  leave  the  glass,  so  that  the 
emerging  beam,  if  received  upon  a 

FIG.  149.-Diagram  "^straUngthe  dispersion  of  light      whUe  ^^  ^^  of  forming  a 

spot  of  white  light,  produces  a  band 

of  color  known  as  the  solar  spectrum.  The  colors  of  the  spectrum,  though 
commonly  spoken  of  as  seven  in  number,  really  form  a  continuous  series  from 
the  extreme  red  to  the  extreme  violet,  these  colors  corresponding  to  ether  vibra- 
tions with  rates  of  392,000,000,000,000  and  757,000,000,000,000  in  1  second, 
and  wave  lengths  of  0.7667  and  0.3970  micromilli meters2  respectively. 

Colors,  therefore,  are  sensations  caused  by  the  impact  upon  the  retina  of 
certain  ether  waves  having  definite  frequencies  and  wave-lengths,  but  these 
are  not  the  only  peculiarities  of  the  ether  vibration  which  influence  the  retinal 
sensation.  The  energy  of  the  vibration,  or  the  vis  viva  of  the  vibrating  mole- 
cule, determines  the  "  intensity  "  of  the  sensation  or  the  brilliancy  of  the  light.3 

1  The  vibrations  of  electrical  energy   utilized  in  wireless  telegraphy  are  probably  inter- 
mediate in  their  rate  between  those  of  sound  and  light. 

2  One  micromillimeter  =  0.001  millimeter  —  one  /". 

8  The  energy  of  vibration  capable  of  producing  a  given  subjective  sensation  of  intensity 
varies  with  the  color  of  the  light,  as  will  be  later  explained  (see  p.  340). 


THE  SENSE    OF    VISION.  333 

Furthermore,  the  sensation  produced  by  the  impact  of  ether  waves  of  a  definite 
length  will  vary  according  as  the  eye  is  simultaneously  affected  by  a  greater  or 
less  amount  of  white  light.  This  modification  of  the  sensation  is  termed  its 
degree  of  "  saturation/7  light  being  said  to  be  completely  saturated  when  it  is 
"  monochromatic  "  or  produced  by  ether  vibrations  of  a  single  wave-length. 

The  modifications  of  light  which  taken  together  determine  completely  the 
character  of  the  sensation  are,  then,  three  in  number — viz. :  1.  Color,  depend- 
ent upon  rate  of  vibration  or  length  of  the  ether  wave  ;  2.  Intensity,  dependent 
upon  the  energy  of  the  vibration ;  3.  Saturation,  dependent  upon  the  amount 
of  white  light  mingled  with  the  monochromatic  light.  These  three  qualitative 
modifications  of  light  must  now  be  considered  in  detail. 

Color. — In  our  profound  ignorance  of  the  nature  of  the  process  by  which, 
in  the  rods  and  cones,  the  movements  of  the  ether  waves  are  converted  into  a 
stimulus  for  the  optic  nerve-fibres,  all  that  can  be  reasonably  demanded  of  a 
color  theory  is  that  it  shall  present  a  logically  consistent  hypothesis  to  account 
for  the  sensations  actually  produced  by  the  impact  of  ether  waves  of  varying 
rates,  either  singly  or  combined,  upon  different  parts  of  the  retina.  Some  of 
the  important  phenomena  of  color  sensation  of  which  every  color  theory  must 
take  account  may  be  enumerated  as  follows : 

1.  Luminosity  is  more  readily  recognized  than  color.     This  is  shown  by 
the  fact  that  a  colored  object  appears  colorless  when  it  is  too  feebly  illuminated, 
and  that  a  spectrum  produced  by  a  very  feeble  light  shows  variations  of  inten- 
sity with  a  maximum  nearer  than  normal  to  the  blue  end,  but  no  gradations 
of  color.    A  similar  lack  of  color  is  noticed  when  a  colored  object  is  observed 
for  too  short  a  time  or  when  it  is  of  insufficient  size.     In  all  these  respects 
the  various  colors  present  important  individual  differences  which  will  be  con- 
sidered later. 

2.  Colored  objects  seen  with  increasing  intensity  of  illumination  appear 
more  and  more  colorless,  and  finally  present  the  appearance  of  pure  white. 
Yellow  passes  into  white  more  readily  than  the  other  colors. 

3.  The  power  of  the  retina  to  distinguish  colors  diminishes  from  the  centre 
toward  the  periphery,  the  various  colors,  in  this  respect  also,  differing  mate- 
rially from  each  other.     Sensibility  to  red  is  lost  at  a  short  distance  from  the 
macula  lutea,  while  the  sensation  of  blue  is  lost  only  on  the  extreme  lateral 
portions  of  the  retina.     The  relation  of  this  phenomenon  to  the  distribution 
of  the  rods  and  cones  in  the  retina  will  be  considered  in  connection  with  the 
perception  of  the  intensity  of  light. 

Color-mixture. — Since  the  various  spectral  colors  are  produced  by  the  dis- 
persion of  the  white  light  of  the  sun,  it  is  evident  that  white  light  may  be 
reproduced  by  the  reunion  of  the  rays  corresponding  to  the  different  colors,  and 
it  is  accordingly  found  that  if  the  colored  rays  emerging  from  a  prism,  as  in 
Fig.  149,  are  reunited  by  suitable  refracting  surfaces,  a  spot  of  white  light  will  be 
produced  similar  to  that  which  would  have  been  caused  by  the  original  beam 
of  sunlight.  But  white  light  may  be  produced  not  only  by  the  union  of  all 
the  spectral  colors,  but  by  the  union  of  certain  selected  colors  in  twos,  threes, 


334  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

fours,  etc.  Any  two  spectral  colors  which  by  their  union  produce  white  are 
said  to  be  "  complementary  "  colors.  The  relation  of  these  pairs  of  comple- 
mentary colors  to  each  other  may  be  best  understood  by  reference  to  Figure  150. 


p 

FIG.  150.— Color  diagram. 

Here  the  spectral  colors  are  supposed  to  be  disposed  around  a  curved  line, 
as  indicated  by  their  initial  letters,  and  the  two  ends  of  the  curve  are  united 
by  a  straight  line,  thus  enclosing  a  surface  having  somewhat  the  form  of  a  tri- 
angle with  a  rounded  apex.  If  the  curved  edge  of  this  surface  be  supposed  to 
be  loaded  with  weights  proportionate  to  the  luminosity  of  the  different  colors, 
the  centre  of  gravity  of  the  surface  will  be  near  the  point  W.  Now,  if  a 
straight  line  be  drawn  from  any  point  on  the  curved  line  through  the  point 
W  and  prolonged  till  it  cuts  the  curve  again,  the  colors  corresponding  to  the 
two  ends  of  this  straight  line  will  be  complementary  colors.  Thus  in  Figure 
150  it  will  be  seen  that  the  complementary  color  of  red  is  bluish-green,  and 
that  of  yellow  lies  near  the  indigo.  It  is  also  evident  that  the  complementary 
color  of  green  is  purple,  which  is  not  a  spectral  color  at  all,  but  a  color 
obtained  by  the  union  of  violet  and  red.  The  union  of  a  pair  of  colors 
lying  nearer  together  than  complementary  colors  produces  an  intermediate  color 
mixed  with  an  amount  of  white  which  is  proportionate  to  the  nearness  of  the 
colors  to  the  complementary.  Thus  the  union  of  red  and  yellow  produces 
orange,  but  a  less  saturated  orange  than  the  spectral  color.  The  union  of  two 
colors  lying  farther  apart  than  complementary  colors  produces  a  color  which 
borders  more  or  less  upon  purple. 

The  mixing  of  colors  to  demonstrate  the  above-mentioned  effects  may  be 
accomplished  in  three  different  ways  : 

1.  By  employing  two  prisms  to  produce  two  independent  spectra,  and  then 
directing  the  colored  rays  which  are  to  be  united  so  that  they  will  illuminate 
the  same  white  surface. 

2.  By  looking  obliquely  through  a  glass  plate  at  a  colored  object  placed 
behind  it,  while  at  the  same  time  light  from  another  colored  object,  placed  in 
front  of  the  glass,  is  reflected  into  the  eye  of  the  observer,  as  shown  in  Figure 
151.     Here  the  transmitted  light  from  the  colored  object  A  and  the  reflected 
light  from  the  colored  object  B  enter  the  eye  at  C  from  the  same  direction, 
and  are  therefore  united  upon  the  retina. 

3.  By  rotating  before  the  eye  a  disk  on  which  the  colors  to  be  united  are 
painted  upon  different  sectors.     This  is  most  readily  accomplished  by  using 


THE  SENSE    OF    VISION.  335 

a  number  of  disks,  each  painted  with  one  of  the  colors  to  be  experimented 
with,  and  each  divided  radially  by  a  cut  running  from  the  centre  to  the  circum- 
ference. The  disks  can  then  be  lapped  over  each  other  and  rotated  together,  and 
in  this  way  two  or  more  colors  can  be  mixed  in  any  desired  proportions.  This 
method  of  mixing  colors  depends  upon 
the  property  of  the  retina  to  retain  an 
impression  after  the  stimulus  causing  JV 


it  has  ceased  to  act — a  phenomenon  of  / 

great  importance  in  physiological  optics,  / 

and  one  which  will  be  further  discussed  / 


in  connection  with  the  subject  of  "  after-                  /                               \ 
images."  ^/ \fl 

The    physiological    mixing  of  Colors      FIG.  151.— Diagram  to  illustrate  color  mixture  by 

cannot  be  accomplished  by  the  mixture 

of  pigments  or  by  allowing  sunlight  to  pass  successively  through  glasses  of 
different  colors,  for  in  these  cases  rays  corresponding  to  certain  colors  are 
absorbed  by  the  medium  through  which  the  white  light  passes,  and  the  phe- 
nomenon is  the  result  of  a  process  of  subtraction  and  not  addition.  Light 
reaching  the  eye  through  red  glass,  for  instance,  looks  red  because  all  the  rays 
except  the  red  rays  are  absorbed,  and  light  coming  through  green  glass  appears 
green  for  a  similar  reason.  Now,  when  light  is  allowed  to  pass  successively 
through  red  and  green  glass  the  only  rays  which  pass  through  the  red  glass 
will  be  absorbed  by  the  green.  Hence  no  light  will  pass  through  the  combi- 
nation of  red  and  green  glass,  and  darkness  results.  But  when  red  and  green 
rays  are  mixed  by  any  of  the  three  methods  above  described  the  result  of  this 
process  of  addition  is  not  darkness,  but  a  yellow  color,  as  will  be  understood 
by  reference  to  the  color  diagram  on  p.  334.  In  the  case  of  colored  pigments 
similar  phenomena  occur,  for  here  too  light  reaches  the  eye  after  rays  of  cer- 
tain wave-lengths  have  been  absorbed  by  the  medium.  This  subject  will  be 
further  considered  in  connection  with  color-theories.1 

Color-theories. — From  what  has  been  said  of  color-mixtures  it  is  evident 
that  every  color  sensation  may  be  produced  by  the  mixture  of  a  number  of 
other  color  sensations,  and  that  certain  color  sensations — viz.  the  purples — can 
be  produced  only  by  the  mixture  of  other  sensations,- since  there  is  no  single 
wave-length  corresponding  to  them.  Hence  the  hypothesis  is  a  natural  one 
that  all  colors  are  produced  by  the  mixture  in  varying  proportions  of  a  certain 
number  of  fundamental  colors,  each  of  which  depends  for  its  production  upon 
the  presence  in  the  retina  of  a  certain  substance  capable  of  being  affected 
(probably  through  some  sort  of  a  photo-chemical  process)  by  light  of  a  certain 
definite  wave-length.  A  hypothesis  of  this  sort  lies  at  the  basis  of  both  the 
Young-Helmholtz  and  the  Hering  theories  of  color  sensation. 

The  former  theory  postulates  the  existence  in  the  retina  of  three  substances 
capable  of  being  affected  by  red,  green,  and  violet  rays,  respectively — i.  e.  by 
the  three  colors  lying  at  the  three  angles  of  the  color  diagram  given  on  p.  334 

1  For  an  interesting  discussion  of  modern  theories  of  color-vision,  see  the  address  of  Professor 
Frank  P.  Whitman  on  "  Color-vision,"  Science,  Sept,  9,  1898. 


336  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

s . 

— and  regards  all  other  color  sensations  as  produced  by  the  simultaneous  affec- 
tion of  two  of  these  substances  in  varying  proportions.  Thus  when  a  ray  of 
blue  light  falls  on  the  retina  it  stimulates  the  violet-  and  green-perceiving  sub- 
stances, and  produces  a  sensation  intermediate  between  the  two,  while  simul- 
taneous stimulation  of  the  red-  and  green-perceiving  substances  produces  the 
sensations  corresponding  to  yellow  and  orange ;  and  when  the  violet-  and  red- 
perceiving  substances  are  affected  at  the  same  time,  the  various  shades  of 
purple  are  produced.  Each  of  these  three  substances  is,  however,  supposed  to 
be  affected  to  a  slight  extent  by  all  the  rays  of  the  visible  spectrum,  a  suppo- 
sition which  is  rendered  necessary  by  the  fact  that  even  the  pure  spectral 
colors  do  not  appear  to  be  perfectly  saturated,  as  will  be  explained  in  connec- 
tion with  the  subject  of  saturation.  Furthermore,  the  disappearance  of  color 
when  objects  are  very  feebly  or  very  brightly  illuminated  or  when  they  are 
seen  with  the  lateral  portions  of  the  retina  (as  described  on  p.  333)  necessitates 
the  additional  hypotheses  that  these  three  substances  are  all  equally  affected  by 
all  kinds  of  rays  when  the  light  is  of  either  very  small  or  very  great  intensity 
or  when  it  falls  on  the  extreme  lateral  portions  of  the  retina,  and  that  they 
manifest  their  specific  irritability  for  red,  green,  and  violet  rays  respectively 
only  in  light  of  moderate  intensity  falling  not  too  far  from  the  fovea  centralis 
of  the  retina. 

The  modifications  of  the  Young- Hem holtz  theory  introduced  by  these  sub- 
idiary  hypotheses  greatly  diminish  the  simplicity  which  was  its  chief  claim  to 
acceptance  when  originally  proposed.  Moreover,  there  will  always  remain  a 
psychological  difficulty  in  supposing  that  three  sensations  so  different  from  each 
other  as  those  of  red,  green,  and  violet  can  by  their  union  produce  a  fourth 
sensation  absolutely  distinct  from  any  of  them — viz.  white. 

The  fact  that  in  the  Hering  theory  this  difficulty  is  obviated  has  contributed 
greatly  to  its  acceptance  by  physiologists.  In  this  theory  the  retina  is  supposed 
to  contain  three  substances  in  which  chemical  changes  may  be  produced  by  etherj 
vibrations,  but  each  of  these  substances  is  supposed  to  be  affected  in  two  op] 
site  ways  by  rays  of  light  which  correspond  to  complementary  color  sensa-' 
tions.  Thus  in  one  substance— viz.  the  white-black  visual  substance — kata- 
bolic  or  destructive  changes  are  supposed  to  be  produced  by  all  the  rays  of  the 
visible  spectrum,  the  maximum  effect  being  caused  by  the  yellow  rays,  while 
anabolic  or  constructive  changes  occur  when  no  light  at  all  falls  upon  the 
retina.  The  chemical  changes  of  this  substance  correspond,  therefore,  to  the 
sensation  of  luminosity  as  distinguished  from  color.  In  a  second  substance  red 
rays  are  supposed  to  produce  katabolic,  and  green  rays  anabolic  changes,  while 
a  third  substance  is  similarly  affected  by  yellow  and  blue  rays.  These  two 
substances  are  therefore  spoken  of  as  red-green  and  yellow-blue  visual  sub- 
stances respectively. 

It  has  been  sometimes  urged  as  an  objection  to  this  theory  that  the  effect  of 
a  stimulus  is  usually  katabolic  and  not  anabolic.  This  is  true  with  regard  to 
muscular  contraction,  from  the  study  of  which  phenomenon  most  of  our  know- 
ledge of  the  effect  of  stimulation  has  been  obtained,  but  it  should  be  remem- 


THE  SENSE    OF    VISION.  337 

bered  that  observations  on  the  augmentor  and  inhibitory  cardiac  nerves  have 
shown  us  that  nerve-stimulation  may  produce  very  contrary  effects.  There 
seems  to  be,  therefore,  no  serious  theoretical  difficulty  in  supposing  that  light 
rays  of  different  wave-lengths  may  produce  opposite  metabolic  effects  upon  the 
substances  in  which  changes  are  associated  with  visual  sensations. 

A  more  serious  objection  lies  in  the  difficulty  of  distinguishing  between  the  /  j 
sensation  of  blackness,  which,  on  Hering's  hypothesis,  must  correspond  to  active  I     ^ 
anabolism  of  the  white-black  substance,  and  the  sensation  of  darkness  (such  as    )   \ 
we  experience  when  the  eyes  have  been  withdrawn  for  some  time  from  the  /   ^ 
influence  of  light),  which  must  correspond  to  a  condition  of  equilibrium  of   ) 
the   white-black    substance   in  which    neither  anabolism  nor   katabolism    is  / 
occurring. 

Another  objection  to  the  Hering  theory  is  to  be  found  in  the  results  of 
experiments  in  comparing  grays  or  whites  produced  by  mixing  different  colored 
rays  under  varying  intensities  of  light.  The  explanation  given  by  Hering  of 
the  production  of  white  through  the  mixture  of  blue  and  yellow  or  of  red  and 
green  is  that  when  either  of  these  pairs  of  complementary  colors  is  mixed 
the  anabolic  and  the  katabolic  processes  balance  each  other,  leaving  the  corre- 
sponding visual  substance  in  a  condition  of  equilibrium.  Hence,  the  white- 
black  substance  being  alone  stimulated,  the  result  will  be  a  sensation  of  white 
corresponding  to  the  intensity  of  the  katabolic  process  caused  by  the  mixed 
rays.  Now,  it  is  found  that  when  blue  and  yellow  are  mixed  in  certain  pro- 
portions on  a  revolving  disk  a  white  can  be  produced  which  will,  with  a  certain 
intensity  of  illumination,  be  undistinguishable  from  a  white  produced  by  mix- 
ing red  and  green.  If,  however,  the  intensity  of  the  illumination  is  changed, 
it  will  be  found  necessary  to  add  a  certain  amount  of  white  to  one  of  the  mix- 
tures in  order  to  bring  them  to  equality.  On  the  theory  that  complementary 
colors  produce  antagonistic  processes  in  the  retina  it  is  difficult  to  understand 
why  this  should  be  the  case.1 

A  color  theory  which  is  in  some  respects  more  in  harmony  with  recent/ 
observations  in  the  physiology  of  vision  has  been  proposed  by  Mrs.  C.  LJ  jh  /• 
Franklin.     In  this  theory  it  is  supposed  that,  in  its  earlier  periods  of  deA 
velopment,  the  eye  is  sensitive  only  to  luminosity  and  not  to  color — i.  e.  it 
possesses  only  a  white-black  or  (to  use  a  single  word)  a  gray-perceiving  sub- 
stance which  is  affected  by  all  visible  light  rays,  but  most  powerfully  by  those 
lying  near  the  middle  of  the  spectrum.     The  sensation  of  gray  is  supposed  to 
be  dependent  upon  the  chemical  stimulation  of  the  optic  nerve-terminations  by 
some  product  of  decomposition  of  this  substance. 

In  the  course  of  development  a  portion  of  this  gray  visual  substance  becomes 
differentiated  into  three  different  substances,  each  of  which  is  affected  by  rays 
of  light  corresponding  to  one  of  the  three  fundamental  colors  of  the  spectrum 
—viz.  red,  green,  and  blue.     This  differentiation  may  be  supposed  to  occur  in  \ 
the  cones  rather  than  in  the  rods,  which  thus  become  organs  specially  adapted  j 

1  The  renewal  of  the  rod  pigment  in  a  dim  light  may  afford  an  explanation  of  this  phenom- 
enon (see  C.  Ladd  Franklin  :  Psychological  Review,  v.  311). 
VOL.  II.— 22 


338  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

for  the  perception  of  color  (see  p.  342).  When  a  ray  of  light  intermediate 
between  two  of  the  fundamental  colors  falls  upon  the  retina,  the  visual  sub- 
stances corresponding  to  these  two  colors  will  be  affected  to  a  degree  pro- 
portionate to  the  proximity  of  these  two  colors  to  that  of  the  incident  ray. 
Since  this  effect  is  exactly  the  same  as  that  which  is  produced  when  the  retina 
is  acted  upon  simultaneously  by  light  of  two  fundamental  colors,  we  are  incap- 
able of  distinguishing  in  sensation  between  an  intermediate  wave-length  and 
a  mixture  in  proper  amounts  of  two  fundamental  wave-lengths. 

When  the  retina  is  affected  by  two  or  more  rays  of  such  wave-lengths  that 
all  three  of  the  color  visual  substances  are  equally  affected,  the  resulting  decom- 
position will  be  the  same  as  that  produced  by  the  stimulation  of  the  gray  visual 
substance  out  of  which  the  color  visual  substances  were  differentiated,  and  the 
corresponding  sensation  will  therefore  be  that  of  gray  or  white. 

It  will  be  noticed  that  the  important  feature  of  this  theory  is  that  it  pro- 
vides for  the  independent  existence  of  the  gray  visual  substance,  while  pt  the 
same  time  the  stimulation  of  this  substance  is  made  a  necessary  result  of  the 
mixture  of  certain  color  sensations. 

Another  color  theory  has  recently  been  brought  forward  by  Prof.  G.  E. 
Miiller,1  who  substitutes  for  Hering's  antagonistic  processes  of  assimilation  and 
dissimilation  the  conception  of  "  reversible  chemical  actions " — i.  e.  actions 
in  which  the  products  of  a  chemical  change  can  be  used  for  the  reconstruc- 
tion of  the  original  substance. 

Color-blindness. — The  fact  that  many  individuals  are  incapable  of  distin- 
guishing between  certain  colors — i.  e.  are  more  or  less  "  color-blind  " — is  one 
of  fundamental  importance  in  the  discussion  of  theories  of  color  vision.  By 
far  the  most  common  kind  of  color-blindness  is  that  in  which  certain  shades 
of  red  and  green  are  not  recognized  as  different  colors.  The  advocates  of  the 
Young-Helmholtz  theory  explain  such  cases  by  supposing  that  either  the  red 
or  the  green  perceiving  elements  of  the  retina  are  deficient,  or,  if  present,  are 
irritable,  not  by  rays  of  a  particular  wave-length,  but  by  all  the  rays  of  the 
visible  spectrum.  In  accordance  with  this  view  these  cases  of  color-blindness 
are  divided  into  two  classes — viz.  the  red-blind  and  the  green-blind — the  basis 
for  the  classification  being  furnished  by  more  or  less  characteristic  curves  repre- 
senting the  variations  in  the  luminosity  of  the  visible  spectrum  as  it  appears 
to  the  different  eyes.  There  are,  however,  cases  which  cannot  easily  be  brought 
under  either  of  these  two  classes.  Moreover,  it  has  been  proved  in  cases  of 
monocular  color-blindness,  and  is  admitted  even  by  the  defenders  of  the  Helm- 
holtz  theory,  that  such  persons  see  really  only  two  colors — viz.  blue  and  yellow. 
To  such  persons  the  red  end  of  the  spectrum  appears  a  dark  yellow,  and  the 
green  portion  of  the  spectrum  has  luminosity  without  color. 
y  A  better  explanation  of  this  sort  of  color-blindness  is  given  in  the  Hering 
theory  by  simply  supposing  that  in  such  eyes  the  red-green  visual  substance  is 
deficient  or  wholly  wanting,  but  the  theory  of  Mrs.  Franklin  accounts  for  the 
phenomena  in  a  still  more  satisfactory  way ;  for,  by  supposing  that  the  differ- 
\f  l  Zeitschrift  fur  Psychohgie  und  Physiologic  der  Sinnesorgane,  1875  and  1897. 


THE  SENSE    OF    VISION.  339 

entiation  of  the  primary  gray  visual  substance  has  first  led  to  the  formation 
of  a  blue  and  a  yellow  visual  substance,  and  that  the  latter  has  subsequently 
been  differentiated  into  a  red  and  a  green  visual  substance,  color-blindness  is 
readily  explained  by  supposing  that  this  second  differentiation  has  either  not 
occurred  at  all  or  has  taken  place  in  an  imperfect  manner.  It  is,  in  other 
words,  an  arrest  of  development. 

Cases  of  absolute  color-blindness  occasionally  occur.  To  such  persons 
nature  appears  colorless,  all  objects  presenting  simply  differences  of  light  and 
shade. 

In  whatever  Avay  color-blindness  is  to  be  explained,  the  defect  is  one  of 
considerable  practical  importance,  since  it  renders  those  affected  by  it  incapable 
of  distinguishing  the  red  and  green  lights  ordinarily  used  for  signals.  Such 
persons  are,  therefore,  unsuitable  for  employment  as  pilots,  railway  engineers, 
etc.,  and  it  is  now  customary  to  test  the  vision  of  all  candidates  for  employment 
in  such  situations.  It  has  been  found  that  no  satisfactory  results  can  be 
reached  by  requiring  persons  to  name  colors  which  are  shown  them,  and  the 
chromatic  sense  is  now  commonly  tested  by  what  is  known  as  the  "  Holmgren 
method,"  which  consists  in  requiring  the  individual  examined  to  select  from  a 
pile  of  worsteds  of  various  colors  those  shades  which  seem  to  him  to  resemble 
standard  skeins  of  green  and  pink.  When  examined  in  this  way  about  4  per 
cent,  of  the  male  and  one-quarter  of  1  per  cent,  of  the  female  sex  are  found  to 
be  more  or  less  color-blind.  The  defect  may  be  inherited,  and  the  relatives 
of  a  color-blind  person  are  therefore  to  be  tested  with  special  care.  Since 
females  are  less  liable  to  be  affected  than  males,  it  often  happens  that  the 
daughters  of  a  color-blind  person,  themselves  with  normal  vision,  have  sons 
who  inherit  their  grandfather's  infirmity. 

Although  in  all  theories  of  color  vision  the  different  sensations  are  supposed 
to  depend  upon  changes  produced  by  the  ether  vibrations  of  varying  rates 
acting  upon  different  substances  in  the  retina,  yet  it  should  be  borne  in  mind 
that  we  have  at  present  no  proof  of  the  existence  of  any  such  substances.  The 
visual  purple — or,  to  adopt  Mrs.  Franklin's  more  appropriate  term,  "  the  rod 
pigment" — was  at  one  time  thought  to  be  such  a  substance,  but  for  the  reasons 
above  given  cannot  be  regarded  as  essential  to  vision.1 

That  a  centre  for  color  vision,  distinct  from  the  visual  centre,  exists  in  the 
cerebral  cortex  is  rendered  probable  by  the  occurrence  of  cases  of  hemianopsia 
for  colors,  and  also  by  the  experiments  of  Heidenhain  and  Cohn  on  the  influ- 
ence of  the  hypnotic  trance  upon  color-blindness. 

Intensity. — The  second  of  the  above-mentioned  qualitative  modifications  of 
light  is  its  intensity,  which  is  dependent  upon  the  energy  of  vibrations  of  the 
molecules  of  the  luminiferous  ether.  The  sensation  of  luminosity  is  not,  how- 
ever, proportionate  to  the  intensity  of  the  stimulus,  but  varies  in  such  a  way 
that  a  given  increment  of  intensity  causes  a  greater  difference  in  sensation  with 

1  In  a  recently  developed  theory  by  Ebbinghaus  (Zeitschrift  fur  Psychologic  und  Physiologic 
der  Sinnesorgane,  v.  145)  a  physiological  importance  in  relation  to  vision  is  attached  to  this 
substance  in  connection  with  other  substances  of  a  hypothetical  character. 


340  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

feeble  than  with  strong  illuminations.  This  phenomenon  is  illustrated  by  the 
disappearance  of  a  shadow  thrown  by  a  candle  in  a  darkened  room  on  a  sheet 
of  white  paper  when  sunlight  is  allowed  to  fall  on  the  paper  from  the  opposite 
direction.  In  this  case  the  absolute  difference  in  luminosity  between  the 
shadowed  and  unshadowed  portions  of  the  paper  remains  the  same,  but  it 
becomes  imperceptible  in  consequence  of  the  increased  total  illumination. 

Although  our  power  of  distinguishing  absolute  differences  in  luminosity 
diminishes  as  the  intensity  of  the  illumination  increases,  yet  with  regard  to 
relative  differences  no  such  dependence  exists.  On  the  contrary,  it  is  found 
within  pretty  wide  limits  that,  whatever  be  the  intensity  of  the  illumination, 
it  must  be  increased  by  a  certain  constant  fraction  of  its  total  amount  in  order 
to  produce  a  perceptible  difference  in  sensation.  This  is  only  a  special  case  of 
a  general  law  of  sensation  known  as  Weber's  law,  which  has  been  formulated 
by  Foster  as  follows  :  "  The  smallest  change  in  the  magnitude  of  a  stimulus 
which  we  can  appreciate  through  a  change  in  our  sensation  always  bears  the 
same  proportion  to  the  whole  magnitude  of  the  stimulus." 

Luminosity  of  Different  Colors. — When  two  sources  of  light  having  the 
same  color  are  compared,  it  is  possible  to  estimate  their  relative  luminosity 
with  considerable  accuracy,  a  difference  of  about  1  per  cent,  of  the  total 
luminosity  being  appreciated  by  the  eye.  When  the  sources  of  light  have 
different  colors,  much  less  accuracy  is  attainable,  but  there  is  still  a  great  differ- 
ence in  the  intensity  with  which  rays  of  light  of  different  wave-lengths  affect 
the  retina.  We  do  not  hesitate  to  say,  for  instance,  that  the  maximum 
intensity  of  the  solar  spectrum  is  found  in  the  yellow  portion,  but  it  is  import- 
ant to  observe  that  the  position  of  this  maximum  varies  with  the  illumina- 
tion. In  a  very  brilliant  spectrum  the  maximum  shifts  toward  the  orange, 
and  in  a  feeble  spectrum  (such  as  may  be  obtained  by  narrowing  the  slit  of 
the  spectroscope)  it  moves  toward  the  green.  Hence  changes  of  intensity  are 
associated  with  changes  of  color,  and,  as  Haycraft l  has  observed,  "  we  cannot 
abstract  '  brightness '  from  our  sensations  of  light  as  we  can  abstract  '  loud- 
ness'  from  our  sensations  of  sound."  The  curves  in  Figure  152  illus- 
trate this  shifting  of  the  maximum  of  luminosity  of  the  spectrum  with  vary- 
ing intensities  of  illumination.  The  abscissas  represent  wave-lengths  in 
millionths  of  a  millimeter,  and  the  ordinates  the  luminosity  of  the  different 
colors  as  expressed  by  the  reciprocal  values  of  the  width  of  the  slit  necessary 
to  give  to  the  color  under  observation  a  luminosity  equal  to  that  of  an  arbi- 
trarily chosen  standard.  The  curves  from  A  to  H  represent  the  distribution 
of  the  intensity  of  light  in  the  spectrum  with  eight  different  grades  of  illumi- 
nation. This  shifting  of  the  maximum  of  luminosity  in  the  spectrum 
explains  the  so-called  "  Purkinje's  phenomenon  " — viz.  the  changing  rela- 
tive values  of  colors  in  varying  illumination.  This  can  be  best  observed 
at  nightfall,  the  attention  being  directed  to  a  carpet  or  a  wall-paper 
the  pattern  of  which  is  made  up  of  a  number  of  different  colors.  As 
the  daylight  fades  away  the  red  colors,  which  in  full  illumination  are 

1  "Luminosity  and  Photometry,"  by  John  Berry  Haycraft:  Journal  of  Physiology,  xxi.  126. 


THE    HENSE    OF    VISION. 


341 


the  most  intense,  becomes  gradually  darker,  and  are  scarcely  to  be  distin- 
guished from  black  at  a  time  when  the  blue  colors  are  still  very  readily 
distinguished. 

Function  of  Rods  and  Cones. — There  is,  as  mentioned  on  p.  337, 
some  reason  to  suppose  that  the  rods  and  cones  have  different  functions. 
That  color  sensation  and  accuracy  of  definition  are  most  perfect  in  the 
central  portion  of  the  retina  is  shown  by  the  fact  that  when  we  desire  to 
obtain  the  best  possible  idea  of  the  form  and  color  of  an  object  we  direct 


3.8- 

3.4- 
3.2 
3. 

2.8- 

2.6  • 

12.4 

2.2 

2. 

1.8 

UK 

1.4 

1.2 

1. 

0.8 

0.6 

0.4 

0.2 


Intensity  H 
O 
F 
E 
D 

B 

A 


670  650  625  605  590  575  555  535   520  505 
BCD  E 


490  470  450  430 

F  O 

FIG.  152.— Diagram  showing  the  distribution  of  the  intensity  of  the  spectrum  as  dependent  upon  the 

degree  of  illumination  (Konig). 


our  eyes  in  such  a  way  that  the  image  falls  upon  the  foxeajcentralis  of  the 
retina.     The  1nmi'r)nai'fry  of  a  faint  object,  however,  seems  greatest  when  we 
look  not  directly  at  it,  but  a  little  tn  ^n^  sHQ  of  it.     This  can  be  readily 
observed  when  we  look  at  a  group  of  stars,  as,  for  example,  the  Pleiades. 
When  the  eyes  are  accurately  directed  to  the  stars  so  as  to  enable  us  to  count 
them,  the  total  luminosity  of  the  constellation  appears  much  less  than  when 
the  eyes  are  directed  to  a  point  a  few  degrees  to  one  side  of  the  object.     Now, 
an  examination  of  the  retina  shows  only  cones  in  the  fovea  centralis.     In  the 
immediately  adjacent  parts  a  small  number  of  rods  are  found  mingled  with 
the  cones.     In  the  lateral  portions  of  the  retina  the  rods  are  relatively  more 
numerous  than  the  cones,  and  in  the  extreme  peripheral  portions  the  rods  alone 
exist.      Hence  this  phenomenon  is  readily  explained   on  the  supposition,  | 
which  is  supported  by  Ramon  y  CajaPs l  recent  observations,  that  the  rods! 
are  a  comparatively  rudimentary  form  of  visual  apparatus,  taking  cognizance II 
1  Zeitschrift  fur  Psychologic  und  Physiologic  der  Sinnesorgane,  xvi.  S.  161. 


f 


342  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


of  the  existence  ^  light  Avith  spfHnl  reference  to  its  varying  intensity. 
and  that  the  cones  are  organs  specially  modified  for  the  localization  of 
stimuli  and  for  the  perception  of  differences  of  wave-lengths.  The  view 
that  the  rods  are  specially  adapted  for  the  perception  of  luminosity  and  the 
cones  for  that  of  color  derives  support  from  the  fact  that  in  the  retina  of  cer- 
tain nocturnal  animals  —  e.  g.  bats  and  owls  —  rods  alone  are  present.  This 
theory  has  been  further  developed  by  Von  Kries,1  who  in  a  recent  article 
describes  the  rods  as  differing  from  the  cones  in  the  following  respects  :  (1) 
They  are  color-blind  —  i.  e.  they  produce  a  sensation  of  simple  luminosity 
whatever  be  the  wave-length  of  the  light-ray  falling  on  them  ;  (2)  they  are 
more  easily  stimulated  than  the  cones,  and  are  particularly  responsive  to  light- 
waves of  short  wave-lengths  ;  (3)  they  have  the  power  of  adapting  themselves 
to  light  of  varying  intensity. 

On  this  theory  it  is  evident  that  we  must  get  the  sensation  of  white  or 
colorless  light  in  two  different  ways  :  (1)  In  consequence  of  the  stimulation 
of  the  rods  by  any  sort  of  light-rays,  and  (2)  in  consequence  of  the  stimula- 
tion of  the  cones  by  certain  combinations  of  light-rays  —  i.  e.  complementary 
colors.  In  this  double  mode  of  white  perception  lies  perhaps  the  explanation 
of  the  effect  of  varying  intensity  of  illumination  upon  the  results  of  color- 
mixtures  which  has  been  above  alluded  to  (see  p.  337)  as  an  objection  to  the 
Hering  theory.  The  so-called  "  Purkinje's  phenomenon,"  described  on  p.  340, 
is  readily  explained  in  accordance  with  this  theory,  for,  owing  to  the  greater 
irritability  of  the  rods,  the  importance  of  these  organs,  as  compared  with  the 
•cones,  in  the  production  of  the  total  visual  sensation  is  greater  with  feeble 
than  with  strong  illumination  of  the  field  of  vision.  At  the  same  time,  the 
power  of  the  rods  to  respond  particularly  to  light-rays  of  short  wave-length 
will  cause  a  greater  apparent  intensity  of  the  colors  at  the  blue  than  at  the  red 
end  of  the  spectrum.  In  this  connection  it  is  interesting  to  note  that  the  phe- 
nomenon is  said  not  to  occur  when  the  observation  is  limited  to  the  fovea 
centralis,  where  cones  alone  are  found.2 

(Saturation.  —  The  degree  of  saturation  of  light  of  a  given  color  depends,  as 
above  stated,  upon  the  amount  of  white  light  mixed  with  it.  The  quality  of 
light  thus  designated  is  best  studied  and  appreciated  by  means  of  experiments 
with  rotating  disks.  If,  for  instance,  a  disk  consisting  of  a  large  white  and  a 
small  red  sector  be  rapidly  rotated,  the  effect  produced  is  that  of  a  pale  pink 
color.  By  gradually  increasing  the  relative  size  of  the  red  sector  the  pink 
color  becomes  more  and  more  saturated,  and  finally  when  the  white  sector  is 
reduced  to  zero  the  maximum  of  saturation  is  produced.  It  must  be  borne 
in  mind,  however,  that  no  pigments  represent  completely  saturated  colors. 
Even  the  colors  of  the  spectrum  do  not  produce  a  sensation  of  absolute 
saturation,  for,  whatever  theory  of  color  vision  be  adopted,  it  is  evident  that 
all  the  color-perceiving  elements  of  the  retina  are  affected  more  or  less  by  all 
the  rays  of  light.  Thus  when  rays  of  red  light  fall  upon  the  retina  they  will 

1  Zeitschrift  fur  Psychologic  und  Physiologic  der  Sinnesorgane,  ix.  81. 

2  von  Kries:  Centralblatt  fur  Physiologic,  1896,  i. 


THE  SENSE    OF    VISION.  343 

I  stimulate  not  only  the  red-perceiving  elements,  but  to  a  slight  extent  also  (to 
use  the  language  of  the  Helmholtz  theory)  the  green-  and  violet-perceiving 
elements  of  the  retina.  The  effect  of  this  will  be  that  of  mixing  a  small 
amount  of  white  with  a  large  amount  of  red  light — i.  e.  it  will  produce  the 
sensation  of  incompletely  saturated  red  light.  This  dilution  of  the  sensation 

(can  be  avoided  only  by  previously  exhausting  the  blue-  and  green-perceiving 
elements  of  the  retina  in  a  manner  which  will  be  explained  in  connection  with 
the  phenomena  of  after-images. 

Retinal  Stimulation. — Whenever  by  a  stimulus  applied  to  an  irritable 
substance  the  potential  energy  there  stored  up  is  liberated  the  following  phe- 
nomena may  be  observed  :  1.  A  so-called  latent  period  of  variable  duration 
during  which  no  effects  of  stimulation  are  manifest ;  2.  A  very  brief  period 
during  which  the  effect  of  the  stimulation  reaches  a  maximum ;  3.  A  period 
of  continued  stimulation  during  which  the  effect  diminishes  in  consequence  of 
the  using  up  of  the  substance  containing  the  potential  energy — i.  e.  a  period 
of  fatigue ;  4.  A  period  after  the  stimulation  has  ceased  in  which  the  effect 
slowly  passes  away. 


FIG.  153.— Diagram  showing  the  effect  of  stimulation  of  an  irritable  substance. 

The  curve  drawn  by  a  muscle  in  tetanic  contraction,  as  shown  in  Figure 

153,  illustrates  this  phenomenon.    Thus,  if  A  D  represents  the  duration  of  the 
stimulation,  A  B  indicates  the  latent  period,  B  C  the  period  of  contraction, 
C  D  the  period  of  fatigue  under  stimulation,  and  D  E  the  after-effect  of 
stimulation  showing  itself  as  a  slow  relaxation.     When  light  falls  upon  the 
retina  corresponding  phenomena  are  to  be  observed. 

Latent  Period. — That  there  is  a  period  of  latent  sensation  in  the  retina 
(i.  e.  an  interval  between  the  falling  of  light  on  the  retina  and  the  beginning 
of  the  sensation)  is,  judging  from  the  analogy  of  other  parts  of  the  nervous 
system,  quite  probable,  though  its  existence  has  not  been  demonstrated. 

Rise  to  Maximum  of  Sensation. — The  rapidity  with  which  the  sensation  of 
light  reaches  its  maximum  increases  with  the  intensity  of  the  light  and  varies 
with  its  color,  red  light  producing  its  maximum  sensation  sooner  than  green 
and  blue.  Consequently,  when  the  image  of  a  white  object  is  moved  across 
the  retina  it  will  appear  bordered  by  colored  fringes,  since  the  various  con- 
stituents of  white  light  do  not  produce  their  maximum  effects  at  the  same 
time.  This  phenomena  can  be  readily  observed  when  a  disk  on  which  a 
black  and  a  white  spiral  band  alternate  with  each  other  (as  shown  in  Figure 

1 54,  A)  is  rotated  before  the  eyes.     The  white  band  as  its  image  moves  out- 
ward or  inward  over  the  retinal  surface  appears  bordered  with  colors  which 


344  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

vary  with  the  rate  of  rotation  of  the  disk  and  with  the  amount  of  exhaustion 
of  the  retina.  Chromatic  effects  due  to  a  similar  cause  are  also  to  be  seen 
when  a  disk,  such  as  is  shown  in  Figure  154,  B  (known  as  Benham's  spectrum 


A  B 

FIG.  154.— Disks  to  illustrate  the  varying  rate  at  which  colors  rise  to  their  maximum  of  sensation. 

top),  is  rotated  with  moderate  rapidity.  The  concentric  bands  of  color  appear 
in  reverse  order  when  the  direction  of  rotation  is  reversed.  The  apparent 
movement  of  colored  figures  on  a  background  of  a  different  color  when  the 
eye  moves  rapidly  over  the  object  or  the  object  is  moved  rapidly  before  the 
eye  seems  to  depend  upon  this  same  retinal  peculiarity.  The  phenomenon 
may  be  best  observed  when  small  pieces  of  bright-red  paper  are  fastened  upon 
a  bright-blue  sheet  and  the  sheet  gently  shaken  before  the  eyes.  The  red 
figures  will  appear  to  move  upon  the  blue  background.  The  effect  may  be 
best  observed  in  a  dimly-lighted  room. 

In  this  connection  should  be  mentioned  the  phenomenon  of  "  recurrent 
images  "  or  "  oscillatory  activity  of  the  retina."  l  This  may  be  best  observed 
when  a  black  disk  containing  a  white  sector  is  rotated  at  a  rate  of  about  one 
revolution  in  two  seconds.  If  the  disk  is  brightly  illuminated,  as  by  sunlight, 

and  the  eye  fixed  steadily  upon  the  axis  of  rota- 
tion, the  moving  white  sector  seems  to  have  a 
shadow  upon  it  a  short  distance  behind  its  ad- 
vancing border,  and  this  shadow  may  be  followed 
by  a  second  fainter,  and  even  by  a  third  still 
fainter  shadow,  as  shown  in  Figure  155.  The 
distance  of  the  shadows  from  each  other  and 
from  the  edge  of  the  sector  increases  with  the  rate 
of  rotation  of  the  disk  and  corresponds  to  a  time 
FIG.  i55.-To  illustrate  the  oscillatory  interval  of  about  0.015".  It  thus  appears  that 

activity  of  the  retina  (Charpentier). 

when  light  is  suddenly  thrown  upon  the  retina 

the  sensation  does  not  at  once  rise  to  its  maximum,  but  reaches  this  point  by 
a  sort  of  vibratory  movement.  The  apparent  duplication  of  a  single  very 
brief  retinal  stimulation,  as  that  caused  by  a  flash  of  lightning,  may  perhaps 
be  a  phenomenon  of  the  same  sort. 

Fatigue  of  Retina. — When  the  eye  rests  steadily  upon  a  uniformly  illu- 
1  Charpentier:  Archives  de  Physiologic,  1892,  pp.  541,  629  ;  and  1886,  p.  677. 


THE  SENSE    OF    VISION.  345 

rninated  white  surface  (e.  g.  a  sheet  of  white  paper),  we  are  usually  unconscious 
of  any  diminution  in  the  intensity  of  the  sensation,  but  it  can  be  shown  that 
the  longer  we  look  at  the  paper  the  less  brilliant  it  appears,  or,  in  other  words, 
that  the  retina  really  becomes  fatigued.  To  do  this  it  is  only  necessary  to  place 
a  disk  of  black  paper  on  the  white  surface  and  to  keep  the  eyes  steadily  fixed 
for  about  half  a  minute  upon  the  centre  of  the  disk.  Upon  removing  the  disk 
without  changing  the  direction  of  the  eyes  a  round  spot  will  be  seen  on  the 
white  paper  in  the  place  previously  occupied  by  the  disk.  On  this  spot  the 
whiteness  of  the  paper  will  appear  much  more  intense  than  on  the  neighboring 
portion  of  the  sheet,  because  we  are  able  in  this  experiment  to  bring  into  direct 
contrast  the  sensations  produced  by  a  given  amount  of  light  upon  a  fresh  and 
a  fatigued  portion  of  the  retina.1 

The  rapidity  with  which  the  retina  becomes  fatigued  varies  with  the  color 
of  the  light.  Hence  when  intense  white  light  falls  upon  the  retina,  as  when 
we  look  at  the  setting  sun,  its  disk  seems  to  undergo  changes  of  color  as  one 
or  another  of  the  constituents  of  its  light  becomes,  through  fatigue,  less  and 
less  conspicuous  in  the  combination  of  rays  which  produces  the  sensation  of 
white. 

The  After-effect  of  Stimulation. — The  persistence  of  the  sensation  after  the 
stimulus  has  ceased  causes  very  brief  illuminations  (e.  g.  by  an  electric  spark)  to 
produce  distinct  effects.  On  this  phenomenon  depends  also  the  above-described 
method  of  mixing  colors  on  a  revolving  disk,  since  a  second  color  is  thrown 
upon  the  retina  before  the  impression  produced  by  the  first  color  has  had  time 
enough  to  become  sensibly  diminished.  The  interval  at  which  successive  stim- 
ulations must  follow  each  other  in  order  to  pro- 
duce a  uniform  sensation  (a  process  analogous 
to  the  tetanic  stimulation  of  a  muscle)  may  be 
determined  by  rotating  a  disk,  such  as  repre- 
sented in  Figure  156,  and  ascertaining  at  what 
speed  the  various  rings  produce  a  uniform  sen- 
sation of  gray.  The  interval  varies  with  the 
intensity  of  the  illumination  from  0.1"  to 
0.033",  and  may,  therefore,  be  used  as  a 
measure  of  the  intensity,  as  in  the  method  of 
u flicker  photometry."2  The  special  advan- 
tage of  this  method  is  that  it  affords  a  means  FIG.  156.-Disk  to  illustrate  the  persistence 
...  .  . .  of  retinal  sensation  (Helmholtz). 

of  determining  the  relative  intensity  of  lights 

of  different  colors.  The  duration  of  the  after-effect  depends  also  upon  the 
length  of  "the  stimulation  and  upon  the  color  of  the  light  producing  it,  the 
most  persistent  effect  being  produced  by  the  red  rays.  In  this  connection  it 
is  interesting  to  note  that  while  with  the  rapidly  vibrating  blue  rays  a  less 

1  Although  the  retina  is  here  spoken  of  as  the  portion  of  the  visual  apparatus  subject  to 
fatigue,  it  should  be  borne  in  mind  that  we  cannot,  in  the  present  state  of  our  knowledge,  dis- 
criminate between  retinal  fatigue  and  exhaustion  of  the  visual  nerve-centres. 

2  Rood  :  American  Journal  of  Science,  Sept.,  1893. 


346  AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 

intense  illumination  suffices  to  stimulate  the  eye,  the  slowly  vibrating  red 
rays  produce  the  more  permanent  impression. 

After-images. — When  the  object  looked  at  is  very  brightly  illuminated  the 
impression  upon  the  retina  may  be  so  persistent  that  the  form  and  color  of  the 
object  are  distinctly  visible  for  a  considerable  time  after  the  stimulus  has  ceased 
to  act.  This  appearance  is  known  as  a  "  positive  after-image/'  and  can  be  best 
observed  when  we  close  the  eyes  after  looking  at  the  sun  or  other  bright  source 
of  light.  Under  these  circumstances  we  perceive  a  brilliant  spot  of  light  which, 
owing  to  the  above-mentioned  difference  in  the  persistence  of  the  impressions 
produced  by  the  various  colored  rays,  rapidly  changes  its  color,  passing  gen- 
erally through  bluish  green,  blue,  violet,  purple,  and  red,  and  then  disappear- 
ing. This  phenomenon  is  apt  to  be  associated  with  or  followed  by  another 
effect  known  as  a  "  negative  after-image."  This  form  of  after-image  is  much 
more  readily  observed  than  the  positive  variety,  and  seems  to  depend  upon  the 
fatigue  of  the  retina.  It  is  distinguished  from  the  positive  after-image  by  the 
fact  that  its  color  is  always  complementary  to  that  of  the  object  causing  it.  In 
the  experiment  to  demonstrate  the  fatigue  of  the  retina,  described  on  p.  345, 
the  white  spot  which  appears  after  the  black  disk  is  withdrawn  is  the  "  nega- 
tive after-image"  of  the  disk,  white  being  complementary  to  black.  If  a 
colored  disk  be  placed  upon  a  sheet  of  white  paper,  looked  at  attentively  for  a 
few  seconds,  and  then  withdrawn,  the  eye  will  perceive  in  its  place  a  spot  of 
light  of  a  color  complementary  to  that  of  the  disk.  If,  for  example,  the  disk 
be  yellow,  the  yellow-perceiving  elements  of  the  retina  become  fatigued  in 
looking  at  it.  Therefore  when  the  mixed  rays  constituting  white  light  are 
thrown  upon  the  portion  of  the  retina  which  is  thus  fatigued,  those  rays  which 
produce  the  sensation  of  yellow  will  produce  less  effect  than  the  other  rays  for 
which  the  eye  has  not  been  fatigued.  Hence  white  light  to  an  eye  fatigued  for 
yellow  will  appear  blue. 

If  the  experiment  be  made  with  a  yellow  disk  resting  on  a  sheet  of  blue 
paper,  the  negative  after-image  will  be  a  spot  on  which  the  blue  color  will 
appear  (1)  more  intense  than  on  the  neighboring  portions  of  the  sheet,  owing 
to  the  blue-perceiving  elements  of  that  portion  of  the  retina  not  being  fatigued  ; 
(2)  more  saturated,  owing  to  the  yellow-perceiving  elements  being  so  far 
exhausted  that  they  no  longer  respond  to  the  slight  stimulation  which  is  pro- 
duced when  light  of  a  complementary  color  is  thrown  upon  them,  as  has  been 
explained  in  connection  with  the  subject  of  saturation. 

Contrast. — As  the  eye  wanders  from  one  part  of  the  field  of  vision  to 
another  it  is  evident  that  the  sensation  produced  by  a  given  portion  of  the 
field  will  be  modified  by  the  amount  of  fatigue  produced  by  that  portion  on 
which  the  eye  has  last  rested,  or,  in  other  words,  the  sensation  will  be  the  result 
of  the  stimulation  by  the  object  looked  at  combined  with  the  negative  after- 
image of  the  object  previously  observed.  The  effect  of  this  combination  is  to 
produce  the  phenomenon  of  successive  contrast,  the  principle  of  which  may  be 
thus  stated  :  Every  part  of  the  field  of  vision  appears  lighter  near  a  darker 
part  and  darker  near  a  lighter  part,  and  its  color  seen  near  another  color 
approaches  the  complementary  color  of  the  latter.  A  contrast  phenomenon 


THE  SENSE    OF    VISION. 


347 


similar  in  its  effects  to  that  above  described  may  be  produced  under  conditions 
in  which  negative  after-images  can  play  no  part.  This  kind  of  contrast  is 
known  as  simultaneous  contrast,  and  may  perhaps  be  explained  on  the  theory 
that  a  stimulation  of  a  given  portion  of  the  retina  produces  in  the  neighboring 
portions  an  effect  to  some  extent  antagonistic  to  that  caused  by  direct  stimulation. 

A  good  illustration  of  the  phenomenon  of  contrast  is  given  in  Figure  157, 
in  which  black  squares  are  separated  by  white  bands  which  at  their  points  of 
intersection  appear  darker  than  where  they  are  bordered  on  either  side  by  the 
black  squares. 

A  black  disk  on  a  yellow  background  seen  through  white  tissue-paper 
appears  blue,  since  the  white  paper  makes  the  black  disk  look  gray  and  the 
yellow  background  pale  yellow.  The  gray  disk  in  contrast  to  the  pale  yellow 
around  it  appears  blue. 


FIG.  157.— To  illustrate  the  phenomenon  of  contrast. 

The  phenomenon  of  colored  shadovjs  also  illustrates  the  principle  of  con- 
trast. These  may  be  observed  whenever  an  object  of  suitable  size  and  shape 
is  placed  upon  a  sheet  of  white  paper  and  illuminated  from  one  direction  by 
daylight  and  from  another  by  gaslight.  Two  shadows  will  be  produced,  one 
of  which  will  appear  yellow,  since  it  is  illuminated  only  by  the  yellowish  gas- 
light, while  the  other,  though  illuminated  by  the  white  light  of  day,  will 
appear  blue  in  contrast  to  the  yellowish  light  around  it. 

Space-perception. — Rays  of  light  proceeding  from  every  point  in  the 
field  of  vision  are  refracted  to  and  stimulate  a  definite  point  on  the  sur- 
face of  the  retina,  thus  furnishing  us  with  a  local  sign  by  which  we  can 
recognize  the  position  of  the  point  from  which  the  light  proceeds. 
Hence  the  size  and  shape  of  an  optical  image  upon  the  retina  enable  us  to 
judge  of  the  size  of  the  corresponding  object  in  the  same  way  that  the  cutane- 


348  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

cms  terminations  of  the  nerves  of  touch  enable  us  to  judge  of  the  size  and 
shape  of  an  object  brought  in  contact  with  the  skin.  This  spatial  perception 
is  materially  aided  by  the  muscular  sense  of  the  muscles  moving  the  eyeball, 
for  we  can  obtain  a  much  more  accurate  idea  of  the  size  of  an  object  if 
we  let  the  eye  rest  in  succession  upon  its  different  parts  than  if  we  gaze  fixedly 
at  a  given  point  upon  its  surface.  The  conscious  effort  associated  with  a  given 
amount  of  muscular  motion  gives,  in  the  case  of  the  eye,  a  measure  of  distance 
similar  to  that  secured  by  the  hand  when  we  move  the  fingers  over  the  surface 
of  an  object  to  obtain  an  idea  of  its  size  and  shape. 

The  perception  of  space  by  the  retina  is  limited  to  space  in  two  dimensions 
— i.  e.  in  a  plane  perpendicular  to  the  axis  of  vision.  Of  the  third  dimension 
in  space — i.  e.  of  distance  from  the  eye — the  retinal  image  gives  us  no  know- 
ledge, as  may  be  proved  by  the  study  of  after-images.  If  an  after-image  of 
any  bright  object — e.  g.  a  window — be  produced  upon  the  retina  in  the  man- 
ner above  described  and  the  eye  be  then  directed  to  a  sheet  of  paper  held  in 
the  hand,  the  object  will  appear  outlined  in  miniature  upon  the  surface  of  the 
paper.  If,  however,  the  eye  be  directed  to  the  ceiling  of  the  room,  the  object 
will  appear  enlarged  and  at  a  distance  corresponding  to  that  of  the  surface 
looked  at.1  Hence  one  and  the  same  retinal  image  may,  under  different  cir- 
cumstances, give  rise  to  the  impression  of  objects  at  different  distances.  We 
must  therefore  regard  the  perception  of  distance  not  as  a  direct  datum  of  vision, 
but,  as  will  be  later  explained,  a  matter  of  visual  judgment. 

When  objects  are  of  such  a  shape  that  their  images  may  be  thrown  suc- 
cessively upon  the  same  part  of  the  retina,  it  is  possible  to  judge  of  their  rela- 
tive size  with  considerable  accuracy,  the  retinal  surface  serving  as  a  scale  to 
which  the  images  are  successively  applied.  When  this  is  not  the  case,  the 
error  of  judgment  is  much  greater.  We  can  compare,  for  instance,  the  relative 
length  of  two  vertical  or  of  two  horizontal  lines  with  a  good  deal  of  precision, 
but  in  comparing  a  vertical  with  a  horizontal  line  we  are  liable  to  make  a  con- 
siderable error.  Thus  it  is  difficult  to  realize  that  the  vertical  and  the  hori- 
zontal lines  in  Figure  158  are  of  the  same  length.  The  error  consists  in  an 

over-estimation  of  the  length  of  the  vertical 
lines  relatively  to  horizontal  ones,  and  appears  to 
depend,  in  part  at  any  rate,  upon  the  small  size 
of  the  superior  rectus  muscle  relatively  to  the 
other  muscles  of  the  eye.  The  difference  amounts 
to  30-45  per  cent,  in  weight  and  40-53  per  cent, 
in  area  of  cross  section.  It  is  evident,  therefore, 
that  a  given  motion  of  the  eye  in  the  upward 
direction  will  require  a  more  powerful  contraction 
.  of  the  weaker  muscle  concerned  in  the  movement 


FIG.  158,-To  illustrate  the  over-esti-    than  will  be  demanded  of  the  stronger  muscles 

mation  of  vertical  lines. 

moving  the  eye  laterally  to  an    equal    amount. 

1  This  power  of  the  surface  of  projection  to  determine  the  apparent  size  and  distance  of  the 
after-image  may  be  to  some  extent  influenced  by  the  will. — Jeffries  :  Journal  of  Boston  Society 
of  Medical  Sciences,  vol.  i.  No.  9. 


THE  SENSE    OF    VISION.  349 

Hence  we  judge  the  upward  motion  of  the  eye  to  be  greater  because  to  accom- 
plish it  we  make  a  greater  effort  than  is  required 
for  a  horizontal  movement  of  equal  extent. 

The  position  of  the  vertical  line  bisecting  the 
horizontal  one  (in  Fig.  158)  aids  the  illusion,  as 
may  be  seen  by  turning  the  page  through  90°,  so 
as  to  bring  the  bisected  line  into  a  vertical  posi- 
tion, or  by  looking  at  the  lines  in  Figure  159,  in 
which  the  illusion  is  much  less  marked  than  in 
Figure  158. 

The  tendency  to  over-estimate  the  length  of 
vertical  lines  is  also  illustrated  by  the  error 
commonly  made  in  supposing  the  height  of  the 
crown  of  an  ordinarv  silk  hat  to  be  greater 

,          .       ,          ,  ,  FIG.  159.— To  illustrate  the  over-estima- 

tnan  its  breadth.  tion  of  verticai  lines. 

Irradiation.  —  Many    other    circumstances 

affect  the  accuracy  of  the  spatial  perception  of  the  retina.  One  of  the  most 
important  of  these  is  the  intensity  of  the  illumination.  All  brilliantly  illumi- 
nated objects  appear  larger  than  feebly  illuminated  ones  of  the  same  size,  as  is 
well  shown  by  the  ordinary  incandescent  electric  lamp,  the  delicate  filament  of 
which  is  scarcely  visible  when  cold,  but  when  intensely  heated  by  the  electric 
current  glows  as  a  broad  band  of  light.  The  phenomenon  is  known  as  "  irra- 
diation," and  seems  to  depend  chiefly  upon  the  above-described  imperfections 
in  the  dioptric  apparatus  of  the  eye,  in  consequence  of  which  points  of  light 
produce  small  circles  of  dispersion  on  the  retina  and  bright  objects  produce 


FIG.  160.— To  illustrate  the  phenomenon  of  irradiation. 

images  with  imperfectly  defined  outlines.  The  white  square  surrounded  by 
black  and  the  black  square  surrounded  by  white  (Figure  160),  being  of  the 
same  size,  would  in  an  ideally  perfect  eye  produce  images  of  the  same  size  on 
the  retina,  but  owing  to  the  imperfections  of  the  eye  the  images  are  not  sharply 
defined,  and  the  white  surfaces  consequently  appear  to  encroach  upon  the  darker 
portions  of  the  field  of  vision.  Hence  the  white  square  looks  larger  than  the 
black  one,  the  difference  in  the  apparent  size  depending  upon  the  intensity  of 
the  illumination  and  upon  the  accuracy  with  which  the  eye  can  be  accommo- 
dated for  the  distance  at  which  the  objects  are  viewed.  The  effect  of  irradi- 
ation is  most  manifest  when  the  dark  portion  of  the  field  of  vision  over  which 
the  irradiation  takes  place  has  a  considerable  breadth.  Thus  the  circular  white 


350 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


spots  in  Figure  161,  when  viewed  from  a  distance  of  three  or  four  meters, 
appear  hexagonal,  since  the  irradiation  is  most  marked  in  the  triangular  dark 
space  between  three  adjacent  circles.  A  familiar  example  of  the  effect  of  irra- 


FIG.  161.— To  illustrate  the  phenomenon  of  irradiation. 

diation  is  afforded  by  the  appearance  of  the  new  moon,  whose  sun-illuminated 
crescent  seems  to  be  part  of  a  much  larger  circle  than  the  remainder  of  the 
disk,  which  shines  only  by  the  light  reflected  upon  it  from  the  surface  of  the 
earth. 


D  E 

FIG.  162.— To  illustrate  the  illusion  of  subdivided  space. 

Subdivided  Space. — A  space  subdivided  into  smaller  portions  by  inter- 
mediate objects  seems  more  extensive  than  a  space  of  the  same  size  not  so  sub- 
divided. Thus  the  distance  from  A  to  B  (Fig.  162)  seems  longer  than  that  from 
B  to  (7,  though  both  are  of  the  same  length,  and  for  the  same  reason  the  square 


THE  SENSE    OF    VISION.  351 

D  seems  higher  than  it  is  broad,  and  the  square  E  broader  than  it  is  high,  the 
illusion  being  more  marked  in  the  case  of  D  than  in  the  case  of  E,  because,  as 
above  explained,  vertical  distances  are,  as  a  rule,  over-estimated. 

The  explanation  of  this  illusion  seems  to  be  that  the  eye  in  passing  over  a 
subdivided  line  or  area  recognizes  the  number  and  size  of  the  subdivisions, 


\  \  \  \  \  \  \ 


\\N\\\\\\\\ 

FIG.  163.—  Zollner's  lines. 

and  thus  gets  an  impression  of  greater  total  size  than  when  no  subdivisions 
are  present. 

A  good  example  of  this  phenomenon  is  afforded  by  the  apparently  increased 
extent  of  a  meadow  when  the  grass  growing  on  it  is  cut  and  arranged  in  hay- 
cocks.1 

The  relations  of  lines  to  each  other  gives  rise  to  numerous  illusions  of 
spatial  perception,  among  the  most  striking  of  which  are  those  afforded  by  the 
so-called  "  Zollner's  lines/'  an  example  of  which  is  given  in  Figure  163.  Here 
the  horizontal  lines,  though  strictly  parallel  to  each  A 

other,  seem  to  diverge  and  converge  alternately,  their 
apparent  direction  being  changed  toward  greater  per- 
pendicularity to  the  short  oblique  lines  crossing  them. 
This  illusion  is  to  be  explained  in  part  by  the  tendency 
of  the  eye  to  over-estimate  the  size  of  acute  and  to 
under-estimate  that  of  obtuse  angles  —  a  tendency  which, 
according  to  Filehne,2  is  due  to  the  fact  that  we  are 
constantly  surrounded  by  square-cornered  objects 
(houses,  furniture,  etc.),  the  right  angles  of  which, 
being  seen  obliquely,  are  projected  onto  our  retinas 
as  acute  or  obtuse  angles.  Knowing  these  angles 

.    .  ,  Fio.  164.—  To  illustrate  illusion 

be  right  angles,  we  are  constantly  applying  mental         of  space-perception. 
corrections  to  our  visual    data,  and  the  habit  thus 

acquired  forces  us  to  regard  all  acute  and  obtuse  angles  as  nearer  to  right 
angles  than  they  really  are.  The  illusion  in  Zollner's  lines  is  more  marked 
when  the  'figure  is  so  held  that  the  long  parallel  lines  make  an  angle  of  about 

1  It  is  interesting  to  note  that  a  similar  illusion  has  been  observed  when  an  interval  of  time 
subdivided  by  audible  signals  is  compared  with  an  equal  interval  not  so  subdivided  (Hall  and 
Jastrow  :  Mind,  xi.  62). 

2  Zeitschrift  far  Psychologic  und  Physiologic  der  Sinnesorgane,  xvii.  S.  16. 


352  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

45°  with  the  horizon,  since  in  this  position  the  eye  appreciates  their  real 
position  less  accurately  than  when  they  are  vertical  or  horizontal.  It  is 
diminished,  but  does  not  disappear,  when  the  eye,  instead  of  being  allowed 


FIG.  165.— To  illustrate  contrast  in  space-perception  (Muller-Lyer). 

to  wander  over  the  figure,  is  fixed  upon  any  one  point  of  the  field  of  vision. 
Hence  the  motions  of  the  eye  must  be  regarded  as  a  factor  in,  but  not  the 
sole  cause  of,  the  illusion. 


PIG.  166.— To  illustrate  contrast  in  space-perception  (Muller-Lyer). 

The  illusion  in  Fig.  164,  where  the  line  d  is  the  real  and  the  line  /  the 
apparent  continuation  of  the  line  a,  is  to  be  explained  partly  by  the  over- 
estimation  of  acute  angles  and  partly,  according  to  Helmholtz,  by  irradiation. 


FIG.  167.— To  illustrate  contrast  in  space-perception  (Muller-Lyer). 

The  fact  that  the  illusion  is  greatly  diminished  by  turning  the  figure  on  its 
side  seems  to  show  that  the  tendency  to  over-estimate  vertical  dimensions 
also  plays  a  part  in  its  production. 


THE   SENSE    OF    VISION.  353 

Our  estimate  of  the  size  of  given  lines,  angles,  and  areas  is  influenced  by 
neighboring  lines,  angles,  and  areas  with  which  they  arc  compared.  This 
influence  is  sometimes  exerted  in  accordance  with  the  principle  of  contrast, 
and  tends  to  make  a  given  extension  appear  larger  in  presence  of  a  smaller. 
and  smaller  in  presence  of  a  larger  extension.  This  effect  is  illustrated  in 


FIG.  168.— To  illustrate  so-called  "  confluxion  "  in  space-perception  (Miiller-Lyer). 

Figure  165,  in  which  the  middle  portion  of  the  shorter  line  appears  larger 
than  the  corresponding  portion  of  the  longer  line,  in  Figure  166,  in  which  a 
similar  effect  is  observed  in  the  case  of  angles,  and  in  Figure  167,  in  which 
the  space  between  the  two  squares  seems  smaller  than  that  between  the  two 
oblong  figures. 


FIG.  169.— To  illustrate  so-called  "confluxion"  in  space-perception  (Miiller-Lyer). 

In  some  cases,  however,  an  influence  of  the  opposite  sort1  seems  to  be 
exerted,  as  is  shown  in  Figure  168,  in  which  the  middle  one  of  three  parallel 
lines  seems  longer  when  the  outside  lines  are  longer,  and  shorter  when  they 
are  shorter  than  it  is  itself,  and  in  Figure  1 69,  in  which  a  circle  appears  larger 
if  surrounded  by  a  circle  larger  than  itself,  and  smaller  if  a  smaller  circle  is 
shown  concentrically  within  it. 


FIG.  170.— To  illustrate  the  influence  of  angles  upon  the  apparent  length  of  lines  (Mtiller-Lyer). 

Lines  meeting  at  an  angle  appear  longer  when  the  included  angle  is  large 
than  when  it  is  small,  as  is  shown  in  Figure  170.  This  influence  of  the 
included  angle  affords  a  partial  explanation  of  the  illusion  shown  in  Figure 
171,  in  which  the  horizontal  line  at  B  seems  longer  than  at  A  ;  but  the  distance 

1  For  this  influence  the  name  "confluxion  "  has  been  proposed  by  Miiller-Lyer,  from  whose 
article  in  the  Archivfur  Physiologic,  1889,  Sup.  Bd.,  the  above  examples  are  taken. 
VOL.  II.— 23 


354 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


between  the  extremities  of  the  oblique  lines  seems  also  to  affect  our  estimate 
of  the  horizontal  line  in  the  same  way  as  the  outside  lines  in  Figure  168 
influence  our  judgment  of  the  length  of  the  line  between  them. 


B 


FIG.  171— Illusion  of  space-perception. 

Einthoven 1  has  recently  explained  this  phenomenon  as  dependent  upon 
indistinct  vision  in  the  lateral  portions  of  the  retina  which  causes  the  blurred 
images  of  the  ends  of  the  line  a  to  appear  nearer  together  than  those  of  the 

line  b.  This  effect  of  indistinctness 
of  outline  can  be  illustrated  by  photo- 
graphing the  lines  more  or  less  out 
of  focus  as  shown  in  Figure  172  a.  A 
similar  explanation  is  given  by  Ein- 
thoven for  the  illusion  of  subdivided 
space  described  on  p.  351. 

Perception  of  Distance. — The 
retinal  image  gives  us,  as  we  have 
seen,  no  direct  information  as  to  the 
distance  of  the  object  from  the  eye. 
This  knowledge  is,  however,  quite  as 
important  as  that  of  position  in  a  plane 
perpendicular  to  the  line  of  vision,  and 
we  must  now  consider  in  what  way  it 
is  obtained.  The  first  fact  to  be  noticed 
is  that  there  is  a  close  connection  be- 
tween the  judgments  of  distance  and 
of  actual  size.  A  retinal  image  of 
a  given  size  may  be  produced  by  a 
small  object  near  the  eye  or  by  a 
large  one  at  a  distance  from  it. 
Hence  when  we  know  the  actual  size 
of  any  object  (as,  for  example,  a 
human  figure)  we  judge  of  its  distance  by  the  size  of  its  image  on  the  retina. 
Conversely,  our  estimate  of  the  actual  size  of  an  object  will  depend  upon 
our  judgment  of  its  distance.  The  fact  that  children  constantly  misjudge 
both  the  size  and  distance  of  objects  shows  that  the  knowledge  of  this  rela- 
tion is  acquired  only  by  experience.  If  circumstances  mislead  us  with  regard 
to  the  distance  of  an  object,  we  necessarily  make  a  corresponding  error  with 
regard  to  its  size.  Thus,  objects  seen  indistinctly,  as  through  a  fog,  are 
judged  to  be  larger,  because  we  suppose  them  to  be  farther  off  than  they 
really  are.  The  familiar  fact  that  the  moon  seems  to  be  larger  when  near  the 
horizon  than  when  near  the  zenith  is  also  an  illustration  of  this  form  of  illu- 

1  Pftuyer's  Archiv,  Ixxi.  S.  1. 


FIG.  172.— Illustrating  Einthoven's  explanation  of 
space  illusions  through  indistinct  vision. 


THE  SENSE   OF    VISION.  355 

sion.  When  the  moon  is  high  above  our  heads  we  have  no  moans  of  esti- 
mating its  distance  from  us,  since  there  are  no  intervening  objects  with  which 
we  can  compare  it.  Hence  we  judge  it  to  be  nearer  than  when,  seen  on  the 
horizon,  it  is  obviously  farther  off  than  all  terrestrial  objects.  Since  the  size 
of  the  retinal  image  of  the  moon  is  the  same  in  the  two  cases,  we  reconcile 
the  sensation  with  its  apparent  greater  distance  when  seen  on  the  horizon  by 
attributing  to  the  moon  in  this  position  a  greater  actual  size. 

If  the  retinal  image  have  the  form  of  a  familiar  object  of  regular  shape — 
c.  (j.  a  house  or  a  table — we  interpret  its  outlines  in  the  light  of  experience 
and  distinguish  without  difficulty  between  the  nearer  and  more  remote  parts  of 
the  object.  Even  the  projection  of  the  outlines  of  such  an  object  on  to  a  plane 
surface  (?'.  e.  a  perspective  drawing)  suggests  the  real  relations  of  the  different 
parts  of  the  picture  so  strongly  that  we  recognize  at  once  the  relative  distances 
of  the  various  portions  of  the  object  represented.  How  powerfully  a  familiar 
outline  can  suggest  the  form  and  relief  usually  associated  with  it  is  well  illus- 
trated by  the  experiment  of  looking  into  a  mask  painted  on  its  interior  to 
resemble  a  human  face.  In  this  case  the  familiar  outlines  of  a  human  face 
are  brought  into  unfamiliar  association  with  a  receding  instead  of  a  projecting 
form,  but  the  ordinary  association  of  these  outlines  is  strong  enough  to  force 
the  eye  to  see  the  hollow  mask  as  a  projecting  face.1  The  fact  that  the  pro- 
jecting portions  of  an  object  are  usually  more  brightly  illuminated  than  the 
receding  or  depressed  portions  is  of  great  assistance  in  determining  their  rela- 
tive distance.  This  use  of  shadows  as  an  aid  to  the  perception  of  relief  pre- 
supposes a  knowledge  of  the  direction  from  which  the  light  falls  on  an  object, 
and  if  we  are  deceived  on  the  latter  we  draw  erroneous  conclusions  with 
regard  to  the  former  point.  Thus,  if  we  look  at  an  embossed  letter  or  figure 
through  a  lens  which  makes  it  appear  inverted  the  accompanying  reversal  of 
the  shadows  will  cause  the  letter  to  appear  depressed.  The  influence  of 
shadows  on  our  judgment  of  relief  is,  however,  not  so  strong  as  that  of  the 
outline  of  a  familiar  object.  In  a  case  of  conflicting  testimony  the  latter 
usually  prevails,  as,  for  example,  in  the  above-mentioned  experiment  with  the 
mask. 

Aided  by  these  peculiarities  of  the  retinal  picture,  the  mind  interprets  it  as 
corresponding  in  its  different  parts  to  points  at  different  distances  from  the  eye, 
and  it  is  interesting  to  notice  that  painters,  whose  work,  being  on  a  plane  sur- 
face, is  necessarily  in  all  its  parts  at  the  same  distance  from  the  eye,  use  similar 
devices  in  order  to  give  depth  to  their  pictures.  Distant  hills  are  painted  with 
indistinct  outlines  to  secure  what  is  called  "  aerial  perspective."  Figures  of 
men  and  animals  are  introduced  in  appropriate  dimensions  to  suggest  the  dis- 
tance between  the  foreground  and  the  background  of  the  picture.  Landscapes 
are  painted  preferably  by  morning  and  evening  light,  since  at  these  hours  the 
marked  shadows  aid  materially  in  the  suggestion  of  distance. 

1  In  the  experiment  the  mask  should  be  placed  at  a  distance  of  about  two  meters  and  one 
eye  closed.  Even  with  both  eyes  open  the  illusion  often  persists  if  the  distance  is  increased  to 
five  or  six  meters. 


356  AN  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 

The  eye,  however,  can  aid  itself  in  the  perception  of  depth  in  ways  which 
the  painter  has  not  at  his  disposaL  By  the  sense  of  effort  associated  with  the 
act  of  accommodation  we  are  able  to  estimate  roughly  the  relative  distance  of 
objects  before  us.  This  aid  to  our  judgment  can,  of  course,  be  employed  only 
in  the  case  of  objects  comparatively  near  the  eye.  Its  effectiveness  is  greater 
for  objects  not  far  from  the  near-point  of  vision,  and  diminishes  rapidly  as  the 
distance  is  increased,  and  disappears  for  distances  more  than  two  or  three  meters 
from  the  eye. 

When  the  head  is  moved  from  side  to  side  an  apparent  change  in  the  rela- 
tive position  of  objects  at  different  distances  is  produced,  and,  as  the  extent  of 
this  change  is  inversely  proportional  to  the  distance  of  the  objects,  it  serves  as 
a  measure  of  distance.  This  method  of  obtaining  the  "  parallax  "  of  objects 
by  a  motion  of  the,  head  is  often  noticeable  in  persons  whose  vision  in  one 
eye  is  absent  or  defective. 

Binocular  Vision. — The  same  result  which  is  secured  by  the  comparison 
of  retinal  images  seen  successively  from  slightly  different  points  of  view  is 
obtained  by  the  comparison  of  the  images  formed  simultaneously  by  any  object 
in  the  two  eyes.  In  binocular  vision  we  obtain  a  much  more  accurate  idea  of 
the  shape  and  distance  of  objects  around  us  than  is  possible  with  monocular 
vision,  as  may  be  proved  by  trying  to  touch  objects  in  our  neighborhood  with 
a  crooked  stick,  first  with  both  eyes  open  and  then  with  one  eye  shut.  When- 
ever we  look  at  a  near  solid  object  with  two  eyes,  the  right  eye  sees  farther 
round  the  object  on  the  right  side  and  the  left  eye  farther  round  on  the  left. 
The  mental  comparison  of  these  two  slightly  different  images  produces  the 
perception  of  solidity  or  depth,  since  experience  has  taught  us  that  those  objects 
only  which  have  depth  or  solidity  can  affect  the  eyes  in  this  way.  Conversely, 
if  two  drawings  or  photographs  differing  from  each  other  in  the  same  way  that 
the  two  retinal  images  of  a  solid  object  differ  from  each  other  are  presented, 
one  to  the  right  and  -the  other  to  the  left  eye,  the  two  images  will  become 
blended  in  the  mind  and  the  perception  of  solidity  will  result.  Upon  this  fact 
depends  the  effect  of  the  instrument  known  as  the  stereoscope,  the  slides  of 
which  are  generally  pairs  of  photographs  of  natural  objects  taken  simultaneous- 


FIG.  173. — To  illustrate  stereoscopic  vision. 

ly  with  a  double  camera,  of  which  the  lenses  are  at  a  distance  from  each  other 
equal  to  or  slightly  exceeding  that  between  the  two  axes  of  vision.  The  prin- 
ciple of  the  stereoscope  can  be  illustrated  in  a  very  simple  manner  by  drawing 
circles  such  as  are  represented  in  Figure  173  on  thin  paper,  and  'fastening  each 


THE  SENSE    OF    VISION. 


357 


pair  across  the  end  of  a  piece  of  brass  tube  about  one  inch  or  more  in  diameter 
and  ten  inches  long.  Let  the  tubes  be  held  one  in  front  of  each  eye  with  the 
distant  ends  nearly  in  contact  with  each  other,  as  shown  in  Figure  174.  If 
the  tubes  are  in  such  a  position  that  the  small  circles  are  brought  as  near  to 
each  other  as  possible,  as  shown  in  Figure  173,  the  retinal  images  will  blend, 

the  smaller  circle  will  seem  to  be  much 
nearer  than  the  larger  one,  and  the  eyes  will 
appear  to  be  looking  down  upon  a  truncated 
cone,  such  as  is  shown  in  Figure  175,  since 
a  solid  body  of  this  form  is  the  only  one 


FIG.  174.— To  illustrate  stereoscopic  vision. 


FIG.  175.— To  illustrate  stereoscopic  vision. 


,  bounded  by  circles  relied  to  each  other  as  those  shown  in  this  experiment. 

Stereoscopic  slides  often  serve  well  to  illustrate  the  superiority  of  binocular 
*>ver  monocular  vision*.  If  the  slide  represents  an  irregular  mass  of  rocks  or 
ice,  it  is  often-very  Difficult  by  looking  at  either  of  the  pictures  by  itself  to 
determine  the  relative  distance  of  the  various  objects  represented,  but  if  the 
slide  is  placed  in  the  stereoscope  the  true  relation  of  the  different  parts  of  the 
picture  becomes  at  once  apparent. 

Since  the  comparison  of  two  slightly  dissimilar  images  received  on  the  two 
retinas  is  the  essential  condition  of  stereoscopic  vision,  it  is  evident  that  if  the 
two  pictures  are  identical  no  sensation  of  relief  can  be  produced.  Thus,  when 
two  pages  printed  from  the  same  type  or  two  engravings  printed  from  the  same 
plate  are  united  in  a  stereoscope,  the  combined  picture  appears  as  flat  as  either 
of  its  components.  If,  however,  one  of  the  pictures  is  copied  from  the  other, 
even  if  the  copy  be  carefully  executed,  there  will  be  slight  differences  in  the 
distances  between  the  lines  or  in  the  spacing  of  the  letters  which  will  cause 
apparent  irregularities  of  level  in  the  different  portions  of  the  combined  pic- 
ture. Thus,  a  suspected  banknote  may  be  proved  to  be  a  counterfeit  if,  when 
placed  in  a  stereoscope  by  the  side  of  a  genuine  note,  the  resulting  combined 
picture  shows  certain  letters  lying  apparently  on  different  planes  from  the  rest. 

Pseudoscopic  Vision. — If  the  pictures  of  an  ordinary  stereoscopic  slide  be 
reversed,  so  that  the  picture  belonging  in  front  of  the  right  eye  is  presented  to 
the  left  eye,  and  vice  versd,  the  stereoscopic  gives  place  to  what  is  called  a  pseudo- 
scopic  effect — /.  e.  we  perceive  not  a  solid  but  a  hollow  body.  The  effect  is  best 


358  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

obtained  with  the  outlines  of  geometrical  solids,  photographs  of  coins  or  medals 
or  of  objects  which  may  readily  exist  in  an  inverted  form.  Where  the  photo- 
graphs represent  objects  which  cannot  be  thus  inverted,  such  as  buildings  and 
landscapes,  the  pseudoscopic  effect  is  not  readily  produced — another  example 
of  the  power  (see  p.  355)  of  the  outline  of  a  familiar  object  to  outweigh  other 
sorts  of  testimony. 

A  pseudoscopic  effect  may  be  readily  obtained  without  the  use  of  a  stereo- 
scope by  simply  converging  the  visual  axes  so  that  the  right  eye  looks  at  the 
left  and  the  left  eye  at  the  right  picture  of  a  stereoscopic  slide.  The  eyes  may 
be  aided  in  assuming  the  right  degree  of  convergence  by  looking  at  a  small 
object  like  the  head  of  a  pin  held  between  the  eyes  and  the  slide  in  the  manner 
described  on  p.  312.  Figure  173,  viewed  in  this  way,  will  present  the  appear- 
ance of  a  hollow  truncated  cone  with  the  base  turned  toward  the  observer.  A 
stereoscopic  slide  with  its  pictures  reversed  will,  of  course,  when  viewed  in  this 
way,  present  not  a  pseudoscopic,  but  a  true  stereoscopic,  appearance,  as  shown 
by  Figures  138  and  139. 

Binocular  Combination  of  Colors. — The  effect  of  binocularly  combin- 
ing two  different  colors  varies  with  the  difference  in  wave-length  of  the  colors. 
Colors  lying  near  each  other  in  the  spectrum  will  generally  blend  together 
and  produce  the  sensation  of  a  mixed  color,  such  as  would  result  from  the 
union  of  colors  by  means  of  the  revolving  disk  or  by  the  method  of  reflected 
and  transmitted  light,  as  above  described.  Thus  a  red  and  a  yellow  disk 
placed  in  a  stereoscope  may  be  generally  combined  to  produce  the  sensation 
of  orange.  If,  however,  the  colors  are  complementary  to  each  other,  as  blue 
and  yellow,  no  such  mixing  occurs,  but  the  field  of  vision  seems  to  be  occupied 
alternately  by  a  blue  and  by  a  yellow  color.  This  so-called  "  rivalry  of  the 
fields  of  vision  "  seems  to  depend,  to  a  certain  extent,  upon  the  fact  that  in 
order  to  see  the  different  colors  with  equal  distinctness  the  eyes  must  be  dif- 
ferently accommodated,  for  it  is  found  that  if  the  colors  are  placed  at  different 
distances  from  the  eyes  (the  colors  with  the  less  refrangible  rays  being  at  the 
greater  distance),  the  rivalry  tends  to  disappear  and  the  mixed  color  is  more 
easily  produced. 

An  interesting  effect  of  the  stereoscopic  combination  of  a  black  and  a 
white  object  is  the  production  of  the  appearance  of  a  metallic  lustre  or  polish. 
If,  for  instance,  the  two  pictures  of  a  stereoscopic  slide  represent  the  slightly 
different  outlines  of  a  geometrical  solid,  one  in  black  upon  white  ground  and 
the  other  in  white  upon  black  ground,  their  combination  in  the  stereoscope 
will  produce  the  effect  of  a  solid  body  having  a  smooth  lustrous  surface. 
The  explanation  of  this  effect  is  to  be  found  in  the  fact  that  a  polished  surface 
reflects  the  light  differently  to  the  two  eyes,  a  given  point  appearing  bril- 
liantly illuminated  to  one  eye  and  dark  to  the  other.  Hence  the  stereoscopic 
combination  of  black  and  white  is  interpreted  as  indicating  a  polished  surface, 
since  it  is  by  means  of  a  polished  surface  that  this  effect  is  usually  produced. 

Corresponding1  Points. — When  the  visual  axes  of  both  eyes  are  directed 
to  the  same  object  two  distinct  images  of  that  object  are  formed  upon  widely 


THE  SENSE    OF    VISION.  359 

separated  parts  of  the  nervous  system.  Yet  but  a  single  object  is  perceived. 
The  phenomenon  is  the  same  as  that  which  occurs  when  a  grain  of  sand  is 
held  between  the  thumb  and  finger.  In  both  cases  we  have  learned  (chiefly 
through  the  agency  of  muscular  movements  and  the  nerves  of  muscular  sense) 
to  interpret  the  double  sensation  as  produced  by  a  single  object. 

Any  two  points,  lying  one  in  each  retina,  the  stimulation  of  which  by  rays 
of  light  gives  rise  to  the  sensation  of  light  proceeding  from  a  single  object  are 
said  to  be  "  corresponding  points."  Now,  it  is  evident  that  the /owes  centrcdes 
of  the  two  eyes  must  be  corresponding  points,  for  an  object  always  appears 
single  when  both  eyes  are  fixed  upon  it.  That  double  vision  results  when  the 
images  are  formed  on  points  which  are  not  corresponding  may  be  best  illus- 
trated by  looking  at  three  pins  stuck  in  a  straight  rod  at  distances  of  35,  45, 
and  55  centimeters  from  the  end.  If  the  end  of  the  rod  is  held  against  the 
nose  and  the  eyes  directed  to  each  of  the  three  pins  in  succession,  it  will  be 
found  that,  while  the  pin  looked  at  appears  single,  each  of  the  others  appears 
double,  and  that  the  three  pins  therefore  look  like  five. 

The  two  fovece  centrales  are  not,  of  course,  the  only  corresponding  points. 
In  fact,  it  may  be  said  that  the  two  retinas  correspond  to  each  other,  point  for 
point,  almost  as  if  they  were  superposed  one  upon  the  other  with  the  fovea? 
together.  The  exact  position  of  the  points  in  space  which  are  projected  on  to 
corresponding  points  of  the  two  retinas  varies  with  the  position  of  the  eyes. 
The  line  or  surface  in  which  such  points  lie  is  known  as  the  "  horopter."  A 
full  discussion  of  the  horopter  would  be  out  of  place  in  this  connection,  but 
one  interesting  result  of  its  study  may  be  pointed  out — viz.  the  demonstration 
that  when,  standing  upright,  we  direct  our  eyes  to  the  horizon  the  horopter  is 
approximately  a  plane  coinciding  with  the  ground  on  which  we  stand.  It  is 
of  course  important  for  security  in  walking  that  all  objects  on  the  ground 
should  appear  single,  and,  as  they  are  known  by  experience  to  be  single,  the 
eye  has  apparently  learned  to  see  them  so. 

Since  the  vertical  meridians  of  the  two  eyes  represent  approximately  rows 
of  corresponding  points,  it  is  evident  that  when  two  lines  are  so  situated  that 
their  images  are  formed  each  upon  a  vertical  meridian  of  one  of  the  eyes,  the 
impression  of  a  single  vertical  line  will  be  produced,  for  such  a  line  seen  bin- 
ocularly  is  the  most  frequent  cause  of  this  sort  of  retinal  stimulation.  This 
is  the  explanation  commonly  given  of  the  singular  optical  illusion  which  is 
produced  when  lines  drawn  as  in  Figure  176  are  looked  at  with  both  eyes  fixed 
upon  the  point  of  intersection  of  the  lines  and  with  the  plane  in  which  the 
visual  axes  lie  forming  an  angle  of  about  20°  with  that  of  the  paper,  the  dis- 
tance of  the  lines  from  the  eyes  being  such  that  each  line  will  lie  approximately 
in  the  same  vertical  plane  with  one  of  the  visual  axes.  Under  these  circum- 
stances each  line  will  form  its  image  on  a  vertical  meridian  of  one  of  the  eyes, 
and  the  combination  of  these  images  results  in  the  perception  of  a  third  line, 
not  lying  in  the  plane  of  the  paper,  but  apparently  passing  through  it  more  or 
less  vertically,  and  swinging  round  its  middle  point  with  every  movement  of 
the  head  or  the  paper.  In  this  experiment  it  will  be  found  that  the  illusion 


360 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


of  a  Hue  placed  vertically  to  the  plane  of  the  paper  does  not  entirely  dis- 
appear when  one  eye  is  closed.     Hence  it  is  evident  that  there  is,  as  Mrs. 


FIG.  177.— Monocular  illusion  of  vertical  lines. 

C.  L.  Franklin  has  pointed  out,1  a  strong  tendency  to  regard 
lines  which  form  their  images  approximately  on  the  vertical 
meridian  of  the  eye  as  themselves  vertical.  This  tendency 
is  well  shown  when  a  number  of  short  lines  converging 
toward  a  point  outside  of  the  paper  on  which  they  are 
drawn,  as  in  Figure  177,  are  looked  at  with  one  eye  held 
a  short  distance  above  the  point  of  convergence.  Even 
when  the  lines  are  not  convergent,  but  parallel,  so  that  their 
images  cannot  fall  upon  the  vertical  meridian  of  the  eye,  the 
illusion  is  not  entirely  lost.  It  will  be  found,  for  instance, 
that  when  the  Zollner  lines,  as  given  in  Figure  163,  are 
looked  at  obliquely  with  one  eye  from  one  corner  of  the 
FIG.  176.— Binocu-  figure,  the  short  lines  which  lie  nearly  in  a  plane  with  the 
ttcaUine011  OJ  &  Ver  vlsus^  axis  appear  to  stand  vertically  to  the  plane  of  the 

paper. 

In  this  connection  it  may  be  well  to  allude  to  the  optical  illusion  in  conse- 
quence of  which  certain  portraits  seem  to  follow  the  beholder  with  the  eyes. 
This  depends  upon  the  fact  that  the  face  is  painted  looking  straight  out  from 
the  canvas  — i.  e.  with  the  pupil  in  the  middle  of  the  eye.  The  painting  being 
upon  a  flat  surface,  it  is  evident  that,  from  whatever  direction  the  picture  is 
viewed,  the  pupil  will  always  seem  to  be  in  the  middle  of  the  eye,  and  the 
eye  will  consequently  appear  to  be  directed  upon  the  observer.  The  phenom- 
enon is  still  more  striking  in  the  case  of  pictures  of  which  the  one  repre- 
sented in  Figure  178  may  be  taken  as  an  example.  Here  the  soldier's  rifle 

1  Am.  Journal  of  Psychology,  vol.  i.  p.  99. 


THE  SENSE    OF    VISION.  361 

is  drawn  as  it  appears  to  an  eye  looking  straight  down  the  barrel,  and,  as  this 

foreshortening  is  the  same  in  all  positions  of  the  observer,  it  is  evident  that 

when  such  a  picture  is  hung  upon  the  wall 

of  a  room  the  soldier  will  appear  to  be 

aiming  directly  at  the  head  of  every  person 

present. 

In  concluding  this  brief  survey  of  some 
of  the  most  important  subjects  connected 
with  the  physiology  of  vision  it  is  well  to 
utter  a  word  of  caution  with  regard  to  a 
danger  connected  with  the  study  of  the  sub- 
ject. This  danger  arises  in  part  from  the 
fact  that  in  the  scientific  study  of  vision  it 
is  often  necessary  to  use  the  eyes  in  a  way 
quite  different  from  that  in  which  they  are 
habitually  employed,  and  more  likely,  there- 
fore, to  cause  nervous  and  muscular  fatigue. 
We  have  seen  that  in  any  given  position  of 
the  eye  distinct  definition  is  limited  to  an  FIG.  iTs.-niusion  of  lines  always  pointing 

*  toward  observer. 

area  which  bears  a  very  small  proportion  to 

the  whole  field  of  vision.  Hence  in  order  to  obtain  an  accurate  idea  of  the 
appearance  of  any  large  object  our  eyes  must  wander  rapidly  over  its  whole 
surface,  and  we  use  our  eyes  so  instinctively  and  unconsciously  in  this  way 
that,  unless  our  attention  is  specially  directed  to  the  subject,  we  find  it  diffi- 
cult to  believe  that  the  power  of  distinct  vision  is  limited  to  such  a  small 
portion  of  the  retina.  In  most  of  the  experiments  in  physiological  optics, 
however,  this  rapid  change  of  direction  of  the  axis  of  vision  must  be  carefully 
avoided,  and  the  eye-muscles  held  immovable  in  tonic  contraction. 

Our  eyes,  moreover,  like  most  of  our  organs,  serve  us  best  when  we  do  not 
pay  too  much  attention  to  the  mechanism  by  which  their  results  are  brought 
about.  In  the  ordinary  use  of  the  eyes  we  are  accustomed  to  neglect  after- 
images, intraocular  images,  and  all  the  other  imperfections  of  our  visual  appa- 
ratus, and  the  usefulness  of  our  eyes  depends  very  much  upon  our  ability  thus 
to  neglect  their  defects.  Now,  the  habit  of  observing  and  examining  these 
defects  that  is  involved  in  the  scientific  study  of  the  eye  is  found  to  interfere 
with  our  ability  to  disregard  them.  A  student  of  the  physiology  of  vision 
who  devotes  too  much  attention  to  the  study  of  after-images,  for  instance,  may 
render  his  eyes  so  sensitive  to  these  phenomena  that  they  become  a  decided 
obstacle  to  ordinary  vision. 


362  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


B.  THE  EAR  AND  HEARING. 

Anatomy  and  Histology  of  the  Ear. — The  organ  of  hearing  may  con- 
veniently be  divided  into  three  parts :  (1)  The  external  ear,  including  the 
pinna  or  auricle  and  the  external  auditory  meatus  ;  (2)  the  middle  ear,  called 
the  "  tympanic  cavity  "  or  tympanum ;  and  (3)  the  internal  ear,  or  labyrinth. 
The  labyrinth  is  situated  in  the  dense  petrous  bone,  and  it  contains  a  mem- 
branous sac  of  complex  form  which  receives  the  peripheral  terminations  of  the 
auditory  nerve.  This  sac,  therefore,  is  to  the  ear  what  the  retina  is  to  the  eye  • 
as  the  lens,  cornea,  etc.  of  the  eye  are  simply  physical  media  for  the  production 
of  sharp  images  on  the  retina,  so  all  parts  of  the  organ  of  hearing  are  devoted 
solely  to  the  accurate  transmission  of  the  energy  of  air-waves  to  the  internal 
ear. 

The  External  Ear. — The  pinna  or  auricle,  commonly  known  simply  as 
the  "  ear"  (Fig.  179),  is  a  peculiarly  wrinkled  sheet  of  tissue,  consisting  essen- 


FIG.  179.-Diagram  of  organ  of  hearing  of  left  side  (Quain,  after  Arnold) :  1,  the  pinna;  2,  bottom  of 
concha ;  2-2',  meatus  externus ;  3,  tympanum ;  above  3,  the  chain  of  ossicles ;  3',  opening  into  the  mastoid 
cells ;  4,  Eustachian  tube ;  5,  meatus  internus,  containing  the  facial  (uppermost)  and  auditory  nerves ; 
6,  placed  on  the  vestibule  of  the  labyrinth  above  the  fenestra  ovalis ;  a,  apex  of  the  petrous  bone ;  b, 
internal  carotid  artery ;  c,  styloid  process ;  d,  facial  nerve,  issuing  from  the  stylo-mastoid  foramen ;  e, 
mastoid  process  ;  /,  squamous  part  of  the  bone. 

tially  of  yellow  elastic  cartilage  covered  with  skin,  and  forming  at  the  entrance 
of  the  auditory  meatus  a  cup-shaped  depression  called  the  "  concha." 

The  concha,  and  to  some  extent  the  whole  auricle,  serves  a  useful  purpose 
in  collecting,  like  the  mouth  of  a  speaking-trumpet,  the  waves  of  sound  falling 
upon  it ;  but  in  many  of  the  lower  animals  the  concha  is  relatively  larger  than 
in  man,  and,  their  ears  being  freely  movable,  the  auricle  becomes  of  greater 
physiological  importance. 

External  Auditory  Meatus. — In  man  the  external  auditory  meatus  or  audi- 
tory canal  is  about  one  and  a  quarter  inches  in  length,  and  it  extends  from 


THE   SENSE    OF    HKMtlNG. 


363 


the  bottom  and  anterior  edge  of  the  concha  to  the  membrana  tympani,  or 

tympanic  membrane.  Starting 
from  the  bottom  of  the  concha, 
the  general  direction  of  the  audi- 
tory canal  is  first  obliquely  up- 
ward and  backward  for  about 
half  an  inch,  and  then  inward 
and  forward.  Therefore,  to  look 
into  the  ear  or  to  introduce  the 
aural  speculum  the  canal  must  be 
0  straightened  by  pulling  the  pinna 
upward  and  backward.  The 


14 


FIG.  180. — Tympanum  of  left  ear,  with  ossicles  in  situ 
(after  Morris) :  1,  suspensory  ligament  of  malleus ;  2,  head 
of  malleus  ;  3,  epitympanic  region ;  4,  external  ligament 
of  malleus  ;  5,  processus  longus  of  incus ;  6,  base  of  stapes ; 
7,  processus  brevis  of  malleus;  8,  head  of  stapes;  9,  os 
orbiculare;  10,  manubrium ;  11,  Eustachian  tube  ;  12,  exter- 
nal auditory  meatus:  13,  membrana  tympani;  14,  lower 
part  of  tympanum. 


canal-wall  is  cartilaginous 
movable  for  about  half  an  inch 
from  the  exterior,  but  is  osseous 
for  the  rest  of  its  extent;  it  is 
lined  by  a  reflexion  of  thin  skin, 
on  whose  surface,  in  the  cartilag- 
inous part  of  the  canal,  open  the 
ducts  of  numerous  sebaceous  and 
ceruminous  glands. 

Tympanum. — The  middle  ear, 
or  tympanum  (Figs.  179,  180),  is 
shut  off  from  the  auditory  canal 
by  the  tympanic  membrane.  It 

is  an  air-holding  cavity  of  irregular  shape  in  the  petrous  bone,  and  it  is  broader 

behind  and  above  than  it  is  below  and 

in  front.     Posteriorly  it  is  in  open  com- 
munication with   the  complex  system  of 

air-cavities  in  the  mastoid   bone  known 

as  the   mastoid   antrum  and   the  mastoid 

ectk.    Anteriorly  it  is  continuous  with  the 

pharynx    through   the    Eustachian   tube. 

The  inner  wall  slants  somewhat  outward 

from    top   to   bottom,  and    it   is  formed 

chiefly  by  part  of  the  bony  envelope  of 

the  internal  ear.    The  surface  of  this  wall 

is  pierced  by  two  apertures,  the  fenestra 

GW//X,  or  oval  window,  and  the  fenestra 

rotunda,  or  round  window,   leading  into 

the  cavity  of  the  bony  labyrinth ;  in  life 

each  fenestra  is  covered  by  a  thin  sheet 

of  membrane,  and  the  foot  of  the  stapes 

is  fastened  by  a  ligamentous  fringe  in  the 

oval  window.     The  outer  wall  of  the  middle  ear  is  made  up  of  the  tympanic 


FIG.  181.— Otoscopic  view  of  left  membrana 
tympani  (Morris):   I,  membrana  flaccid  a ;  2,2', 

foldg  bounding  the  former ;  3, reflection  from 
processus  brevis  of  malleus ;  4,  processus  lon- 

gus   of  incus    (occasionally  seen):    ft,  mem- 

brana  tympani .  6f  umbo  and  end  of  manu- 
t>rium;  7,  pyramid  of  light. 


364 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


membrane  and  the  ring  of  bone  into  which  this  membrane  is  inserted. 
The  roof  is  formed  by  a  thin  plate  of  bone,  the  tegmen,  which  separates  it 
from  the  cranial  cavity,  and  the  narrow  floor,  concave  upward,  is  just  above 
the  jugular  fossa.  The  cavity  is  lined  by  mucous  membrane  continuous  with 
that  of  the  Eustachian  tube  and  the  pharynx,  and  the  membrane,  like  that 
of  the  Eustachian  tube,  is  ciliated  except  over  the  surfaces  of  the  ossicles  and 
the  tympanic  membrane.  Suppurative  inflammation  of  the  middle  ear  may 
not  only  involve  the  mastoid  cells,  but  may  also  cause  absorption  of  the  thin 
plate  of  bone  forming  the  roof  of  the  tympanic  cavity  and  the  mastoid 
antrum.  In  this  and  in  other  ways  inflammation  may  extend  from  the  tym- 
panic to  the  cranial  cavity,  making  otitis  media,  or  inflammation  of  the  middle 
ear,  the  commonest  source  of  pyogenic  affections  of  the  brain.1 

Tympanic  Membrane,  or  Drum-skin. — The  membrana  tympani  (Figs.  181, 
182)  is  a  somewhat  oval  disk  whose  longer  axis  is  directed  from  behind  and  above 

downward  and  forward,  and 
whose  length  is  about  nine 
millimeters.  The  membrane 
is  inserted  obliquely  to  the 
axis  of  the  auditory  canal, 
so  that  the  floor  of  the  canal 
is  longer  than  its  roof.  The 
membrana  tympani,  though 
so  thin  as  to  be  semi-trans- 
parent, is  composed  of  three 
layers  of  tissue.  Externally 
it  is  covered  by  a  thin  plate 
of  skin ;  internally,  by  mu- 
cous membrane ;  and  between 
these  lies  the  proper  sub- 


FIG.  182.2— Tympanum  of  right  side  with  ossicles  in  place,  viewed 
from  within  (after  Morris) :  1,  body  of  incus ;  2,  suspensory  ligament 
of  malleus ;  3,  ligament  of  incus ;  4,  head  of  malleus ;  5,  epitym- 

panic  cavity ;  6,  chorda  tympani  nerve ;  7,  tendon  of  tensor  tympani  FIG.  183.— The  chain  of  auditory 

muscle;  8,  foot-piece  of  stirrup;  9,  os  orbiculare;  10,  manubrium ;  ossicles,  anterior  view  (after  Tes- 

11,  tensor  tympani  muscle;  12,  membrana  tympani;  13,  Eustachian  tut) :  1,  head  of  malleus;  2,  long 

tube-  process  of  incus ;  3,  stapes. 

stance  (membrana  propria)  of  the  membrane,  made  up  chiefly  of  fibrous  tissue. 
The  greater  number  of  the  fibres  of  the  membrana  propria  radiate  from  near 
the  centre  to  the  periphery  of  the  membrane ;  but  there  are  also  circular  fibres 
of  elastic  tissue  which  are  most  numerous  in  a  ring  near  the  attached  margin 
of  the  membrane.  The  surface  of  the  tympanic  membrane  is  not  flat,  but  is 
funnel-shaped,  with  the  apex  of  the  funnel  pointing  inward.  Moreover,  lines 
1Macewen  :  Pyogenic  Diseases  of  the  Brain  and  Spinal  Cord,  1893. 
2  Figs.  180, 181,  and  182  are  taken  by  permission  from  Morris's  Text-Book  of  Anatomy,  Phila.,  1893. 


TKE  SENSE    OF  HEARING. 


365 


drawn  from  the  centre  to  the  margin  of  the  membrane  would  not  be  straight, 
but  would  be  curved  slightly,  with  the  convexity  outward,  this  shape  being 
due  to  the  tension  of  the  elastic  circular  fibres  of  the  membrane.  The  mem- 
brane, throughout  the  greater  part  of  its  circumference,  is  inserted  in  a  groove 
in  a  bony  ring  set  in  the  wall  of  the  auditory  canal,  but  a  small  arc  at  its 
superior  portion  is  attached  directly  to  the  wall  of  the  canal.  The  segment  of 
membrane  corresponding  to  this  arc,  known  as  the  membrana  flacrida,  lacks 
the  tenseness  of  the  rest  of  the  drum-skin. 

Viewed  through  the  aural  speculum,  the  normal  tympanic  membrane  has 
a  pearly  lustre  (Fig.  181).  The  handle  of  the  malleus,  or  manubrium,  inserted 
within  its  fibrous  layer,  can  be  seen  as  an  opaque  ridge  running  from  near  the 
upper  anterior  margin  downward  and  backward  and  ending  in  the  umbo,  or 
central  depression,  where  the  membrane  is  drawn  considerably  inward  by  the 
tip  of  the  manubrium.  It  is  from  this  point  that  the  radial  fibres  of  the  mem- 
brana propria  diverge. 

At  the  top  of  the  manubrium  is  a  shining  spot  which  is  the  reflection 
from  the  short  process  of  the  malleus  where  it  presses  against  the  membrane. 
From  this  point  two  delicate  folds  of  the  membrane  run  to  the  periphery — 
one  forward  and  the  other  backward.  They  form  the  lower  border  of  the 
membrana  flaccida,  or  ShrapneWs  membrane,  in  which  there  is  less  fibrous  tissue 
than  in  the  remaining  part  of  the  membrane,  and  the  cutaneous  and  mucous 
layers  are  also  less  tense  than  elsewhere.  A  bright  reflection  of  triangular 
shape,  known  as  the  "  pyramid  of  light,"  is  seen  in  the  lower  quadrant  of  the 
tympanic  membrane.  The  apex  of  this 
bright  triangle  is  at  the  tip  of  the  manu- 
brium, and  its  base  is  on  or  near  the 
periphery  of  the  membrane. 

Auditory  Ossicles.  —  The  tympanic 
membrane  is  put  into  relation  with  the 
internal  ear  by  a  chain  of  bone,  the 
auditory  ossicles,  known  as  the  malleus, 
the  incus,  and  the  stapes,  so  called  from 
their  fancied  resemblance  to  a  hammer,  an 
anvil,  and  a  .stirrup  (Figs.  180,  182, 183). 
The  malleus  (Fig.  184)  is  18  to  19  milli- 
meters long;  it  presents  a  rounded  head,  FlG.184._Malleu80ftherightside:A>anterior 

grooved  O11  one  side  for  artfculation  with  face;  B,  internal  face  (after  Testut):  1,  capitu- 
.  i  •  ,  -i  -11  iji  lum  or  head  of  malleus ;  2,  cervix  or  neck ;  3, 

the  incus,  a  short  neck,  and  a  long  handle    procewmg  brevis .  4)  pr0ceS8us graciiis ;  5, manu- 

Or  manubrium,  which    is   inserted    in    the     brium;  6,  grooved  articular  surface  for  incus; 

.  ,>  7,  tendon  of  m.  tensor  tympani. 

tissue  oi  the  tympanic  membrane  irom 

a  point  on  its  upper  periphery  to  a  little  below  its  centre.  The  processus 
brevis  of  the  malleus  is  a  low  conical  projection  which  springs  from  the  top 
of  the  manubrium  and  presses  directly  against  that  segment  of  the  tympanic 
membrane  known  as  the  membrana  flaccida,  through  which  it  can  be  seen 
shining  on  inspection  with  the  ear-speculum.  The  processus  graciiis,  or  pro- 


366 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


cessus  Folianus,  long  and  slender,  arises  from  an  eminence  just  below  the 
neck  of  the  malleus,  and,  passing  forward  and  outward,  is  inserted  in  the 
Glaserian  fissure  in  the  wall  of  the  tympanum.  The  malleus  is  held  in  posi- 
tion partly  by  ligaments;  the  suspensory  or  superior  ligament  passes  downward 
and  outward  from  the  roof  of  the  tympanum  to  be  inserted  into  the  head  of 
the  malleus.  The  main  portion  of  the  anterior  ligament  is  attached  to  the 
neck  of  the  malleus  just  above  the  processus  gradlis  ;  it  embraces  the  latter, 

and,  passing  forward,  finds  its  origin 
in  the  anterior  wall  of  the  tympanum 
and  in  the  Glaserian  fissure.  Another 
division  of  this  ligament,  the  external 
ligament,  arises  and  is  attached  more 
externally  than  that  just  described. 


Ig.inc 


represents  a  nearly  horizontal  section  of  the  tym- 
panum,  carried  through  the  heads  of  the  malleus 
and  incus  :  M,  malleuf  ;  1,  incus  ;  t,  articular  tooth 


.  of  the  malleus  serve  to 

keep  its  head  in  position.       The  exter- 

i  i  •  .».   i     •  uiu  j.i 

na}  lament,  being  attached  above  the 

of  incus  ;  Ig.a  and  Ig.e,  external  ligament  of  mal-  axis  of  rotation  of  the  hammer,  pre- 
leus;  Ig.inc,  ligament  of  the  incus  ;  the  line  a-x  rep-  ,i  u  j  /»  j.v»  i  p 

resents  the  axis  of  rotation  of  the  two  ossicles.  vents    tne     head-    oi     this    bcme    *rom 

moving  too  far  inward,  and  the  manu- 

brium  from  being  pushed  too  far  outward.  The  superior  ligament,  owing  to 
its  oblique  course,  restrains  the  head  of  the  hammer  from  moving  too  far 
outward. 

The  incus,  ambos,  or  anvil-bone  (Fig.  186)  is  shaped  somewhat  like  a  bicus- 
pid tooth.     Its  thicker  portion  is  hollowed  on  the  surface  and  is  covered  with 

cartilage  for  articulation  with  the 
head  of  the  malleus.  It  has  two 
processes,  a  long  and  a  short, 
which  project  at  right  angles  to 


FIG.  186.— The  incus  of  the  right  side :  A,  anterior  face ;  B, 
internal  face  (after  Testut) :  1,  body  of  incus ;  2,  processus 
brevis ;  3,  processus  longus  ;  4,  articular  curface  for  the  mal- 
leus ;  5,  a  convex  tubercle,  processus  lenticularis,  for  articu- 
lation with  stapes ;  6,  rough  surface  for  attachment  of  the 
ligament  of  the  incus. 


FIG.  187.— The  stapes  (after  Testut) :  1, 
base1 ;  2,  anterior  cms ;  3,  posterior  crus ; 
4,  articulating  surface  of  head  of  the 
bone ;  5,  cervix  or  neck. 


each  other;  the  former  has  a  length  of  4J  millimeters,  and  the  latter  a  length 
of  3  to  3J  millimeters.  When  in  position  the  long  process  descends  nearly 
parallel  with  the  manubritim,  but  it  has  less  than  three-fourths  the  length  of 
the  latter.  The  free  end  of  the  long  process  is  turned  sharply  inward  at  right 
angles,  and  terminates  in  a  round  projection,  the  os  orbiculare,  which  is  provided 
with  cartilage  for  articulation  with  the  head  of  the  stapes.  The  short  process  is 


THE  SENSE    OF  HEAI!IX<;.  367 

conical  in  shape  and  is  thicker  than  the  long  process.  It  has  a  horizontal  posi- 
tion, and  is  attached  by  a  thick  ligament  to  the  posterior  wall  of  the  tympanum. 

The  stapes  (Fig.  187)  articulates  with  the  end  of  the  long  process  of  the 
incus ;  its  plane  is  horizontal  and  about  at  right  angles  to  that  process.  It 
measures  3  to  4  millimeters  in  length  and  about  2J  millimeters  in  breadth. 
The  base  of  the  stapes  is  somewhat  oval  in  shape,  the  superior  margin  being 
convex  and  the  inferior  being  slightly  concave.  It  is  set  in  the  fenestra  ovalis, 
an  aperture  measuring  about  3  millimeters  by  1J  millimeters,  and  is  held  in 
place  by  a  narrow  membrane  made  up  of  radial  fibres  of  connective  tissue. 
When  in  position,  the  inner  face  of  the  base  of  the  stirrup  is  covered  with 
lymphatic  endothelium  and  is  washed  by  the  perilymph  of  the  internal  ear; 
the  outer  face,  like  the  other  tympanic  bones  and  the  wall  of  the  cavity,  is 
covered  by  thin  mucous  membrane. 

Movement  of  the  Ossicles. — The  malleus-incus  articulation  is  so  arranged 
that  with  outward  movements  of  the  manubrium  the  head  of  the  malleus 
glides  freely  in  the  joint ;  but  the  lower  margins  of  the  articulating  surfaces 
project  in  such  a  way  that  the  prominences  lock  together  when  the  manubrium 
moves  inward.  Thus,  in  inward  movements  of  the  tympanic  membrane  and 
its  attached  manubrium,  the  malleus  and  the  incus  move  together  like  one 
rigid  piece  of  bone,  the  motions  of  the  manubrium  and  the  long  process  of  the 
incus  being  parallel.  Of  the  malleus-incus  articulation  Helmholtz1  says: 
"  In  its  action  it  may  be  compared  with  the  joints  of  the  well-known  Breguet 
watch-keys,  which  have  rows  of  interlocking  teeth,  offering  scarcely  any  resist- 
ance to  revolution  in  one  direction,  but  allowing  no  revolution  whatever  in  the 
other."  In  the  outward  movements  the  locking  teeth  or  projections  are  prob- 
ably still  kept  in  apposition,  under  ordinary  circumstances,  through  the  elastic 
reaction  of  the  ligament  and  the  stapedial  attachment  of  the  incus.  Should, 
however,  the  tympanic  membrane  be  forced  unduly  outward,  as  by  increase  of 
pressure  within  the  tympanum  or  by  rarefaction  of  air  in  the  auditory  meatus, 
the  incus  only  follows  the  malleus  for  a  certain  distance,  the  latter  completing 
its  motion  by  gliding  in  the  joint.  There  is  thus  no  danger  of  the  stapes  being 
torn  out  of  the  oval  window.  The  hammer  and  the  anvil,  suspended  by  their 
ligaments,  move  freely  about  an  axis  one  end  of  which  is  found  at  the  origin  of 
the  anterior  part  of  the  anterior  ligament  of  the  malleus,  and  the  other  end  in 
the  origin  of  the  ligament  which  is  continuous  with  the  short  process  of  the  incus 
(Fig.  185).  In  inward  motions  of  the  tympanic  membrane  the  ossicles  move  like 
a  single  bone  around  the  axis  of  suspension ;  and  as  the  distance  measured  from 
the  axis  of  rotation  to  the  tip  of  the  manubrium,  where  the  power  is  applied,  is 
about  one  and  one-half  times  the  distance  to  the  end  of  the  long  process  of  the 
incus,  where  the  effect  is  produced,  the  motions  transmitted  to  the  stapes  can  have 
but  two-thirds  the  amplitude  of  the  movements  of  the  tip  of  the  manubrium,  but 
have  one  and  one-half  times  their  force.  It  will  be  noticed  that  a  large  pro- 
portion of  the  mass  of  both  anvil  and  hammer  is  found  above  their  axis  of  rota- 
tion ;  this  upper  portion  acts  as  a  counterpoise  to  the  parts  below  which  are  directly 
1  Sensations  of  Tone,  trans,  by  Ellis,  1885,  p.  133. 


368  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

concerned  in  the  lever  action.  The  bony  lever  being  thus  balanced,  it  is  less 
difficult  to  understand  its  known  sensitiveness  to  impulses  that  are  inconceivably 
weak.  The  tense  tympanic  membrane,  by  reason  of  its  funnel  shape,  resists 
strong  inward  compression  ;  hence  the  stapes  is  prevented  from  being  pressed 
too  far  inward.  The  maximum  amplitude  of  motion  of  the  stapes  in  the 
fenestra  is  very  small,  being  only  about  -fa  millimeter  to  -fa  millimeter,  while 
that  of  the  centre  of  the  tympanic  membrane  is  about  fa  millimeter  to  ^ 
millimeter. 

The  functional  movements  of  the  auditory  ossicles  are  not  molecular  but 
are  molar  vibrations,  the  chain  of  bones  moving  in  a  body.  The  sole  purpose 
of  this  apparatus  of  the  middle  ear  is  to  transmit  exactly  the  variations  of 
pressure  in  the  air  of  the  external  auditory  meatus  to  the  peri  lymph  which 
bathes  the  foot  of  the  stapes — in  other  words,  to  convert  air-waves  into  a 
similar  series  of  water-waves.  In  the  words  of  Helmholtz,1  "  The  mechanical 
problem  which  the  apparatus  within  the  drum  of  the  ear  had  to  solve  was  to 
transform  a  motion  of  great  amplitude  and  little  force,  such  as  impinges  on 
the  drum-skin,  into  a  motion  of  small  amplitude  and  great  force,  such  as  had 
to  be  communicated  to  the  fluid  in  the  labyrinth. " 

The  adaptation  of  the  apparatus  of  the  middle  ear  to  this  end  is  worthy 
of  careful  consideration.  In  the  first  place,  it  will  be  noticed  that  the  area 
of  the  fenestra  ovalis  which  receives  the  impulses  of  the  stapes  is  but  a  small 
fraction  of  the  surface  of  the  tympanic  membrane  on  which  the  air-waves 
impinge,  the  latter  area  being  some  fifteen  to  twenty  times  greater  than  the 
former,  so  that  the  energy  of  air-motion  is,  in  a  fashion,  concentrated.  In  the 
second  place,  as  previously  observed,  the  lever  mechanism  of  the  auditory 
ossicles  is  such  that  the  movements  of  the  end  of  the  long  process  of  the  incus 
have  two-thirds  the  amplitude  of  those  of  the  tip  of  the  manubrium,  but 
about  one  and  one-half  times  their  force.  It  should  also  be  noticed  that  the 
membrane  fastening  the  foot  of  the  stapes  in  the  fenestra  is  somewhat  less 
tense  on  the  upper  side,  so  that  the  top  of  the  oval  foot-piece  has  a  freer 
motion  than  the  bottom,  and  the  head  of  the  stirrup  rises  slightly  with  inward 
motions.  In  the  third  place,  it  has  been  demonstrated  by  Helmholtz2  that  the 
shape  of  the  tympanic  membrane  peculiarly  adapts  it  for  transforming  weak 
movements  of  wide  amplitude  into  strong  ones  of  small  compass.  For  this 
membrane  is  not  a  simple  funnel  depressed  inwardly,  but  the  radii  are  slightly 
curved  with  the  convexity  outward,  a  shape  chiefly  due  to  the  tension  of  the 
elastic  circular  fibres  of  the  membrane  on  its  inner  face,  these  being  most 
numerous  toward  the  circumference.  Air-waves  beating  upon  this  convexity 
flatten  the  curve  somewhat,  and  their  whole  energy  must  be  concentrated,  with 
increased  intensity  but  loss  of  motion,  at  the  central  point  of  the  membrane. 
This  effect  may  be  illustrated  by  holding  a  slightly-curved  brass  wire,  several 
inches  in  length,  with  its  plane  perpendicular  to  the  surface  of  a  table  and 
supported  on  its  ends.  When  one  end  of  the  wire  is  held  immovable,  up-and- 
down  motions  of  the  arch  are  transferred  to  the  free  end  with  diminished 
1  Op.  cit.,  p.  134.  2  Op.  cit. 


TEE  SENSE    OF  HEARING.  369 

amplitude.  The  wire  represents  a  single  radial  fibre  of  the  tympanic  mem- 
brane, and  the  funnel  shape  of  this  membrane  is  adapted  to  concentrating  this 
motion  of  the  radial  fibres  upon  the  manubrium.  The  same  effect  is  illus- 
trated by  the  fact  that  when  a  string  or  a  rope  is  stretched  between  two  points, 
no  matter  how  tightly,  it  always  sags  at  its  middle;  the  weight  of  the  cord, 
however  slight,  is  sufficient  to  give  it  a  curved  course,  and  produces  a  corre- 
sponding traction  on  the  points  of  support. 

Eustachian  Tube. — That  the  tympanic  membrane  may  maintain  its 
freedom  of  motion,  it  is  obviously  necessary  that  the  average  atmospheric 
pressure  on  each  side  of  it  should  remain  the  same.  This  equality  of  pressure 
is  maintained  through  the  medium  of  the  Eustachian  tube,  a  somewhat  trumpet- 
shaped  canal  which,  beginning  in  the  lower  anterior  walls  of  the  tympanum, 
runs  downward,  forward,  and  inward,  and  terminates  in  a  slit  in  the  side  of 
the  upper  part  of  the  pharynx.  The  Eustachian  tube  is  lined,  like  the  walls 
of  the  tympanum,  with  ciliated  epithelium,  the  cilia  working  in  such  a  way 
as  to  carry  into  the  pharynx  such  secretions  as  may  arise  from  the  mucous 
membrane  of  the  middle  ear.  The  pharyngeal  opening  of  the  Eustachian  tube 
is  probably  normally  closed,  but  it  may  easily  be  made  to  open  by  increase  or 
decrease  of  air-pressure  within  the  pharynx,  as  may  be  produced  by  closing 
the  nose  and  mouth  and  either  forcing  air  into  the  pharynx  by  strong  expiration 
or  rarefying  it  by  suction.  In  the  former  case  the  air-pressure  within  the 
tympanum  is  increased,  and  in  the  latter  it  is  diminished.  When  air  is  thus 
made  to  enter  or  to  leave  the  tympanum,  a  sensation  of  a  sudden  snap  and 
a  dull  crackling  noise  in  the  ear  is  experienced.  The  lower  end  of  the  tube 
is  normally  opened  during  the  act  of  swallowing,  and  it  is  at  this  moment  that 
the  intra-  and  extra-tympanic  air-pressures  are  equalized. 

Muscles  of  the  Middle  Ear. — Two  muscles  are  devoted  to  adjusting  the 
tension  of  the  auditory  mechanism  of  the  middle  ear.  The  tensor  tympani  is 
lodged  within  a  groove  which  is  just  above  and  about  parallel  with  the  Eusta- 
chian tube.  It  terminates  externally  in  a  long  tendon  which  bends  nearly  at 
right  angles  round  the  outer  edge  of  the  groove  and  is  inserted  into  the 
handle  of  the  malleus  near  the  neck.  Contraction  of  the  tensor  tympani 
thus  pulls  inward  the  tympanic  membrane,  increases  its  tension,  and  some- 
what dampens  its  vibrations.  At  the  same  time  a  strain  is  put  upon  the 
chain  of  ossicles,  the  toothed  processes  of  the  malleus  and  incus  are  brought 
more  closely  together,  and  the  foot  of  the  stapes  is  pressed  into  the  oval  win- 
dow, increasing  the  pressure  upon  the  fluids  of  the  internal  ear.  It  is  si  id 
that  the  relaxed  tympanic  membrane,  particularly  after  section  of  the  tensor 
tympani  muscle,  is  thrown  into  sympathetic  vibration  with  comparative  ease, 
and  is  in  this  condition  best  adapted  to  respond  to  weak  ai'-rial  impulses  and 
to  the  periodic  waves  of  musical  notes.  When  the  membrane  is  tense  its 
vibrations  are  damped,  and  it  is  particularly  fitted  to  transmit  noises  and  con- 
sonantal sounds,  and  thus  the  muscle  involved  would  seem  important  to  the 
clear  transmission  of  ordinary  speech,  though  its  effect  would  be  to  decn  u-<- 
the  acuteness  of  hearing.  According  to  Hensen,1  the  tensor  tympani  muscle 

1  Hermann's  Handbuch  der  Physioloyie,  1880. 
VOT,.  TT.— 24 


370  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

is  excited  to  reflex  contraction  by  the  initial  waves  of  a  sound,  resulting  in  a 
closer  union  of  the  toothed  processes  of  the  malleus  and  incus,  so  that  there  is 
less  loss  of  motion  in  the  subsequent  vibrations.  But  Ostermann  l  believes 
the  muscle  to  be  chiefly  a  protective  mechanism  which  by  its  contraction 
prevents  oscillation  of  so  wide  an  amplitude  as  to  be  hurtful,  and  that  its 
reflex  action  is  called  forth  chiefly  by  very  loud  noises  (PL  1,  Fig.  1).  The 
stapedius  is  a  small  muscle  imbedded  in  the  inner  wall  of  the  tympanum, 
near  the  fenestra  ovalis.  Its  tendon,  passing  forward,  is  inserted  into  the 
neck  of  the  stapes.  Contraction  of  the  muscle  would  cause  a  slight  rotation 
of  the  stapes  round  a  vertical  axis,  so  that  the  hinder  part  of  the  foot  of  the 
ossicle  would  be  pressed  more  deeply  into  the  fenestra,  while  the  remaining 
portion  would  be  drawn  out  of  it.  Its  action  probably  reduces  the  pressure 
in  the  cavity  of  the  perilymph,  and  thus  is  antagonistic  to  that  of  the  tensor 
tympani  (PL  1,  Fig.  2,  A,  B). 

Vibrations  of  the  Tympanic  Membrane. — It  is  a  general  physical  law 
that  every  elastic  body  can  be  made  to  vibrate  more  easily  at  one  definite  rate 
than  at  any  other.  The  musical  tone  represented  by  this  rate  of  vibration  is 
known  as  the  prime  or  fundamental  tone  of  the  body.  Membranes  have  funda- 
mental tones  (see  p.  383),  whose  pitch  is  determined  by  their  area,  thickness, 
and  tension,  but  they  differ  from  rods  and  strings  in  being  less  strictly  confined 
to  a  single  fundamental  tone  in  their  vibration.  The  tympanic  membrane  is 
quite  peculiar  in  that  it  can  hardly  be  said  to  have  a  definite  fundamental  tone. 
It  would  obviously  be  a  great  imperfection  in  an  organ  of  hearing  were  cer- 
tain sounds  intensified  by  it  out  of  proportion  to  others,  as  would  be  the  case 
if  the  tympanic  membrane  had  a  marked  fundamental  tone  of  its  own.  This 
is  prevented  in  the  case  of  the  membrana  tympani  probably  both  by  reason  of 
the  peculiar  form  of  its  surface  and  its  structure,  and  also  because  its  oscilla- 
tions are  damped  by  the  pressure  of  the  malleus  held  in  position  by  the  other 
mechanisms  of  the  tympanum.  When  the  tympanic  membrane  is  perforated 
or  is  wholly  removed,  without  destructive  inflammatory  changes  in  the  middle 
ear,  sounds  are  still  heard,  though  usually  with  diminished  loudness.  A 
musician  who  had  suffered  this  accident  was  no  longer  able  to  play  his  violin, 
probably  because  sounds  of  different  pitch  ceased  to  be  perceived  in  their  true 
relations  of  loudness.  We  may  thus  conclude  that  the  function  of  the  tym- 
panic membrane  is  not  only  to  guard  against  injury  to  the  delicate  mem- 
branes of  the  fenestra}  and  the  internal  ear,  but  also  to  transmit  to  the  ossicles 
sonorous  vibrations  with  their  true  proportion  of  intensity.  The  membranes 
covering  the  round  and  oval  windows  of  the  internal  ear  have  no  means  of 
damping  sympathetic  vibrations  (see  p.  385),  and,  should  complex  air-waves 
strike  directly  upon  them,  they  would,  probably,  by  sympathetic  resonance, 
respond  more  powerfully  to  tones  of  certain  pitch  than  to  any  others. 

The  sensation  of  sound  may  be  excited  by  conduction  through  the  bones 
of  the  skull  as  well  as  in  the  ordinary  way.  Thus,  a  tuning-fork  set  vibrating 
and  held  between  the  teeth  or  on  the  forehead  is  heard  perfectly,  and  more 

1  Archivfur  Anatomic  und  Physiologic,  1898,  S.  75. 


THE  SENSE    OF  HEARING. 


371 


loudly  when  the  ears  are  closed  than  when  open.  The  vibrations  thus  con- 
ducted probably  partly  affect  the  internal  ear  directly,  and  partly  indirectly  by 
setting  in  oscillation  the  tympanic  membrane.  It  is  said  that  when  the  sound 
of  a  tuning-fork  held  close  to  the  ear  dies  away,  it  may  again  be  heard  if  the 
handle  of  the  fork  be  prosed  against  the  teeth.  When  the  tone  now  fails, 
it  once  more  becomes  audible  if  one  of  the  ear-passages  is  lightly  closed,  and 
the  sound  seems  to  be  on  the  side  which  is  closed.  The  sensation  failing,  it 
may  again  be  aroused  if  the  appropriately  formed  handle  of  the  fork  be 
inserted  in  the  auditory  meatus.1 

Normal  individuals  differ  greatly  in  their  keenness  of  hearing,  and  tests 
show  frequently  disparity  in  the  sensibility  of  the  two  ears.  The  hearing 
ability  of  children  is  said  to  improve  up  to  the  age  of  twelve  years.  There 
is  no  functional  relation  between  keen  hearing  and  sensibility  to  pitch.2 

The  Internal  Ear,  or  Labyrinth. — The  internal  ear  is  the  site  of  the  true 
organ  of  hearing.  The  membranous  labyrinth  (PL  1,  Fig.  4  ;  Fig.  191)  is  a  com- 
plicated system  of  membranous  tubes  and  sacs,  in  which  terminate  at  particular 
points  the  filaments  of  the  auditory  nerve ;  it  is  contained  within  a  chamber, 
the  bony  labyrinth,  hollowed  out  in  the  petrous  bone.  The  cavity  of  the  bony 
labyrinth  (Figs.  188,  189)  consists  of  a  median  part,  the  vestibule,  which  is  pro- 
longed posteriorly  in  the  system  of  semicircular  canals  and  anteriorly  in  the 
cochlea.  The  vestibule  is  a  space  which  measures  about  one-fifth  of  an  inch 
in  diameter,  and  it  is  perforated  in  its  outer  wall  by  an  oval  opening  known 
as  the  fenestra  ovalis.  The  semicircular  canals  are  three  tubes  of  circular 


5  9        10 

FIG.  188.— Right  bony  labyrinth,  viewed  from 
outer  side :  the  figure  represents  the  appearance 
produced  by  removing  the  petrous  bone  down  to 
the  denser  layer  immediately  surrounding  the 
labyrinth  (from  Quain,  after  Sommering) :  1,  2, 3, 
the  superior,  posterior,  and  horizontal  semicir- 
cular canals ;  4,  5,  6,  the  ampullae  of  the  same ; 
7,  the  vestibule ;  8,  the  fenestra  ovalis ;  9,  fenestra 
rotunda;  10,  first  turn  of  the  cochlea;  11,  second 
turn ;  12,  apex. 


FIG.  189.— Interior  view  of  left  bony  labyrinth  after 
removal  of  the  superior  and  external  walls  (from 
Quain,  after  Sommering) :  1,  2,  3,  the  superior,  pos- 
terior, and  horizontal  semicircular  canals ;  4,  fovea 
hemi-elliptica ;  5,  fovea  hemispherica ;  6,  common 
opening  of  the  superior  and  posterior  semicircular 
canals ;  7,  opening  of  the  aqueduct  of  the  vestibule ; 
8,  opening  of  the  aqueduct  of  the  cochlea;  9,  the 
scala  vestibuli ;  10,  scala  tympani ;  the  lamina  spiralis 
separating  9  and  10. 


section,  known  respectively  as  the  anterior  or  superior,  the  posterior,  and  the 

1  Rinne,  quoted  by  Ilensen  :  Hermann 's  Handbuzh  der  Physiologic,  3880,  Bd.  iii.  Th.  2,  S.  26. 

2  Seashore  :    "Studies  in  Psychology,"  Bulletin  University  of  Iowa,  1899. 


372 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


external  or  horizontal  semicircular  canal.  Their  planes  are  at  right  angles  to 
one  another,  so  that  they  occupy  the  three  possible  dimensions  of  space.  The 
external  canal  lies  in  a  nearly  horizontal  plane,  while  the  other  two  approach 
the  vertical.  Each  canal  is  dilated  at  one  extremity  into  a  globular  cavity 

which  is  more  than  twice  the  diameter  of  the 
canal  itself,  and  which  is  known  as  the  am- 
pulla. The  anterior  and  posterior  canals 
unite  near  the  ends  not  provided  with  am- 
pullae, and  they  enter  the  vestibule  as  a  com- 
mon tube.  Anteriorly  the  cavity  of  the 
4  vestibule  is  continued  as  a  tube  of  complex 
internal  structure  which  is  coiled  upon  itself 
two  and  one-half  times,  and  which,  from  its 
resemblance  to  the  shell  of  a  snail,  is  known 
as  the  cochlea  (PL  1,  Fig.  3).  The  osseous 
cochlea  may  be  conceived  as  formed  by  a  bony  tube  turned  about  a  bony  central 
pillar,  the  modiolm,  which  diminishes  in  diameter  from  the  base  to  the  apex 
of  the  cochlea.  From  the  modiolus  a  bony  shelf  stretches  into  the  cavity  of 
the  tube,  incompletely  dividing  it  into  two  tubular  chambers,  winding  round 
the  modiolus  like  a  circular  staircase,  the  upper  of  which  chambers  we  shall 


FIG.  190.— Diagram  of  the  osseous  cochlea 
laid  open  (after  Quain) :  1,  scala  vestibuli ; 
2,  lamina  spiralis  ;  3,  scala  tympani ;  4,  cen- 
tral pillar  or  modiolus. 


—9 


FIG.  191.— Diagram  of  right  membranous  labyrinth  seen  from  the  external  side  (after  Testut) :  1,  utri- 
cle ;  2,  3,  4,  superior,  posterior,  and  horizontal  semicircular  canals  ;  5,  saccule  ;  6,  ductus  endolymphat- 
icus,  with  7,  7',  its  twigs  of  origin ;  8,  saccus  endolymphaticus ;  9,  canalis  cochlearis,  with  9',  its  vestibular 
cul-de-sac,  and  9",  its  blind  extremity ;  10,  canalis  reuniens. 

soon  learn  to  know  as  the  scala  vestibuli,  and  the  lower  chamber  as  the  scala 
tympani  (Fig.  190 ;  PI.  1,  Fig.  3).  The  bony  shelf  mentioned  above  as  partly 
bisecting  the  cochlear  tube  has,  of  course,  like  the  latter,  a  spiral  course,  and  is 
known  as  the  lamina  spiralis ;  its  importance  as  a  supporter  of  the  auditory- 
nerve  filaments  will  soon  be  seen. 

Contained  within  the  cavity  of  the  bony  labyrinth,  and  parallel  with  its  walls, 
is  the  membranous  labyrinth,  in  which  are  found  the  essential  structures  of  the 
organ  of  hearing  (PL  lj  Fig.  4  ;  Fig.  191).  The  membranous  labyrinth  is  filled 
with  a  somewhat  watery,  mucin-holding  fluid,  the  endolymph,  while  a  similar 
fluid,  the  perilymph,  is  found  outside  it  and  within  the  osseous  labyrinth.  The 


THK  XEXHi:  o/'  y//-;.i/,'/.v<v.  ;;:;; 

perilymph  space,  which  is  lined  by  lymphatic  epithelium,  is  in  communication, 
along  the  sheath  of  the  auditory  nerve,  with  the  subdiiral  and  subarachnoid 
lymph-areas  of  the  brain.  Numerous  sheets  and  bars  of  connective  tissue  cross 
from  the  wall  of  the  bony  to  that  of  the  membranous  labyrinth  and  help  support 
the  latter.  That  part  of  the  membranous  labyrinth  lying  within  the  vestibule 
is  composed  of  two  separate  sacs — a  larger  posterior,  known  as  the  utricle  or 
utriculus,  and  a  smaller,  more  anterior,  known  as  the  saccule  or  sacculus.  The 
plane  of  division  between  the  two  sacs  ends  opposite  the  fenestra  ovalis  (PI.  1, 
Fig.  4).  Though  the  sacs  are  quite  separate,  their  cavities  are  indirectly  continu- 
ous, through  the  union  of  two  small  tubes  arising  from  either  sac,  which  tubes 
unite  to  form  the  ductus  endolymphnticits,  a  tube  running  inward  through  a 
canal  in  the  petrosal  bone  and  ending  blindly  in  a  dilated  flattened  extremity, 
the  saccus  endofymphaticus,  this  being  supported  between  the  layers  of  the 
dura  mater  within  the  cavity  of  the  skull  (PL  1,  Fig.  4).  Bundles  of  audi- 
tory-nerve fibres  penetrate  the  wall  of  each  sac.  The  utricle  gives  rise  to  the 
membranous  semicircular  canals,  which  communicate  with  it  at  five  points, 
it  being  remembered  that  the  anterior  and  posterior  canals  fuse  into  a  single 
tube  at  the  ends  not  provided  with  ampullae,  and  that  they  have  a  common 
entrance  into  the  utricle.  The  saccule  is  continuous  by  a  narrow  tube,  the 
canalis  reuniens,  with  that  division  of  the  membranous  labyrinth  contained 
within  the  cochlea  and  known  as  the  canalis  cochlearis.  The  auditory  nerve 
really  consists  of  two  distinct  divisions  having  separate  origins  and  different 
distributions.  One  of  these  branches  passes  finally  to  the  cochlea,  and  the 
other  to  the  vestibule  and  the  semicircular  canals.  The  nerve  approaches  the 
labyrinth  by  way  of  a  canal  known  as  the  meatus  audilorius  internusy  and 
on  reaching  the  angle  between  the  vestibule  and  the  base  of  the  cochlea  the 
cochlear  division  passes  to  the  cochlea.  The  remainder  of  the  nerve  consists 
of  two  divisions,  the  superior  of  which  is  distributed  to  the  utricle  and  to  the 
ampullae  of  the  anterior  and  horizontal  semicircular  canals ;  the  inferior  branch 
supplies  the  saccule  and  the  posterior  semicircular  canal.  The  inner  wall  of 
both  utricle  and  saccule  is  developed  at  a  particular  spot  into  a  low  mound, 
the  macula  acustica,  made  up  of  an  accumulation  of  the  connective-tissue  ele- 
ments of  the  membranous  wall  and  covered  by  a  peculiarly  modified  epithe- 
lium, the  auditory  epithelium  (Fig.  192).  All  the  auditory-nerve  filaments  that 
enter  the  saccule  and  utricle  respectively  pass  to  these  mounds  and  there  enter 
into  relation  with  the  auditory  epithelium. 

As  the  auditory-nerve  endings  are  confined  to  a  particular  area  in  the 
utricle  and  the  saccule,  so  the  nerve-fibres  supplying  the  semicircular  canals 
arc  limited  to  a  certain  part  of  the  ampulla  of  each  canal.  The  tissue  of  the 
wall  of  the  ampulla  is  developed  into  a  ridge  projecting  into  the  cavity  in  a 
direction  across  its  long  axis.  This  ridge,  present  in  each  ampulla,  is  called 
the  crista  acustica;  it  is  capped  by  a  thick  layer  of  columnar  epithelial  cells, 
the  auditory  epithelium,  which  thins  away  at  the  border  of  the  crista  into 
the  sheet  of  flattened  cells  by  which  the  rest  of  the  ampulla  is  lined.  The 
auditory  cells  (Fig.  192)  are  said  to  be  of  two  kinds — one,  cylindrical  in 


374 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


shape  and  reaching  only  part  way  to  the  basement  membrane,  the  hair-cells  ; 
the  other,  narrow  and  elongated,  the  supporting  or  sustentacular  cells.  The 
former  are  peculiar  in  the  fact  that  from  their  free  ends  there  project  long, 
stiff,  hair-like  processes.  The  filaments  of  the  ampullary-nerve  branches 
pass  through  the  cristae  and  encircle  the  bodies  of  the  hair-cells.  The  cells 
•  covering  the  maculae  acusticce  have 

essentially  the  same  structure  as  those 
just  described,  though  in  the  maculae 
the  auditory  hairs  are  shorter  than  in 
the  cristae.  Seated  on  the  free  surface 
of  the  macular  epithelium  is  a  fibrous 
mass  which  is  said  to  be  a  normal 
structure,  and  not,  like  a  somewhat 
similar  mass  found  covering  the  eristse 
in  post-mortem  section,  a  coagulum 
due  to  the  method  of  preparation. 
Imbedded  in  the  membrane  over  the 
maculae  of  both  saccule  and  utricle 
small  crystals,  otoliths  or  oto- 


are 


FIG.  192.— Diagram  showing  the  epithelial  cells  of 
a  macula  or  a  crista  (after  Foster) :  1,  cylinder  or 
hair-cell ;  2,  the  same,  enveloped  in  a  nest  of  nerve- 
fibrils  ;  3,  4,  5,  forms  of  rod-  or  spindle-cells. 


coma,  composed  chiefly  of  carbonate 
of  lime.  Otoconia  are  also  found 
less  constantly  in  the  ampullae  and 
even  in  the  perilymph  space  of  the  cochlea.  In  fishes  there  are  large  masses 
of  calcareous  matter,  otoliths,  attached  to  the  wall  of  the  auditory  sac. 

General  Anatomy  of  the  Cochlea. — By  far  the  most  complex  structure  of 
the  ear  is  found  in  the  cochlea  (PI.  1 .  Figs.  1,  3,  4 ;  Figs.  1 88-191).  The  bony 
cochlea  continues  from  the  anterior  wall  of  the  vestibule,  and  in  the  upright  posi- 
tion of  the  head  the  axis  of  the  modiolus  is  nearly  horizontal,  pointing,  from  base 
to  apex,  outward  and  slightly  down  and  forward,  the  base  of  the  cochlea  being 
formed  by  the  inner  surface  of  the  petrous  bone.  The  membranous  cochlea, 
canalis  or  ductus  cochlearis,  is  a  tube  of  nearly  triangular  cross-section  which 
winds  round  the  modiolus  from  base  to  apex  (Fig.  193).  The  base  or  outer  side 
of  this  triangle  is  attached  closely  to  the  bony  wall  of  the  cochlea ;  the  upper 
side,  supposing  the  modiolus  to  be  vertical  with  its  apex  above,  is  made  of  a  thin 
sheet  of  cells  known  as  the  membrane  of  Reissner  ;  the  lower  side  is  made  up 
partly  of  the  bony  margin  of  the  lamina  spiralis  and  partly  of  a  membrane, 
radially  striated,  stretched  across  from  the  edge  of  the  spiral  lamina  to  the  side 
wall  of  the  cochlea ;  this  is  called  the  basilar  membrane,  membrana  basilaris. 
The  coiled  tube  forming  the  bony  cochlea  is  thus  divided  by  the  lamina  spiralis 
and  the  canalis  cochlearis  into  three  tubes  which  wind  spirally  and  parallel 
round  the  modiolus.  The  canalis  cochlearis  contains  endolymph,  and  its  cav- 
ity ends  blindly  above  and  below,  but  is  continuous  by  way  of  the  narrow 
canalis  reuniens  with  that  of  the  saccule.  The  tubes  above  and  below  the 
canalis  cochlearis  are  perilymph-spaces ;  it  will  be  noticed  that  there  is  no 
such  space  on  the  outer  side  of  the  membranous  cochlea. 


THE  SENSE    OF  HEARING. 


375 


The  upper  tube,  when  followed  down  to  the  base  of  the  cochlea,  is  found 
to  open  freely  into  the  vestibule  of  the  labyrinth  ;  it  is  therefore  known 
as  the  scala  vestibuli.  The  lower  tube  ends  blindly  at  the  base  of  the 
oK-hlea,  but,  where  this  part  bulges  into  the  tympanum  as  the  "promontory" 
of  its  inner  wall,  it  is  perforated  by  the  aperture  known  as  the  feneatra 
rotunda,  whose  proper  membrane  alone  prevents  the  perilymph  from  escaping 


FIG.  193.— Diagram  of  a  transverse  section  of  a  whorl  of  the  cochlea  (after  Foster) :  Sc.  V,  scala  vestib- 
uli; Sc.  T,  scala  tympani;  C.Chi,  canalis  cochlearis;  Lam.sp,  lamina  spiralis;  Gg.sp,  ganglion  spirale; 
n.aud,  auditory  nerve ;  m. R,  membrane  of  Reissner ;  Str.v,  stria  vascularis ;  Lg.sp,  ligamentum  spirale ; 
U,  lymphatic  epithelioid  lining  of  basilar  membrane  on  the  tympanic  side ;  m.b,  basilar  membrane ; 
Org.C,  organ  of  Corti;  L.t,  labium  tympanicum;  Ib,  limbus ;  L.v,  labium  vestibulare;  m.t,  tectorial 
membrane. 

into  the  middle  ear.  This  tube  is  therefore  known  as  the  scala  tympani. 
From  its  central  position  the  membranous  cochlear  canal  is  frequently  known 
as  the  scala  media.  The  scala  vestibuli  and  the  scala  tympani  both  decrease  in 
size  as  they  wind  from  the  base  to  the  apex  or  cupola  of  the  cochlea ;  the 
membranous  cochlear  canal,  on  the  contrary,  increases  in  section  from  base  to 
apex  until  near  the  top ;  hence  the  width  of  the  basilar  membrane  and  the 


376  AN  AMERICAN   TEXT-BOOK  .OF  PHYSIOLOGY. 

length  of  its  radial  fibres  increase  from  below  upward.  The  scala  vestibuli 
and  the  scala  tympani  have  no  communication  except  through  a  small  aperture 
under  the  cupola  of  the  cochlea,  known  as  the  helicotrema ;  this  is  bounded 
by  the  hook-like  termination,  the  hamulm,  of  the  bony  lamina  spiral  is,  which 
forms  the  greater  part  of  a  ring  completed  by  the  pointed  blind  extremity  of 
the  canalis  cochlearis  fastened  above  it  to  the  cupola. 

The  Transmission  of  Vibrations  through  the  Labyrinth. — Vibrations 
of  the  tympanic  membrane  are  transmitted  as  pulses  of  very  small  amplitude  to 
the  membrane  covering  the  fenestra  ovalis.  The  relatively  considerable  body  of 
perilymph  bathing  the  inner  face  of  this  membrane  must  be  thus  set  in  motion, 
and  there  starts  a  fluid-wave  which  is  free  to  make  its  way  throughout  the 
perilymph-spaces  of  the  vestibule  and  the  semicircular  canals.  It  may  pass 
from  the  vestibule  along  the  scala  vestibuli  to  its  top,  through  the  helicotrema, 
and  back  by  way  of  the  scala  tympani,  at  whose  bottom  it  finally  surges 
against  the  membrane  covering  the  fenestra  rotunda;  or  the  wave  may  be 
transmitted  directly  across  the  membranous  cochlea.  The  fluids  of  the  laby- 
rinth being  physically  incompressible,  the  function  of  the  fenestra  rotunda  as 
a  sort  of  safety-valve  seems  evident.  Politzer  inserted  a  glass  tube  in  the 
round  window,  and  found  that  fluid  in  the  tube  rose  when  strong  air-pressure 
was  brought  to  bear  on  the  outer  side  of  the  tympanic  membrane.  The  cavity 
of  the  membranous  labyrinth  (PI.  1,  Fig.  4)  is  nowhere  in  communication  with 
the  perilymph-space  about  it,  and  we  must  therefore  assume  that  the  irritation 
of  the  auditory  cells  seated  in  its  wall  must  depend  on  vibrations  transmitted 
from  the  perilymph  directly  through  the  membranous  sacs  and  tubes. 

Like  the  perilymph-space,  the  cavity  of  the  membranous  labyrinth  is  in 
communication  throughout,  though  in  certain  situations  the  connection  of 
adjacent  parts  is  very  indirect.  Thus,  though  the  semicircular  canals  open 
freely  at  both  ends  into  the  utricle,  the  utricle  and  saccule  are  only  brought 
into  union  by  the  two  narrow  tubes  that  unite  to  form  the  ductus  endolym- 
phaticus.  It  will  be  noted  that  by  means  of  this  duct  the  membranous  laby- 
rinth is  really  continued  into  the  cranial  cavity.  The  saccule  in  turn  is 
continuous  with  the  scala  media  of  the  cochlea  by  way  of  the  canalis  reuniens. 

The  Membranous  Cochlea  and  the  Organ  of  Corti  (Figs.  193-195). — 
The  cochlear  division  of  the  auditory  nerve,  together  with  the  nutrient  blood- 
vessels, penetrates  the  modiolus  at  its  base  and  runs  up  through  the  spongy 
interior  of  the  bony  pillar.  As  the  nerve  ascends  through  the  modiolus  its 
fibres  are  gradually  all  diverted  to  run  in  a  radial  direction  between  the  bony 
plates  of  the  lamina  spiralis,  to  terminate  in  the  organ  of  Corti  of  the  canalis 
cochlearis.  A  collection  of  nerve-cells  is  interposed  in  the  course  of  the  audi- 
tory fibres  at  the  base  of  the  lamina  spiralis. 

A  complete  view  of  the  nerves  of  the  cochlea  would  show  a  central  pillar 
of  nerve-fibres  diminishing  in  thickness  from  below  upward,  and  winding 
round  this  pillar  a  spiral  sheet  of  radially-disposed  nerve-fibres  containing, 
near  their  point  of  departure  from  the  central  pillar,  a  spiral  line  of  ganglion- 
cells  ;  this  collection  of  cells  is  therefore  known  as  the  ganglion  spirale.  The 


THE   SENSE    OF  HEARING.  -,77 

thin,  free  edge  of  the  bony  hnnimr  x/;/m//.s-  is,  in  the  recent  state,  thickened  by 
a  development  of  connective  tissue  forming  a  promontory  known  as  (he  limbus. 
The  free  edge  of  the  limbus  is  in  turn  shaped  in  such  a  way  as  to  make  a  short, 
sharp  projection  in  the  plane  of  the  upper  surface  of  the  lamina  and  a  longer 
projection  in  the  plane  of  its  lower  surface,  leaving  the  free  margin  between 
them  hollowed  out.  The  upper  projection,  which  is  known  as  the  vestibular 
lip,  labium  vcdibn/are,  serves  for  the  attachment  of  the  tectorial  membrane, 
membrana  tectoria,  presently  to  be  described.  The  lower  projection  is  called 
the  tympanic  lip  (labium  tympanicum) ;  to  it  is  attached  the  inner  margin  of 
the  basilar  membrane,  on  whose  inner  half  is  seated  the  very  complex  struct- 
ure known  as  the  organ  of  Corti. 

The  basilar  membrane  is  a  thin  sheet  of  fibrillated  connective  tissue  stretched 
tightly  between  the  tympanic  lip  of  the  limbus  on  the  inside  and  the  spiral 
ligament  (see  p.  379)  on  the  outside.  The  more  median  part  of  the  membrane, 
which  supports  the  organ  of  Corti,  is  thin  and  rigid  and  is  fibrillated  in  a 
radial  direction.  The  outer  part,  which  is  first  thicker  and  then  thinner  again 
near  its  point  of  attachment,  is  distinctly  composed  of  radial  fibres  cemented 
together ;  the  isolated  fibres  are  characterized  by  being  stiff  and  brittle. 

The  organ  of  Corti  (Figs.  193,  194)  has  as  its  supporting  basis  a  series  of 
peculiarly  modified  epithelial  cells,  known  as  the  rods  of  Corti  (Fig.  195,  B,  Br), 
which  are  disposed  along  the  edge  of  the  spiral  lamina  in  two  rows,  an  inner 
and  an  outer.  The  inner  rods  have  their  feet  on  the  basilar  membrane  near  its 
median  attachment ;  they  lean  outward  and  upward,  and  at  their  upper  extrem- 
ity join  or  articulate  with  the  heads  of  the  outer  rods,  whose  feet  are  fastened  to 
the  basilar  membrane  more  externally.  The  two  rows  of  rods  are  thus  joined 
together  like  the  rafters  of  a  house,  and  enclose  beneath  them  a  canal  known 
as  the  tunnel  of  the  organ  of  Corti.  The  inner  rods  are  more  numerous  than 
the  outer,  so  that  the  latter  are  fastened  rather  between  than  to  the  ends  of  the 
former.  Leaning  against  the  inner  or  median  side  of  the  inner  row  of  rods 
is  a  single  row  of  hair-cells  (Fig.  194),  much  like  those  described  as  seated  on 
the  maculae  and  cristse  of  the  labyrinth,  to  which  hair-cells  filaments  of  the 
auditory  nerve  are  distributed.  Closely  applied  to  the  single  row  of  hair- 
cells,  on  the  inner  side,  are  several  rows  of  columnar  cells  gradually  decreas- 
ing in  size  toward  the  median  line,  and  beneath  the  whole  is  a  group  of  nuclei. 
External  to  the  outer  row  of  rods,  and  separated  from  it  by  a  space,  are  four 
parallel  rows  of  hair-cells  known  as  the  cells  of  Corti ;  their  bodies  do  not 
reach  downward  as  far  as  the  basilar  membrane,  and  just  below  each  row  is  a 
bundle  of  nerve-fibres  which  have  traversed  the  tunnel  of  Corti  and  then  have 
changed  their  direction  from  a  radial  to  a  longitudinal  or  spiral  one.  These 
fibres,  and  others  having  a  more  direct  course,  one  by  one  end  in  clusters 
encircling  the  individual  hair-cells. 

Four  rows  of  peculiarly-modified  columnar  cells,  the  cells  of  Deiters,  are 
inserted  closely  between  the  cells  of  Corti,  the  outermost  row  being  external 
to  the  fourth  row  of  Corti.  These  cells  rest  below  on  the  basilar  membrane. 
Still  external  to  these  groups  of  cells  is  a  series  of  rows  of  tall  columnar  cells 


378  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

of  simple  character  supported  upon  the  basilar  membrane,  and  rapidly  decreas- 
ing in  height  externally  into  a  layer  of  cuboidal  epithelium  covering  the  outer 
part  of  the  basilar  membrane.  The  rods  of  Corti  are  peculiarly  shaped  at  the 
top,  the  upper  extremity  of  each  being  bent  at  an  angle  so  as  to  project  exter- 
nally and  parallel  with  the  basilar  membrane ;  these  projections  are  the  pha- 
langar  processes  of  the  rods,  the  phalanges  of  the  inner  row  overlapping  those 
of  the  outer  row.  These  phalangar  processes  of  the  rods  form  the  points  of 
attachment — in  fact,  the  beginning — of  the  reticulate  membrane  (membrana 
reticulata),  a  peculiar  cuticular,  network-like  structure  formed  of  rings  and 
cross-bars,  having  the  appearance  of  certain  vegetable  tissues  seen  under  the 
microscope.  The  reticulate  membrane  stretches  across  the  outer  rows  of  hair- 


m.t 


iig 

n.aud        l.t  i.spn    t.spn        o.spn        ^  __    _ . 

Ig.sp 

FIG.  194.— Diagram  of  the  organ  of  Corti  (from  Foster,  after  Retzius) :  i.r,  inner  rod  of  Corti ;  o.r,  outer 
rod  of  Corti;  i.hc,  inner  hair-cell;  n.c,  the  group  of  nuclei  beneath  it;  o.hc,  outer  hair-cells,  or  cells  of 
Corti ;  C.D,  the  twin  cells  of  Deiters  (four  rows) ;  n.aud,  the  auditory  nerve  perforating  the  tympanic  lip, 
l.t,  and  lost  to  view  among  the  nuclei  beneath  the  inner  hair-cells ;  i.spn,  the  inner  spiral  strand  of  nerve- 
fibrils  ;  t.spn,  the  spiral  strand  of  the  tunnel ;  o.spn,  the  outer  spiral  strand  belonging  to  the  first  row  of 
outer  hair-cells  ;  the  three  succeeding  spiral  strands  belonging  to  the  three  other  rows  are  also  shown  ; 
nerve-fibrils  are  shown  stretching  radially  across  the  tunnel ;  H.c,  Hensen's  cells ;  Cl.c,  Claudius'  cells  ; 
U,  lymphatic  epithelioid  lining  on  the  side  toward  the  scala  tympani ;  Icj.sp,  ligamentum  spirale ;  c,  cells 
lining  the  spiral  groove,  overhung  by  the  vestibular  lip,  l.v ;  m.t,  tectorial  membrane  ;  a  fragment,  torn 
from  it,  remains  attached  to  the  organ  of  Corti  just  outside  the  outermost  row  of  hair-cells. 

cells,  the  body  of  each  of  which  is  enclosed  and  is  held  at  its  top  within  a  ring 
of  the  network  (Fig.  195,  D). 

Each  of  the  cells  of  Deiters,  described  above,  is  continued  upward  in  a 
process  which  is  attached  to  a  cross-bar  or  a  ring  of  the  reticulate  membrane 
next  outside  its  companion-cell  of  Corti.  The  inner  or  median  line  of  the 
Deiters  cell  is  also  modified  into  a  cuticular  thread  fused  below  to  the  basilar 
membrane  and  above  to  a  ring  of  the  reticulate  membrane.  Thus  the  audi- 
tory hair-cells  of  Corti  may  be  regarded  as  suspended  from  the  reticulate  mem- 
brane, which  in  turn  is  supported  by  the  cuticular  processes  of  the  cells  of 
Deiters.  which  rest  upon  the  basilar  membrane,  and  by  the  phalangar  pro- 
cesses of  the  rods  of  Corti.  The  physical  contact  of  the  cells  of  Corti  with 
those  of  Deiters  is  so  intimate — if,  indeed,  their  substance  is  not  continuous — 
that  impulses  generated  in  the  one  can  probably  easily  be  communicated  to 
the  other. 

The  upper  wall  of  the  canalis  cochlearis  is  made  of  a  sheet  of  homogenous, 
fibrillated  connective  tissue  covered  with  flat  cells,  and  stretches  from  the 
limbus  of  the  spiral  lamina  outward  and  upward  to  the  side  wall  of  the 


THE   SENSE    OF  HEARING. 


379 


cochlea.  It  is  known  as  the  membrane  of  Reismer.  The  periosteal  con- 
nective tissue  of  the  bony  wall  of  the  cochlea  is  generally  well  developed 
within  the  area  enclosed  between  the  membrane  of  Reissner  and  the  membrana 
basilaris;  it  is  particularly  thick  at  the  line  of  division  between  the  scala  media 
and  the  scala  tympani,  where  it  forms  a  projecting  ridge  at  the  outer  attach- 
ment of  the  basilar  membrane.  This  ridge  is  the  spiral  ligament ;  an  exten- 


m.b 


FIG.  195.— Diagram  of  the  constituents  of  the  organ  of  Corti  (from  Foster,  after  Retzius) :  A,  inner  hair 
cell;  A',  the  head,  seen  from  above;  B,  inner,  B',  outer,  rod  of  Corti;  ph,  in  each,  is  the  phalangar  pro- 
cess ;  c,  the  twin  outer  hair-cell ;  C.c,  the  cell  of  Corti ;  h;  its  auditory  hairs ;  n,  its  nucleus ;  x,  Hensen's 
body ;  D.c,  cell  of  Deiters ;  n',  its  nucleus ;  ph.p,  its  phalangar  process ;  fil,  the  cuticular  filament ;  m.b, 
basilar  membrane ;  m.r,  reticulate  membrane ;  c',  the  head  of  a  cell  of  Corti,  seen  from  above ;  D,  the 
organ  of  Corti,  seen  from  above ;  i.hc,  the  heads  of  the  inner  hair-cells ;  i.r.h,  the  head  and  phalangar  pro 
cess  of  the  inner  rod ;  o.r.h,  the  head  of  the  outer  rod,  with  ph.p,  its  phalangar  process,  covered  to  the  left 
hand  by  the  inner  rods,  but  uncovered  to  the  right ;  o.h.c,  the  heads  of  the  cells  of  Corti,  supported  by 
the  rings  of  the  reticulate  membrane ;  ph,  one  of  the  phalangse  of  the  reticulate  membrane. 

sion  from  it,  gradually  decreasing  in  thickness,  reaches  into  both  the  vestibular 
and  the  tympanic  scala. 

A  thick  layer  of  both  columnar  and  cuboidal  epithelium  lines  the  con- 
nective tissue  forming  the  outer  wall  of  the  canalis  cochlearis.  This  epithe- 
lium is  peculiar  in  that  the  blood-vessels  of  the  underlying  connective  tissue 
penetrate  between  the  epithelial  cells  themselves.  The  tectvrial  membrane 
(membrana  tectoria)  is  a  sheet  of  radial ly-fibri Hated  tissue,  thin  at  its  point  of 
attachment  to  the  vestibular  lip  of  the  limbus,  and  becoming  thicker  and  then 
thinner  again  as  it  stretches  out  over  the  organ  of  Corti,  reaching  as  far  as  the 
most  external  row  of  hair-cells.  It  is  said  to  lie  in  actual  contact  with  the 
rods  of  Corti  and  the  free  ends  of  the  hair-cells,  and  it  has  been  presumed  to 
serve  as  a  damper  for  the  vibrations  imparted  to  the  organ  of  Corti. 


380  AN  AMERICAN  TEXT-BOOK   OF   PHYSIOLOGY. 

The  researches  of  Howard  Ayers l  have  led  him  to  conclusions  concerning 
the  minute  anatomy  of  the  ear  materially  different  from  those  just  presented. 
Thus,  Ayers  asserts  that  the  so-called  membrana  tectoria  is  nothing  more 
than  the  matted  mass  of  hairs  "  which  spring  from  the  tops  of  the  hair-cells 
and  form  a  waving  plume  on  the  crest  of  the  ridge  of  the  organ  of  Corti." 
He  also  holds  the  membrana  reticulata  and  several  other  structures 
described  by  different  authors  to  be  nothing  more  than  artefacts  produced  by 
the  methods  of  preserving  and  manipulating  the  specimens.  According  to 
Ayers,  the  cochlear  nerves  end  in  the  hair-cells  and  not  freely  between  them, 
and  they  are  probably  continuous  with  the  auditory  hairs. 

Theory  of  Auditory  Sensation. — It  can  hardly  be  doubted  that  the 
nervous  structures  of  the  cochlea  form  an  organ  of  special  sense  for  the  per- 
ception of  musical  tones  and  probably  of  noises  as  well.  But  no  trustworthy 
conclusion  can  be  maintained  as  to  the  precise  mode  of  action  of  the  auditory 
apparatus.  The  fact  that  the  rods  of  Corti  are  absent  from  the  cochleae  of 
birds,  which  evidently  are  capable  of  appreciating  musical  tones,  shows  that 
these  structures  may  be  accessory,  but  are  not  essential  parts  of  the  sensory 
apparatus.  Starting  from  the  fact  that  the  basilar  membrane  splits  readily 
in  a  radial  direction,  in  which,  moreover,  it  is  tightly  stretched  between  its 
attachments,  Helmholtz2  long  ago  proposed  the  theory  that  the  basilar 
membrane  behaves  toward  vibrations  reaching  it  like  a  series  of  stretched 
strings.  As  the  wires  of  a  piano  have  different  rates  of  vibration  according 
to  their  length,  and  respond  sympathetically  to  correspondingly  different 
notes  sounded  in  their  neighborhood,  so  it  has  been  supposed  that  different 
radial  fibres  of  the  basilar  membrane  are  set  into  sympathetic  vibration  by 
different  rates  of  vibration  in  the  fluids  bathing  them.  These  vibrations 
must  be  imparted  to  the  structures  in  the  organ  of  Corti,  and  the  irritation 
of  the  nerves  connected  with  the  cells  of  Corti  is  a  natural  sequel.  It  may 
be  repeated  that,  though  the  canal  of  the  bony  cochlea  as  a  whole  diminishes 
in  diameter  from  base  to  cupola,  the  canal  of  the  membranous  cochlea,  the 
scala  media  with  its  lower  wall  or  basilar  membrane,  increases  in  diameter. 
Thus  the  radial  fibres  of  the  basilar  membrane  are  longest  near  the  apex  of 
the  cochela.  The  radial  width  of  the  basilar  membrane,  measured  near  the 
bottom,  middle,  and  top,  respectively,  is  given  as  0.21  millimeter,  0.34  milli- 
meter, and  0.36  millimeter.  The  waves  of  physical  sound  are  thus  supposed 
to  be  analyzed  in  the  peripheral  sense-organ,  each  auditory  nerve-fibre  excit- 
ing in  consciousness  a  tone  of  a  particular  pitch,  and  the  mind  perceiving 
the  simultaneous  effects  of  different  pendular  vibrations  as  notes  of  different 
quality. 

1  Avers  :  Journal  of  Morphology,  May,  1892. 

2  Helmholtz  :   Tonempfindungen,  1877,  S.  240. 


THE  SENSE    OF  HEARING.  381 

0.  THE  RELATION  BETWEEN  PHYSICAL  AND  PHYSIOLOGICAL  SOUND. 

Production  of  Sound-waves. — Sound,  in  its  physiological  meaning,  is  a 
sensation  which  is  the  conscious  appreciation  of  internal  changes  occurring  in 
certain  cells  of  the  cerebral  cortex.  Fibres  of  the  auditory  nerve  come  into 
close  relation  with  these  cells,  and  in  whatever  way  those  fibres  are  excited 
the  result  is  one  and  the  same,  a  sensation  of  sound. 

The  elaborate  apparatus  of  the  middle  and  internal  ear  is  so  constructed 
that  the  energy  of  mechanical  oscillations  in  the  external  air  is  transmitted  to 
the  terminations  of  the  auditory  nerves  in  a  manner  to  excite  them. 

Sound,  in  a  physical  sense,  consists  in  waves  of  alternate  condensation  and 
rarefaction  travelling  in  the  air  from  the  point  of  origin  of  the  sound,  much  as 
waves  radiate  over  the  surface  of  water  from  the  point  where  a  stone  is  dropped. 
Any  sudden  impulse,  such  as  a  puff  of  air,  or  the  vibration  of  a  solid  body, 
as  a  stretched  string  or  a  tuning-fork,  pushes  the  adjacent  molecules  of  air 
against  those  further  removed,  and  this  impulse  produces  an  area,  or  aerial 
shell,  of  increased  density  or  condensation.  The  air  being  perfectly  elastic, 
the  molecules,  relieved  from  pressure,  spring  back  even  beyond  the  position 
of  equilibrium,  and  leave  an  area  of  decreased  density  or  rarefaction.  Thus 
a  wave,  consisting  of  a  shell  of  condensation  succeeded  by  a  shell  of  corre- 
sponding rarefaction,  moves  through  the  air.  This  single  air-wave  is  the 
simplest  element  of  physical  sound.  When  a  number,  no  matter  how  great, 
of  sound-waves  simultaneously  excite  the  same  particle  of  air,  the  resultant 
motion  of  that  particle  is  the  algebraic  sum  of  all  the  motions  imparted  to  it 
by  the  single  sound-waves  considered  separately.  As  any  elastic  body,  when 
set  vibrating,  continues  its  oscillations  for  a  time,  so  is  it  probable  that  strictly 
isolated  air-waves  do  not  occur.  Any  elastic  body,  such  as  a  stretched  string, 
or  a  tuning-fork,  when  set  in  vibration,  sends  out  from  itself  a  series  of  air- 
waves which  succeed  one  another  at  a  rate  identical  with  the  rate  of  vibration 
of  the  elastic  body.  Such  a  regular  succession  of  air-waves  striking  upon  the 
tympanic  membrane  sets  the  latter  into  correspondingly  regular  oscillations 
and  produces  in  the  auditory  apparatus  the  sensation  of  musical  tone. 

Loudness  and  Musical  Pitch. — The  more  vigorous  the  vibrations  of  the 
oscillating  body,  the  more  forcibly  are  the  air-molecules  which  are  struck  by  it 
driven  forward ;  and  the  greater  their  excursion  or  amplitude  of  movement, 
the  greater  is  the  force  with  which  the  tympanic  membrane  is  driven  inward 
when  the  moving  air-wave  strikes  it.  The  loudness  of  the  tone  manifestly 
depends  upon  the  extent  of  motion  of  the  tympanic  membrane,  as  does  this  on 
the  amplitude  of  air-motion.  Different  elastic  bodies  have  different  natural 
rates  of  oscillation.  The  more  rapid  the  rate,  the  more  frequent  is  the  succes- 
sion of  air-waves  that  strike  upon  the  ear.  It  is  said  that  the  apparent  pitch 
of  a  tone  is  raised  when  its  intensity  is  lowered,  and  that  such  an  elevation 
of  pitch  may  equal  one-fifth  of  a  tone.1  Musical  pitch  is  determined  by  the 
number  of  air-waves  which  pass  a  given  point  in  a  unit  of  time,  or,  in 
other  words,  by  the  rate  of  vibration  of  the  sound-producing  body.  When 

1  Broca:  Jahresbericht  der  Physioloyie,  1897,  S.  111. 


382  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  vibration-rate  increases  the  pitch  is  elevated,  and  vice  versd.  If  some  body 
capable  of  producing  sound  should  have  its  rate  of  vibration  changed  grad- 
ually from  5  or  10  vibrations  per  second  to  50,000  per  second,  no  sensation 
of  sound  would  be  aroused  until  the  vibrations  reached  the  rate  of  about  from 
16  to  24  per  second.  The  droning  note  of  the  16-foot  organ-pipe  and  the 
lowest  bass  of  the  piano  represent  a  vibration-rate  of  33  per  second.  In 
most  persons  sounds  cease  to  be  audible  when  the  air-waves  have  a  fre- 
quency of  16,000  per  second,  though  to  some  the  note  produced  by  40,000 
vibrations  is  perceptible.  It  seems  clear  that  some  animals  hear  tones  whose 
pitch  is  so  elevated  as  to  make  them  inaudible  to  human  ears.  When  a  mov- 
ing bell  or  whistle,  as  of  a  locomotive,  rapidly  approaches,  its  pitch  seems  to 
rise,  and  then  to  fall  as  it  recedes.  The  reason  for  this  variation  is  that  the 
motion  of  the  locomotive  adds  to  or  subtracts  from  the  number  of  sound- 
waves reaching  the  ear  in  a  given  time.  In  musical  execution  and  in  the 
ordinary  uses  of  life  the  limits  in  the  pitch  of  sounds  are  much  narrower. 
Thus,  as  just  stated,  the  lowest  bass  of  the  piano  (C^  represents  a  vibration- 
rate  of  33  in  a  second,  while  the  highest  treble  (cr////)  has  that  of  4224.  As 
to  the  absolute  number  of  vibrations  necessary  to  produce  the  sensation  of 
sound,  it  has  been  found  that  2  or  3  vibrations  excite  the  sensation  of  a  mere 
stroke ;  4  or  5  vibrations  are  necessary  to  give  a  tone ;  and  some  20  or  40  are 
required  to  develop  the  full  musical  qualities  of  a  tone.1  That  is  to  say,  when 
a  musical  tone  falls  upon  the  ear  its  characteristics  cannot  be  appreciated  until 
20  to  40  vibrations  have  been  completed. 

Thus,  from  a  physical  scale  representing  aerial  vibrations  of  indefinitely 
various  rapidity  the  mind  selects  and  appreciates  as  sound  a  very  small 
fraction. 

Tympanic  Membrane  as  an  Organ  of  Pressure-sense. — There  is  good 
reason  to  suppose  that  variations  in  air-pressure  succeeding  one  another  too 
slowly  or  too  irregularly  to  produce  sound-sensation  are  still  of  great  import- 
ance in  the  extensive  realm  of  sensations  which  but  obscurely  excite  our  con- 
sciousness. Slow  inward  movements  of  the  tympanic  membrane  may  still 
give  rise  to  a  perception  of  external  changes.  Thus,  a  blind  man  has  been 
able  to  say  correctly  that  he  has  passed  by  a  fence,  and  whether  it  be  of  solid 
board  or  of  open  picket.  If  any  one  with  closed  eyes  holds  a  book  at  half-arm's 
length  in  front  of  the  ear,  a  different  sensation  will  be  experienced  according 
as  the  book  is  turned  flat  or  edgewise  to  the  face ;  the  feeling  is  one  of  "  shut- 
in-ness  "  or  "  open-ness/7  respectively.  The  air  is  in  ceaseless  agitation,  and 
its  waves,  striking  against  various  objects,  must  be  reflected  to  the  ear  with  an 
intensity  dependent  on  the  position  and  the  physical  character  of  the  reflecting 
media.  We  may  assert  that  the  tympanic  membrane  is  the  peripheral  organ 
of  a  pressure-sense  by  which  we  become  more  or  less  accurately  aware  of  the 
nature  and  position  of  surrounding  objects,  irrespective  of  the  sensations  of 
sight  and  hearing.  Whether  that  group  of  sensations  depends  on  the  excite- 

1  Mach:  Physikalischen  Notizen  Lotos,  Aug.,  1873;  V.  Kries  und  Auerbach :  Du  Bois-Rey- 
montfs  Archiv  fur  Physiologic,  1877,  S.  297 ;  Helmholtz :  Sensations  of  Tone,  translated  by  Ellis. 


THE  SENSE   OF  HEARING.  383 

ment  of  tactile  nerves  in  the  tympanic  membrane  or  of  the  auditory  filaments 
in  the  internal  ear  is  yet  uncertain.1  Such  sensations  probably  form  an  import- 
ant quota  of  that  complex  system  of  sensations  which  do  not  obtrude  themselves 
on  consciousness,  but  which,  nevertheless,  bring  information  from  the  outer 
world,  and  have  an  intimate  association  with  the  more  or  lass  reflex  move- 
ments that  preserve  the  equilibrium  of  the  body. 

Overtones  and  Quality  of  Sound. — We  have  thus  far  considered  only 
simple  tones  produced  by  simple  vibrations  of  elastic  bodies.  Thus,  a  stretched 
string  plucked  at  its  middle  vibrates  throughout  its  whole  length,  the  greatest 
amplitude  of  movement  being  at  the  middle  point,  which  moves  to  and  fro 
like  a  pendulum.  It  is  very  rare  that  a  body  set  vibrating  confines  itself  to 
a  single  pendular  movement.  Thus,  a  stretched  string  when  struck  not  only 
moves  as  a  single  cord,  but  the  string  may  break  up,  as  it  were,  into  two  halves, 
each  vibrating  independently,  but  with  twice  the  rate  of  movement  of  the 
whole  length  of  string.  Not  only  is  this  the  case,  but  the  string  in  its  vibra- 
tion also  breaks  up  into  chords  of  one-third,  one-fourth,  one-fifth,  etc.  of  its 
original  length,  giving  rise  to  vibrations  three,  four,  and  five  times  as  rapid  as 
those  produced  by  the  whole  string.  In  musical  phrase,  the  middle  c  of  the 
piano,  when  this  key  is  struck,  gives  not  only  a  note  c  representing  132  vibra- 
tions, but  also  its  octave  cf  of  264  vibrations,  the  fifth  above  this  of  396 
vibrations,  the  second  octave,  528,  the  third  above  this,  660,  and  so  on.  The 
vibration  of  a  string,  then,  sends  to  the  ear  a  complex  series  of  tones  each  of 
which  represents  a  simple  pendular  motion  of  the  air.  The  lowest  tone,  that 
produced  by  the  slowest  rate  of  vibration  of  the  string  as  a  whole,  is  known 
as  the  fundamental  tone. 

The  pitch  of  the  fundamental  tone  determines  our  estimate  of  the  pitch 
of  the  whole  complex  note.  The  other  tones  produced  by  segmental  vibration 
of  the  string  are  known  as  partial  tones,  upper  partials,  or^-overtones  The 
fundamental  tone  is  usually  stronger  than  its  accompanying  Wertoties,  the 
successively  higher  upper  partials  diminishing  rapidly  in  intensity.  Some 
musical  instruments  produce  notes  with  a  longer  series  of  overtones  than  do 
others ;  the  human  voice  is  particularly  rich  in  overtones.  Instruments  differ 
also  in  the  greater  or  lesser  strength  and  in  the  relative  prominence  of  the 
individual  overtones  accompanying  the  fundamental.  It  is  the  number  and  the 
relative  prominence  of  the  overtones  in  a  musical  note  that  determine  its  quality. 
Thus,  a  violin,  a  cornet,  and  a  piano,  though  sounding  a  note  of  the  same 
pitch,  would  never  be  mistaken  the  one  for  the  other ;  our  discrimination  of 
their  notes  depends  simply  upon  the  difference  in  the  relative  strength  and  the 
number  of  their  overtones,  the  fundamental  tone  being  the  same  throughout. 
The  brilliancy  and  richness  of  musical  notes  is  dependent  on  their  wealth  of 
upper  partials.  It  is  believed  that  a  sound-producing  body,  like  a  stretched 
string,  does  not  send  to  the  ear  a  separate  set  of  waves  representing  each  of  its 
segmental  vibrations,  but  that  all  the  waves  aroused  by  it  fuse  together  into 
a  single  series  of  waves  of  peculiar  form.  Such  a  composite  wave  may  be 
1  W.  James :  Psychology,  1890,  vol  ii.  p.  140. 


384 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


represented  graphically  by  depicting  under  one  another  a  series  of  waves  having 
two,  three,  four,  etc.  times  the  rate  of  succession  of  the  curve  indicating  the 
fundamental  tone.  If  a  vertical  line  be  drawn  across  the  series  representing 
the  vibration-rates  of  the  various  tones,  and  an  algebraic  addition  be  made  of 
the  distance  of  each  point  of  intersection  above  or  below  the  line  of  rest,  the 
result  will  determine  the  position  of  the  composite  curve  on  the  same  vertical 
(Fig.  196).  It  is  evident  that  the  form  of  the  composite  wave  must  change 
with  every  change  in  the  number  and  relative  prominence  of  musical  overtones, 
and  the  movement  imparted  by  it  to  the  tympanic  membrane  and  the  wave 


FIG.  196.— The  curve  B  represents  twice  the  vibration-rate  of  A.  When  the  two  curves  are  combined 
by  the  algebraic  addition  of  their  ordinates,  the  result  is  the  periodic  curve  c  (solid  line),  having  a  dif- 
ferent form  ;  the  dotted  line  of  c  is  a  reproduction  of  A.  If  B  is  displaced  to  the  right  until  e  falls  under 
d  in  A  (change  of  phase),  the  combination  of  A  and  B  will  give  the  curve  D,  the  dotted  line  in  D  repre- 
senting A  as  before.  (After  Helmholtz.) 

generated  in  the  perilymph  must  have  corresponding  differences.  Notes  of 
different  quality  are  produced  by  composite  air-waves  of  different  forms.  But 
waves  differing  in  form  may  still  produce  notes  of  the  same  quality ;  for  if, 
in  the  graphical  figure,  one  or  more  of  the  curves  representing  simple  tones 
be  slid  to  the  right  or  the  left,  the  form  of  the  composite  wave  will  thereby  be 
changed,  but  not  the  quality  of  the  sound  produced  by  it.  In  other  words, 
change  of  phase  of  the  partial  tones  does  not  alter  the  quality  of  the  note.1 
The  quality  of  any  complex  note  may  be  reproduced  by  sounding  together 
a  series  of  tuning-forks  which  have,  respectively,  the  vibration-rate  of  the 
fundamental  tone  and  that  of  one  of  the  overtones  of  the  complex  note. 

Analysis  of  Composite  Tones  by  the  Bar. — According  to  the  theory 
outlined  on  page  380,  the  composite  wave,  beating  against  the  sensitive  organ 
of  the  cochlea,  is  again  analyzed  into  the  elements  composing  it,  one  part  of 
the  basilar  membrane  vibrating  sympathetically  with  one  partial  tone,  another 
with  another.  The  isolated  irritation  of  each  nerve-element  arouses  in  the 
mind  the  idea  of  a  tone  of  a  certain  pitch  and  loudness ;  but  when  a  number 

1  Helmholtz,  op.  cit.,  pp.  30-34. 


THE  SENSE  OF  HEARING.  :isr, 

of  such  elements  are  simultaneously  stimulated,  the  mind  takes  note,  not  of  the 
individual  sensations  thereby  aroused,  but  of  a  resultant  sensation  formed  by 
the  fusion  of  these. 

That  apparently  simple  tones  are  actually  made  up  of  a  number  of  partials, 
having  rates  of  vibration  which  form  simple  multiples  of  the  fundamental 
tone,  may  easily  be  demonstrated  at  the  open  piano.  If  any  note,  as  c  in  the 
bass  clef,  be  struck  while  the  key  of  its  octave  c  is  depressed,  and  then  the 
struck  string  be  damped,  it  will  be  found  that  the  octave  c  rings  out  with  its 
proper  note.  So  in  turn  the  g  above  that,  the  second  octave  and  the  e  above 
that,  may  be  made  to  sound  when  the  lower  c  is  struck,  because  each  of  these 
strings  is  so  tuned  that  its  fundamental  note  has  the  same  vibration-rate  as 
one  of  the  overtones  of  the  lower  c.  A  note  sung  near  the  piano  may  in 
the  same  way  be  analyzed  more  or  less  completely  into  its  component  tones. 
The  organ  of  hearing  certainly  has  some  such  power  of  musical  analysis,  for 
some  cultivated  ears  can  not  only  follow  any  special  instrument  in  a  play- 
ing orchestra,  but  can  even  distinguish  the  overtones  in  a  single  musical 
note. 

The  ear  has  little  or  no  power  of  distinguishing  difference  of  pitch  in  tones 
of  less  than  40  or  more  than  4000  vibrations  per  second ;  but  in  the  upper 
median  parts  of  the  musical  scale  the  sensitiveness  to  change  of  pitch  is  very 
acute.  Thus,  according  to  Preyer,1  in  the  double-accented  octave  a  difference 
of  pitch  of  one-half  vibration  in  a  second  can  be  detected ;  that  is,  in  the 
octave  included  between  500  and  1000  vibrations  per  second,  1000  degrees  of 
pitch  can  be  perceived. 

Every  elastic  body  is  capable  of  sympathetic  vibration ;  that  is,  air- waves 
beating  upon  it  at  its  own  natural  rate  of  vibration  set  it  into  corresponding 
motion.  In  the  same  manner  a  heavy  pendulum  may  be  forced  into  violent 
movement  by  exceedingly  light  taps  with  the  finger,  the  only  necessary  condi- 
tion being  that  the  impulses  imparted  by  the  finger  be  exactly  timed  to  the 
periodic  motion  of  the  pendulum  or  to  some  multiple  of  it.  A  body  capable 
of  sympathetic  vibration  with  some  particular  tone  is  set  into  vibration  by  that 
tone,  and  reinforces  or  magnifies  it,  whether  the  tone  exists  alone  or  as  the 
fundamental  of  a  complex  note,  or  is  contained  in  the  latter  simply  as  an 
upper  partial. 

The  analysis  of  musical  sounds  is  usually  carried  out  by  the  use  of  resona- 
tors, which  are  hollow  cylinders  or  spheres  of  glass  or  of  metal,  rather  widely 
open  at  one  pole,  and  narrow-pointed  at  the  opposite  end  for  insertion  into 
the  ear.  The  mass  of  enclosed  air  vibrates,  according  to  its  size  and  shape, 
at  some  particular  rate,  and  it  is  very  readily  set  into  sympathetic  vibration 
whenever  its  fundamental  tone  is  contained  in  any  sound  reaching  it.  By  this 
means  it  is  possible  strongly  to  magnify,  and  thus  select,  the  individual  over- 
tones contained  in  a  note.  The  vowel  sounds  of  human  speech  owe  their 
difference  of  quality  to  the  adjustment  in  size  and  shape  of  the  resonant  air- 
chambers  above  the  vocal  cords. 

1  Ueber  die  Grenzen  der  Tonwahrnehmung,  June,  1876. 
VOL.  II.— 25 


386  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Inharmonic  Overtones. — It  will  be  remembered  that  all  the  overtones  con- 
tained in  a  musical  note  are  produced  by  vibrations  which  are  simple  multiples 
of  the  rate  of  the  fundamental  tone.  These  overtones  are  properly  called 
harmonic  upper  partials ;  they  are,  according  to  Helmholtz,  particularly  charac- 
teristic of  stretched  strings  and  narrow  organ-pipes.  But  most  elastic  bodies 
have  proper  tones  which  are  not  exact  multiples  of  the  fundamental,  and 
which  may  be  termed  inharmonic  upper  partials.  The  high-pitched  jingle 
heard  when  a  tuning-fork  is  first  struck  represents  the  inharmonic  upper  par- 
tials of  the  fork.  Stretched  membranes  have  a  great  number  of  such  inhar- 
monic overtones.  Inharmonic  upper  partials,  as  might  be  expected,  rapidly 
die  out  in  a  note  of  which  they  form  a  part.  It  is  evident  that  inharmonic 
proper  tones,  when  nearly  of  the  same  pitch,  must  interfere  with  one  another 
and  repress  the  development  of  a  well-marked  fundamental  tone. 

Production  of  Beats. — When  two  tones  of  slightly  different  pitch  are 
sounded  together,  the  more  rapid  vibrations  overtake  the  slower,  so  that  at 
certain  periods  the  crests,  or  phases  of  condensation,  of  two  waves  fall  together, 
and  the  result  is  a  phase  of  increased  condensation  and  louder  sound.  The 
waves  immediately  cease  to  correspond,  and  diverge  more  and  more  until  the 
crest  of  one  falls  upon  the  trough  of  another,  the  result  being  silence,  or  at 
least  great  diminution  in  the  intensity  of  the  sound.  Such  alternate  augmenta- 
tion and  diminution  of  the  waves  give  rise  to  pulses  in  the  sound,  known 
technically  as  beats.  This  is  one  of  the  most  familiar  and  important  phenom- 
ena of  musical  art.  If  two  tuning-forks  on  resonance-boxes  vibrate  in  unison, 
a  piece  of  wax  stuck  to  the  prong  of  one  fork  will  lower  its  tone  and  give  rise 
to  beats.  The  undulating  sound  caused  by  striking  a  bell  or  the  rim  of  a  thin 
glass  tumbler  is  due  to  beats.  When  two  notes  not  included  in  a  perfect  chord 
are  sounded  on  the  piano,  beats  are  heard  not  only  from  the  interference  of  the 
fundamental  tones,  but  of  the  upper  partials  as  well.  It  is  the  absence  of  beats 
in  notes  which  should  be  in  harmony,  as  those  of  the  major  chord,  that  deter- 
mines the  instrument  to  be  in  tune.  When  two  tones  produce  beats,  the 
number  of  beats  in  a  given  time  is  equal  to  the  difference  between  the  number 
of  vibrations  involved  in  the  two  tones  in  the  same  time.  For  example,  a  tone 
produced  by  256  vibrations  in  a  second  sounded  with  one  of  228  vibrations 
would  give  28  beats  in  a  second.  It  is  evident  that  the  frequency  of  beats 
may  be  increased  either  by  increasing  the  interval  between  the  tones  or  by 
striking  tones  of  the  same  interval  in  a  higher  part  of  the  scale.  Beats  which  are 
not  too  frequent — from  four  to  six  in  a  second — have  important  musical  value, 
but  when  they  number  thirty  or  forty  in  a  second  they  become  exceedingly  dis- 
agreeable, irritating  the  ear  in  a  manner  analogous  to  the  effect  of  a  flickering 
light  on  the  eye.  When  sufficiently  near  together  the  beats  no  longer  produce 
an  intermittent  sensation.  The  number  of  beats  in  a  second  required  to  result 
in  this  fusion  increases  as  we  ascend  the  musical  scale,  varying  from  16  beats 
at  c  of  64  vibrations  per  second  to  136  beats  at  c'"  of  1024  vibrations.1  The 
reason  for  this  variation  lies  in  the  progressive  shortening  of  the  waves  as  the 

1  Mayer:  Sound,  1891. 


THE  SENSE  OF  HEARING.  387 

sound  becomes  higher  in  pitch;  for  it  is  obvious  that  as  we  ascend  the  scale, 
and  the  waves  of  sound  become  progressively  shorter,  spaces  would  be  left 
between  the  individual  waves  unless  their  number  were  proportionately- 
increased. 

Harmony  and  Discord. — Tones  are  concordant,  or  harmonize,  when  they 
produce  no  beats  on  being  sounded  together  ;  they  are  discordant  when  beats 
are  produced,  and  the  painful  sense  of  dissonance  increases  in  intensity  up  to 
about  33  beats  per  second.  Perfect  concord  is  obtained  by  blending  notes 
whose  vibrations  are  to  each  other  as  small  whole  numbers. 

Thus,  in  the  major  cord  c  E  G  c 

the  vibration-numbers  are       132  165  198  264 

their  ratios  are  4568 

If  notes  the  ratios  of  whose  vibration-rates  can  be  represented  only  by  large 
whole  numbers  are  combined,  a  discord  is  formed,  for  the  reason  that  their 
upper  partials  interfere  with  one  another  and  cause  beats ;  there  is  no  especial 
virtue  in  the  small  integer.1 

Thus,  in  the  discord  C  D  E 

the  vibration-numbers  are          132  148.5  165 

which  are  not  reducible  to  small  whole  numbers.2 

Combinational  Tones. — When  two  tones  are  sounded  together,  there  is 
produced  a  new,  usually  weaker,  tone,  whose  vibration-number  is  the  numerical 
difference  between  the  vibration-rates  of  the  original  tones.  It  is  therefore 
known  as  a  differential  tone.  Such  tones  may  arise  from  upper  partials  as  well 
as  from  the  fundamentals ;  they  do  not  appear  to  be  formed,  as  might  be  sup- 
posed, by  the  fusion  of  beats.  Other  "  combinational "  tones  of  more  intricate 
relations,  as  well  as  beats,  arise  from  the  interaction  of  vibrations  when  many 
different  notes,  as  those  of  an  orchestra,  are  sounded  together.  To  calculate 
the  physical  result  of  the  combination  of  these  impulses,  which  it  is  the  duty 
of  the  tympanic  membrane  to  transmit,  is  a  problem  of  exceeding  complexity. 

Resume. — To  sum  up  the  subject,  musical  sounds  are  distinguished  in  sen- 
sation by  the  three  factors,  loudness,  pitch,  and  quality,  sometimes  called  color 
or  timbre.  These  sensations  depend  in  turn  on  definite  physical  characters  of 
air-waves :  their  amplitude,  or  the  extent  of  motion  of  the  air-molecules  ;  their 
frequency,  or  rate  of  succession  of  the  waves;  their  form,  which  is  deter- 
mined by  the  pitch  and  relative  predominance  of  the  upper  partials  combined 
with  the  fundamental  tone. 

Fatigue. — That  the  ear  is  subject  to  fatigue  toward  a  note  that  has  been 
sounded  is  easily  demonstrated  in  the  following  way :  Strike  a  single  note  of, 
say,  a  major  chord  on  the  piano,  and  immediately  afterward  sound  the  full 
chord;  the  quality  of  the  latter  will  be  altered  from  its  normal  character, 
owing  to  the  lessened  prominence  of  the  note  which  had  been  struck.3  We 
may  therefore  not  improperly  speak  of  a  successive  contrast  in  auditory  sensa- 

1  Tyndall :  Sound.  2  Waller :  Human  Physiology,  1891. 

3  Foster :  Text-book  of  Physiology,  5th  ed.,  1891. 


388  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

tions,  analogous  to  visual  successive  contrast,  by  which  our  perception  of  every 
sound  is  colored  by  the  sounds  which  have  preceded  it. 

Imperfections  of  the  Ear. — Notwithstanding  the  mechanical  provisions 
for  making  the  external  and  middle  ear  a  perfect  transmitting  apparatus, 
sound-perception  is  more  or  less  modified  by  the  action  of  the  mechanism 
under  certain  conditions.  Thus,  Helmholtz  believed  that  various  combina- 
tional tones  owe  their  origin  chiefly  to  a  periodic  clicking  in  the  joint  between 
the  malleus  and  incus  bones.  The  resonance  of  the  ear  is  a  familiar  fact, 
and  through  it  high-pitched  tones  between  e""  and  g""  are  reinforced  and 
heard  with  undue  loudness.  Certain  hissing  sounds,  the  chirp  of  a  cricket  or 
the  note  of  a  locust,  thus  gain  their  intensity.  This  resonance  probably  is  a 
feature  of  the  external  auditory  meatus,  since  it  is  at  once  destroyed  by  apply- 
ing a  small  resonator  to  the  ear  (Helmholtz). 

Perception  of  Time  Intervals. — The  ear  is  eminently  the  sense  apparatus 
for  determining  small  intervals  of  time.  Flashes  of  light  succeeding  each 
other  at  the  rate  of  twenty-four  in  a  second  are  fused  in  a  continuous  luminous 
impression  by  the  eye,  but  by  the  ear  at  least  one  hundred  and  thirty-two  audi- 
tory impulses  as  beats  may  be  heard  separately  in  a  second.  The  power  which 
the  ear  possesses  of  resolving  complex  air-waves  into  the  host  of  pendular 
vibrations  which  may  enter  into  their  formation  finds  no  analogy  in  the  eye 
(Helmholtz). 

Musical  Tones  and  Noises. — The  important  feature  of  the  physical 
processes  which  give  rise  to  musical  tones  is  their  periodicity.  Every  musical 
tone  is  produced  by  a  regular  succession  of  alternate  rarefactions  and  condensa- 
tions in  the  air.  The  remaining  class  of  sounds,  known  as  noises,  differs  from 
musical  sounds  in  the  respect  that  such  sounds  are  produced  by  an  irregular 
succession  of  air- waves — one  in  which  the  interval  between  phases  of  conden- 
sation and  rarefaction  does  not  remain  constant  as  in  a  musical  note.  Noises 
are  for  the  most  part  made  up  of  short  musical  notes  so  associated  as  not  to 
"  harmonize "  with  one  another.  As  expressed  by  Helmholtz,  the  sensation 
of  a  musical  tone  is  due  to  a  rapid  periodic  motion  of  a  sonorous  body ;  the 
sensation  of  a  noise,  to  non-periodic  motions. 

Functions  of  Different  Parts  of  the  Ear. — Concerning  the  functions  of 
the  different  parts  of  the  internal  ear  in  their  relation  to  sound-perception,  it 
is  generally  believed,  as  previously  stated,  that  the  basilar  membrane  of  the 
cochlea,  with  the  nervous  elements  seated  on  it,  is  the  organ  concerned  in  the 
reception  and  transmission  of  musical  sounds.  There  are  a  sufficient  number 
of  fibres  in  the  basilar  membrane  to  allow  several  to  vibrate  with  every 
audible  tone. 

It  cannot,  however,  too  strongly  be  impressed  that  no  theory  of  physiolog- 
ical action  should  be  accepted  definitively  without  rigid  experimental  proof,  and 
such  evidence  concerning  the  definite  functions  of  the  cochlea  is  almost  wholly 
wanting.  The  sensory  hair-cells  on  the  macula  of  the  saccule  and  the  utricle 
have  been  thought  to  have  the  duty  of  vibrating  in  response  to  any  agitation 
imparted  to  the  perilymph,  without  regard  to  its  periodic  character;  they 


THE  SENSE  OF  HEARING.  389 

might  thus  be  termed  sense  organs  for  the  perception  of  noises.  Evidence 
will  be  adduced  later  (p.  407)  for  the  belief  that  they  are  peripheral  organs 
for  the  preservation  of  static  equilibrium. 

The  hair-cells  on  the  cristae  of  the  ampullae  of  the  semicircular  canals  seem 
to  have  a  special  function  in  giving  rise  to  sensations  caused  by  changing  the 
position  of  the  head  ;  they  thus  are  organs  concerned  with  the  preservation  of 
the  equilibrium  of  the  body. 

Judgment  of  Direction  and  Distance. — The  distance  and  direction  from 
which  sounds  come  to  the  ear  are  not  perceived  directly,  but  our  estimate  of 
them  is  a  judgment  based  on  the  loudness  and  quality  of  the  sound  sensation, 
combined  with  a  power  of  reasoning  from  past  experience.  Thus,  in  seeking  to 
discover  the  direction  whence  a  sound  comes,  it  is  usual  for  an  observer  to  turn 


FIG.  197,-End-bulbs  from  human  conjunctiva  (from  Quain,  after  Longworth) :  A,  ramification  of  nerve- 
fibres  in  the  mucous  membrane,  and  their  termination  in  end-bulbs,  as  seen  with  a  lens ;  B,  end-bulb, 
highly  magnified;  a,  nucleated  capsule ;  b,  core,  the  outlines  of  its  component  cells  not  seen;  c,  entering 
nerve-fibre  branching,  its  two  divisions  to  end  in  the  bulb  at  d. 

the  head  to  the  position  in  which  the  sound  is  heard  loudest,  and  thus  to  form 
an  opinion  as  to  the  direction  whence  it  comes.  Errors  of  judgment  as  to  the 
direction  are  frequent,  owing  to  the  sound  reflected  from  some  object  appearing 
louder  than  that  coming  in  a  direct  line  from  its  source.  It  is  said  that  when 
there  is  total  deafness  in  one  ear  every  sound  seems  to  have  its  origin  on  the 
side  of  the  healthy  ear.  When  the  eyes  are  closed,  sounds  originating  in 
the  median  plane  of  the  head  are  very  imperfectly  localized,  but  tend  to  be 
projected  upward,  and  somewhat  in  front,  since  this  is  the  space  from  which 
most  sounds  come  to  us.1  The  quality  as  well  as  the  loudness  of  a  sound  varies 
according  to  the  distance  of  its  source.  Thus  the  lower  tones  die  away  earliest 
as  a  sound  recedes,  bringing  the  overtones  into  undue  prominence.  The  art  of 

1  Seashore:  Loc.  tit. 


390 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


the  ventriloquist  consists  largely  in  altering  the  quality  of  the  sounds  he  pro- 
duces to  imitate  the  quality  they  would  naturally  have  if  arising  under  the 
conditions  which  he  would  lead  his  hearers  to  believe  to  be  their  origin.  A 
comparatively  feeble  sound  near  at  hand  may  have  the  same  quality  as  a  loud 
one  heard  at  a  distance ;  thus,  a  frog  croaking  in  an  adjoining  room  was  once 
mistaken  by  the  writer  for  a  large  dog  barking  outside  the  building. 

D.  CUTANEOUS  AND  MUSCULAR  SENSATIONS. 

General  Importance  of  the  Cutaneous  and  Muscular  Sensations. — 
Cutaneous  sensations  are  aroused  by  the  operation  of  some  form  of  energy  on 
the  skin,  and  they  include  the  sensations  of  touchy  of  temperature,  and  of 

pain.  By  muscular  sensation  is  meant  the  ap- 
preciation which  we  have  of  the  intensity  and 
direction  of  muscular  effort.  Closely  allied  to 
this  sensation  is  a  general  sensibility  through  which 
we  gain  a  knowledge  of  the  relative  position  of 
the  parts  of  our  bodies,  irrespective  of  movements. 
The  direction,  size,  distance,  and  surface  features 
of  external  objects  are  usually  made  known  to  us 
through  the  sense  of  sight  or  of  hearing.  Yet  these 
fundamental  facts  regarding  the  things  about  us 
do  not  become  a  part  of  knowledge  through  direct 
visual  and  auditory  perception.  Such  knowledge 
is  based  on  complex  judgments  concerning  the 
meaning  of  auditory  and  visual  phenomena  ac- 
cording as  they  have,  in  past  experience,  been 
interpreted  by  tactile  and  muscular  perceptions. 
That  is,  when  reduced  to  its  simplest  terms,  out- 
most practical  and  important  knowledge  of  the 
world  is  the  outgrowth  of  tactile  and  muscular 
perceptions ;  by  and  with  them  all  other  sense- 
perceptions  of  objects  have  been  corrected  and  compared.  Thus,  so  simple 
a  feat  as  the  estimate  of  the  size  of  a  distant  object  is  the  result  of 


FIG.  198.  —  Tactile  corpuscle 
within  a  papilla  of  the  skin  of 
the  hand  (from  Quain,  after  Ran- 
vier) :  n,  n,  two  nerve-fibres  pass- 
ing to  the  corpuscle ;  o,  a,  ter- 
minal varicose  ramifications  of 
the  axis-cylinder  within  the  cor- 
puscle. 


FIG.  199.— Semi-schematic  figure  of  a  neuromuscular  spindle  of  the  first  type,  namely,  with  complex 
nerve-ending  ;  adult  cat :  c.,  capsule  ;  m.  n.  &.,  motor  nerve-bundle  ;  pi.  e.,  plate-ending;  n.  tr.,  nerve-trunk  ; 
pr.  e.,  primary  ending ;  s.e.,  secondary  ending ;  6.  w.,  axial  muscle-fibres.  (From  Ruffini.  Journal  of  Physi- 
ology, vol.  xxiii.) 

a  complex  judgment  based  on  tactile  and   muscular  experience.     Through 
the  sense  of  sight  we  perceive  the  ratio  of  the  visual  angle  subtended  by 


CUTANEOUS   AND    MUSCULAR   SENSATIONS. 


391 


the  object  to  that  of  the  whole  Held  of  vision  ;  but  as  objects  of  different 
size  may  fill  the  same  visual  angle  when  at  different  distances  from  the  eye, 
our  estimate  of  their  size  depends  upon  the  distance  at  which  we  suppose 
them  to  be  situated.  The  distinctness  of  the  surface  features  of  the  body 
afford  the  mind  an  important  clue,  since  experience  shows  that  details  of 
sin-face  in  a  body  become  more  obscure  as  we  recede  from  that  body.  But 
more  important  data  concerning  distance  come  from  the  sense  of  muscular 
innervatiou,  or  feeling  of  the  intensity  of  muscular  contraction,  by  which  we 
estimate  the  degree  of  convergence  of  the  optic 
axes  when  the  object  is  focussed,  and  still  more  by 
the  perception  of  the  amount  of  muscular  effort 
necessary  to  sweep  the  optic  axes  over  the  ground 
surface  intervening  between  the  observer  and  the 
object.  When  objects  approach  the  near-point  of 
vision  the  sense  of  innervation  of  the  pupillary 
muscles  affords  important  evidence  of  their  distance. 

That  fundamental  education  concerning  the  outer 
world  which  engages  the  earliest  years  of  every  child 
consists  in  accumulating  and  systematizing  with 
other  sense-perceptions  tactile  and  muscular  im- 
pressions of  objects.  A  sensation  is  no  sooner  felt 
than  some  muscular  movement  involving  a  definite 
muscular  feeling  is  made  by  which  the  character  of 
the  sensation  is  changed  and  experimentally  tested 
under  different  conditions.  The  physiological  pro- 
cess involved  in  building  up  sense-knowledge,  there- 
fore, embraces  in  alternation  sensation  excited  by 
external  objects,  motion  accompanied  by  muscular 
sensation,  and  change  in  the  original  sensation.  In 
other  words,  the  motor  and  sensory  impulses  form  a 
sort  of  balance,  and  both  are  necessary. 

Ending-  of  Sensory  Nerve-fibres  in  the  Skin. — 
The  afferent  nerves  supplied  to  the  skin  have  several 
modes  of  termination.  In  the  commonest  form  the 
plexus  of  medullated  nerve-fibres  found  in  the  dermis 
close  under  the  epidermis  gives  off  twigs  which,  losing  the  medullary  sheath, 
pierce  the  epidermis  and  here  form  a  network  among  the  cells  of  the  Mal- 
pighian  layer,  the  single  fibres  ending  freely  in  this  position  (Fig.  207).  So 
numerous  are  they  that  it  would  appear  that  every  epithelial  cell  (of  mucous 
membrane  as  well  as  skin)  is  in  contact  with  one  or  more  nerve-fibrils.  These 
axis-cylinder  threads  are  often  varicose  and  usually  end  freely  among  the 
epithelial  cells,  but  in  some  cases  they  are  expanded  at  their  terminations 
into  well-defined  sensory  end-plates.  In  the  corium  and  subcutaneous  con- 
nective tissue  (mesoblastic)  sensory  nerves  may  terminate  in  the  manner  just 
described.  But  they  are  frequently  modified  into  or  form  a  part  of  definite 


FIG.  200. -Magnified  view  of  a 
Pacinian  body  from  the  cat's 
mesentery  (from  Quain,  after 
Ranvier):  n,  stalk  with  nerve- 
fibre  enclosed  in  sheath  of  Henle, 
passing  to  the  corpuscle ;  n',  its 
continuation  through  the  coil,  ra, 
as  a  pale  fibre ;  a,  termination  of 
the  nerve  in  the  distal  end  of  the 
core  (the  terminations  are  not 
always  arborescent);  rf,  lines 
separating  the  tunics  of  the  cor- 
puscles; /,  channel  through  the 
tunics,  traversed  by  the  nerve- 
fibre  ;  c,  external  tunics  of  the 
corpuscle. 


392  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

structures  of  various  forms,  which  may  be  regarded  as  peripheral  sense-organs 
of  the  skin.1  Some  of  these  terminal  organs  are  known  respectively  as  end- 
bulbs,  touch-corpuscles,  and  Pacinian  bodies  (Figs.  197-200).  Each  organ 
consists  of  a  more  or  less  conical  body  in  which  a  nerve-fibre  terminates. 
The  end-bulbs  are  found  only  on  the  dermis  of  the  conjunctiva  and  the  lips, 
and  in  modified  form  on  the  sensitive  surfaces  of  the  genital  organs  (Fig. 
197).  The  touch-corpuscles,  though  apparently  absent  from  the  greater  part 
of  the  body,  occur  in  great  numbers  in  the  skin  of  the  palmar  surface  of  the 
hand  and  that  of  the  fingers,  especially  at  their  tips ;  at  the  edge  of  the  eye- 
lids and  the  lips  ;  on  the  soles  of  the  feet  and  the  toes ;  and  on  the  surface 
of  the  genital  organs.  The  touch- corpuscle  often  occupies  a  papilla  of  the 
dermis  directly  under  the  epidermis  (Fig.  198)..  The  Pacinian  bodies,  which 
are  oval  corpuscles,  larger  than  the  foregoing,  and  easily  visible  to  the 
unaided  eye,  are  found  not  in  the  skin  proper,  but  in  the  subcutaneous  con- 
nective tissue  beneath  it.  They  are  found  in  abundance  beneath  the  skin  of 
the  »alm  of  the  hand  and  the  sole  of  the  foot ;  they  are  also  numerous  along 
th/nerves  of  the  joints,  and  even  among  the  sympathetic  nerves  supplying 
th*  abdominal  organs  (Fig.  200).  "  Ruffini's  endings,  found  in  the  subcu- 
taneous tissue  of  the  finger,  are  formed  by  the  branching  and  anastomosis  of 
terminal  axis-cylinders  inclosed  within  a  special  connective-tissue  envelope. 
Various  other  modifications  of  sensory  nerve  termination  have  been  described. 

1.  Sense  of  Touch. — The  Relations  between  Sensation  and  Stimulus. — 
Many  so-called  "  tactile  sensations,"  such  as  wetness,  hardness,  roughness,  etc., 
are  not  simple  sensations  at  all,  but  are  complex  judgments  built  up  out  of  the 
association  of  certain  tactile,  temperature,  and  muscular  sensations,  and  con- 
veying to  us  a  knowledge  of  the  surface,  substance,  and  form  of  bodies. 

When  analyzed,  the  sense  of  touch  is  nothing  more  than  a  sense  of  pressure 
applied  to  the  skin.  To  test  the  pressure  sensibility  of  the  skin  the  object 
whose  weight  is  to  be  estimated  must  not  be  lifted  in  the  ordinary  way,  for 
that  would  bring  into  play  the  muscular  sensations.  If  the  skin  of  the  hand 
is  to  be  tested,  the  hand  must  be  placed  upon  some  firm  support,  such  as  a 
table,  and  the  weights  be  laid  upon  the  skin.  The  smallest  perceptible  weight 
that  can  thus  be  felt  varies  with  the  situation  to  which  it  is  applied.  Thus, 
the  greatest  sensitiveness  to  pressure  is  found  on  the  forehead,  the  temples, 
the  back  of  the  hand,  and  the  forearm,  where  a  weight  of  .002  gram  (-^ 
grain)  can  be  perceived.  The  weight  must  be  increased  to  .005  to  .015  gram 
to  be  felt  by  the  fingers,  and  to  1.0  gram  when  laid  on  the  finger-nail.2 

The  power  of  discriminating  differences  of  pressure  applied  to  the  skin  is 
tested  by  finding  the  smallest  increase  that  must  be  added  to  a  weight  in  order 
that  it  may  be  perceived  as  being  heavier.  This  increment  is  not,  as  might 
be  supposed,  the  same  for  weights  of  different  value,  but  it  bears  a  distinct 
proportion  to  them.  Thus,  a  weight  of  11  grains  may  just  be  perceptibly 
heavier  than  one  of  10  grains;  but  if  we  start  with  a  weight  of  100  grains, 

1  Of.  Barker  :   The  Nervous  System,  1899,  pp.  361-421. 

2  Aubert  und  Kammler :  Moleschott's  Untersuchungen,  1859,  Bd.  v.  S.  145. 


THE  SENSE    OF  PRESSURE.  393 

a  single  grain  added  to  it  will  arouse  no  difference  of  sensation,  an  increment 
of  10  grains  being  necessary  in  order  that  one  weight  may  appear  heavier 
than  the  other.  This  fact  is  the  basis  for  Weber's  law  of  the  relation  between 
stimulus  and  sensation;  this  law  may  be  formulated  as  follows:  T7ie  amount 
of  stimulus  necessary  to  provoke  a  perceptible  increase  of  sensation  always  bears 
the  same  ratio  to  the  amount  of  stimulus  already  applied.  This  law  is  found 
to  be  only  approximately  correct,  especially  when  very  small  and  very  large 
weights  are  compared.  Fechner  attempted  to  express  more  exactly  the  relation 
between  the  intensity  of  stimulus  and  sensation  in  his  "psycho-physical  law," 
thus :  T/ie  intensity  of  sensation  varies  with  the  logarithm  of  the  stimulus.  In 
other  words,  the  sensation  increases  in  arithmetical  progression,  while  the 
stimulus  increases  in  geometrical  progression.  With  moderate  weights  a 
difference  of  pressure  is  perceptible  when  the  ratio  of  increase  is  smaller  than 
when  either  very  small  or  very  large  weights  are  used ;  that  is,  sensitiveness 
to  pressure-change  is  keenest  under  moderate  stimulation. 

It  is  said  that  the  forehead,  the  lips,  and  the  temples  appreciate  an  increase 
of  ^5-  to  ^5-  of  the  weight  estimated,  while  the  skin  of  the  head,  the  fingers, 
and  the  forearm  requires  an  increase  of  fo  to  -fa  for  its  perception.  In  this 
as  in  other  kinds  of  sensation  it  is  the  difference,  or  variation  of  intensity,  of 
the  sensation  of  which  the  mind  takes  particular  cognizance.  One  touch- 
sensation  is  more  acutely  perceived  when  contrasted  with  another  than  when 
felt  alone.  Weber1  found  the  discrimination  of  pressure-differences  to  be 
finer  when  two  weights  were  laid  in  rapid  succession  on  the  same  skin-area 
than  when  the  weights  were  applied  either  simultaneously  or  successively  to 
different  parts.  If  a  finger  be  dipped  in  a  cup  of  mercury  or  of  water  having 
the  same  temperature  as  the  skin,  the  pressure  will  be  marked  only  at  the 
margin  between  the  air  and  the  fluid,  and  if  the  finger  be  moved  up  and 
down  it  will  seem  as  if  a  ring  were  being  slid  back  and  forth  upon  it.  The 
constant  pressure  of  the  mercury  upon  the  submerged  finger  is  not  felt.  The 
fingers  are  particularly  sensitive  to  intermittent  variations  of  pressure — a 
facility  the  use  of  which  is  manifest  when  the  function  of  these  parts  is 
considered. 

Two  weights,  in  being  tested,  should  press  upon  equal  areas  of  skin  ; 
according  to  Weber,2  if  two  equal  weights  have  different  superficial  expanse, 
that  which  touches  the  larger  skin-surface,  and  thereby  excites  the  greater 
number  of  touch-nerves,  will  appear  to  be  the  heavier.  The  important  part 
played  by  judgment  and  mental  inference  in  such  experiments  is  shown  by 
the  facts  that  when  it  is  sought  to  compare  weights  by  lifting  them  and  with 
the  aid  of  sight,  the  smaller  of  two  equal  weights  seems  to  be  the  heavier ; 
and  of  two  objects  having  the  same  size  and  weight,  that  which  appears  to 
be  the  smaller  seems  heavier.3  The  simultaneous  excitement  of  other  sen- 
sations may  modify  that  of  pressure  ;  thus,  when  two  coins  of  equal  weight, 

1  ''  Tastsinn  und  Gemeingefuhl,"  Wagner's  Handworterbuch  der  Physiologic,  1846. 

2  Quoted  in  Hermann's  Handbuch  der  Physiologic,  Bd.  iii.  2,  S.  336. 

3  Dressier  :  American  Journal  of  Psychology,  1894,  vol.  vi.  No.  3. 


394  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

but  one  warm  and  the  other  cold,  are  laid  upon  the  hand  or  the  forehead, 
the  cold  one  appears  to  be  much  the  heavier. 

There  is  a  sensation  of  after-pressure  depending  for  its  strength  on  the 
amount  of  the  weight  and  the  length  of  time  this  weight  has  been  applied. 
In  fact,  this  after-sensation  may  produce  a  striking  eifect  on  consciousness, 
a  familiar  example  of  which  is  the  persistence  of  the  sense  of  pressure  of 
the  hat-band  after  the  head-covering  is  removed.  Even  light  weights  leave 
an  after-sensation,  and,  in  order  to  be  perceived  as  separate,  must  be  applied  at 
intervals  of  not  less  than  -^$  to  g-j-^-  of  a  second.  It  is  said  that  when  the 
finger  is  applied  to  the  rim  of  a  rotating  wheel  provided  with  blunt  teeth,  the 
separate  teeth  are  no  longer  felt,  and  the  margin  seems  smooth,  when  the  con- 
tacts succeed  each  other  at  the  rate  of  500  to  600  in  a  second.1  Vibrations  of 
a  string  cease  to  be  appreciated  by  the  finger  when  they  have  a  rate  of  between 
1500  and  1600  per  second. 

The  Localization  of  Touch-sensation. — When  a  touch-sensation  is  felt,  the 
mind  inevitably  refers  the  irritation  to  some  particular  part  of  the  surface 
of  the  body,  and  the  sensation  seems  to  be  localized  in  this  area.  On  the 
accurate  localization  of  tactile  sensations  depends  not  only  the  safety  of  the 
individual,  but  also  the  performance  of  the  ordinary  acts  of  life. 

We  may  suppose  that  to  each  area  of  peripheral  distribution  of  tactile 
nerve-fibres  in  the  skin  there  corresponds  an  area  of  tactile  nerve-cells  in  the 
brain.  It  can  hardly  be  doubted  that  the  nerve-cells  are  divided  into  physio- 
logical groups  characterized  by  inherent  and  inborn  quality-differences  in  the 
sensations  aroused  by  their  respective  excitements.  The  reference  of  the  sen- 
sations aroused  by  the  excitement  of  definite  nerve-cells  to  definite  parts  of  the 
periphery  is  a  power  acquired  through  the  physiological  experiences  of  the 
earliest  months  of  life.  Through  the  sense  of  sight  the  seat  of  irritation  is 
recognized,  and  through  muscular  sensation  its  relation  to  surrounding  parts 
is  experimentally  explored,  so  that  cumulative  harmonious  experiences  of  tactile, 
visual,  and  muscular  sensations  finally  bring  into  correspondence  the  various 
areas  with  definite  varieties  of  touch-sensation,  or,  to  use  an  expression  of 
Lotze's,2  every  area  of  the  skin  acquires  a  "  local  sign  "  by  which  it  'is  dis- 
tinguished in  consciousness. 

This  power  of  localization  differs  widely  for  different  parts  of  the  skin. 
The  fineness  of  the  localizing  sense  for  any  skin-area  is  easily  estimated  by 
determining  how  far  apart  the  tips  of  a  pair  of  compasses,  applied  to  the  skin, 
must  be  separated  in  order  to  be  felt  as  two.  For  this  experiment  the  compass- 
points  must  be  smooth,  and  they  should  not  be  applied  heavily.  The  general 
result  of  such  an  inquiry  is  that  the  compass-points  may  be  nearer  together, 
and  still  be  distinguished  as  two,  in  proportion  as  the  surfaces  to  which  they 
are  applied  have  greater  mobility.  Since  it  is  just  such  parts  of  the  body  as 
the  tips  of  the  tongue  and  the  fingers  that  are  chiefly  used  in  determining  the 
position  of  objects,  the  advantage  of  such  an  arrangement  is  obvious.  The 

1  Landois  and  Stirling  :  Human  Physiology,  1886. 

2  Funke,  in  Hermann's  Handbuch  der  Physiologic,  Bd.  iii.  2,  S.  404. 


THE  SENSE    OF  PRESSURE.  395 

skin  can  thus  be  marked  out  in  areas  (tactile  area,?),  within  each  of  which  the 
compass-points  are  felt  as  a  single  object,  but  if  they  are  separated  so  as  to  fall 
beyond  the  borders  of  these  areas,  they  are  at  once  perceived  to  be  two. 

The  following  figures1  represent  the  distances  at  which  the  compass-points 
can  just  be  distinguished  as  double  when  applied  to  various  parts  of  the  body : 

Tip  of  tongue 1.1  mm. 

Palm  of  last  phalanx  of  finger 2.2     " 

Palm  of  second  phalanx  of  finger 4.4     " 

Tip  of  nose 6.6    " 

Back  of  second  phalanx  of  finger 11.1     " 

Back  of  hand 29.8    « 

Forearm 39.6  .  " 

Sternum 44       " 

Back 66       " 

It  will  be  observed  that  accuracy  of  localization  and  sensitiveness  to  pressure 
find  their  most  perfect  manifestations  in  widely  separate  regions  of  the  skin.. 

Tactile  areas  are  found  to  have  a  general  oval  form  with  the  long  axis 
parallel  with  the  long  axis  of  the  member  investigated.  If  the  compass-points, 
separated,  say,  half  an  inch  apart,  be  passed  over  the  skin  of  the  palm  from 
the  middle  of  the  hand  to  the  finger-tips,  the  sensation  will  be  that  of  a  single 
line  gradually  separating  into  two  diverging  lines.  The  result,  of  course, 
depends  on  the  compass-points  passing  successively  through  areas  of  finer 
localization.  If  an  area  be  marked  out  on  a  part  of  the  skin  where  localiza- 
tion is  poor,  within  which  area  two  points  simultaneously  applied  appear  to  be 
one,  a  single  point  moved  within  it  is  still  perceived  to  change  its  place,  and 
two  points  successively  applied  may  be  perceived  to  occupy  different  positions. 
The  mental  fusion  or  separation  of  the  two  compass-points  cannot  depend 
altogether  on  their  being  placed  over  the  terminal  twigs  of  the  same  or  of  two 
adjoining  nerve-fibres,  for,  were  this  the  case,  the  points  could  be  discriminated 
when  separated  by  a  very  small  distance  across  the  line  drawn  between  the 
endings  of  adjoining  nerve-fibres,  while  on  either  side  the  points  would  have 
to  be  much  more  widely  separated  in  the  area  of  distribution  of  a  single  fibre. 
The  important  factor  in  the  mental  separation  of  two  stimulated  points  is,  that 
between  such  points  there  shall  be  found  a  certain  number  of  sensory  elements 
which  are  unstimulated.2  Practice  in  such  experiments  greatly  increases  the 
power  to  localize  impressions.  This  improvement  is  evidently  due  not  to 
the  establishment  of  new  nerves,  but  to  a  more  perfect  discrimination  of  sen- 
sations in  the  nerve-centres.  Dressier 3  found  that  after  practice  for  four 
weeks,  the  compass-points  which  at  the  beginning  had  to  be  separated  18 
millimeters  on  the  skin  of  the  forearm  to  be  distinguished,  could,  at  the  end  of 
the  period,  be  recognized  as  two  when  only  about  4  millimeters  apart.  Almost 
as  great  an  improvement  of  localizing  power  was  gained  by  the  unexercised 

1  Foster's  Physiology,  5th  ed.,  1891. 

2  Weber:  "  Tastsinn  und  Gemeingefiihl,"  Wagner's  Handworterbuch  der  Physiologic,  1846. 

3  Dressier  :  Loc.  cit. 


396  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

corresponding  area  of  the  skin  of  the  opposite  arm,  but  not  by  adjacent  areas  ; 
in  other  words,  the  localizing  power  is  central,  not  peripheral.  Practice 
aroused  in  both  tactile  areas  a  peculiar  quality  of  sensation  by  which  the 
area  was  recognized.  The  improvement  in  localizing  power  is  gradually  lost 
if  unexercised. 

Pressure-points. — It  has  been  found  that  if  a  light  object,  such  as  a  lead- 
pencil,  be  allowed  to  rest  by  a  narrow  extremity  successively  on  different  parts 
of  the  skin,  its  weight  will  appear  very  different  according  to  the  part  which 
is  touched.  If  the  spots  on  which  the  weight  appears  greatest  be  marked  with 
ink,  they  will  be  found  to  have  a  constant  position,  and  the  skin  may  therefore 
be  mapped  out  in  areas  of  pressure-points,  which  are  believed  to  indicate  the 
place  of  ending  of  pressure-nerve  filaments.  The  pressure-points  are  rela- 
tively few  in  number  and  are  principally  collected  about  the  hair-follicles. 

The  Importance  of  the  End-organ. — The  sense  of  touch  or  pressure  is  a 
special  sense ;  that  is,  any  irritation  conveyed  to  the  nerve-centres  in  which 
the  nerves  of  pressure  terminate  gives  rise  to  a  feeling  of  touch,  just  as  dis- 
turbance in  the  visual  or  the  auditory  centre  is  recognized  in  consciousness  as 
a  sensation  of  sight  or  of  sound.  The  complex  anatomical  structures  known  as 
sense-organs  may  be  considered  as  instruments  each  of  which  is  differentiated 
in  a  manner  to  make  it  particularly  irritable  toward  some  special  form  of 
energy.  Thus,  the  retina  is  most  sensitive  to  the  luminiferous  ether ;  the  organ 
of  Corti,  to  waves  of  endolymph,  etc.  To  this  differentiation  of  structure  the 
sensitiveness  of  the  body  to  the  forces  of  nature  is  chiefly  due.  The  peripheral 
ending  of  the  pressure  nerve,  whether  a  naked  axis-cylinder  or  a  touch-corpus- 
cle, is  no  doubt  modified  to  be  particularly  irritable  toward  that  form  of  energy 
manifested  in  the  molecular,  vibration  of  the  tissue  solids,  brought  about  by 
contact  with  foreign  objects.  Hairs,  particularly  those  in  certain  localities  of 
some  animals,  as  the  whiskers  of  the  cat,  appear  to  have  the  function  of  trans- 
mitting mechanical  vibrations  to  the  nerve-endings  in  greater  intensity  than 
could  be  accomplished  through  the  skin  alone. 

No  true  sense  of  touch  is  aroused  by  direct  irritation  of  a  nerve-trunk  or 
exposed  tissue,  and  touch-sensations  do  not  arise  from  irritation  of  the  internal 
surfaces  of  the  body.  A  fluid  of  the  temperature  of  the  body  gives,  when 
swallowed,  no  sensation  in  the  stomach ;  when  cooler  or  warmer  than  the 
body,  there  is  a  sensation  due,  probably,  to  a  transmission  of  temperature 
change  to  the  skin  of  the  abdomen. 

Touch,  Illusions. — Certain  peculiar  errors  in  judgment  may  arise  when 
tactile  sensations  are  associated  in  a  manner  unusual  in  experience.  Thus,  in 
an  experiment  said  to  have  been  devised  by  Aristotle,  if  the  forefinger  and 
the  middle  finger  be  crossed,  a  marble  rolled  between  their  tips  will  appear  to 
be  two  marbles ;  if  the  crossed  finger-ends  be  applied  to  the  tip  of  the  nose, 
there  seems  to  be  two  noses.  The  illusion  is  due  to  the  fact  that  under 
ordinary  circumstances  simultaneous  tactile  sensations  from  the  radial  side  of 
the  forefinger  and  the  ulnar  side  of  the  middle  finger  are  always  caused  by 


THE  KKxxi:  o/--  TI-:MI>I-:I;.[TURE.  397 

two  different  objects.  It  is  a  not  uncommon  surgical  operation  to  replace  a 
loss  of  skin  on  the  nose  by  cutting  a  flap  in  the  skin  of  the  forehead,  without 
injury  to  the  nerves,  and  sliding  the  flap  round  upon  the  nose.  Touching 
the  piece  of  transplanted  skin  gives  the  patient  the  sensation  of  being  touched, 
not  upon  the  nose,  but  upon  the  forehead ;  after  a  time,  however,  a  new  fund 
of  experience  is  accumulated,  and  the  sensation  of  contact  with  the  transplanted 
flap  is  rightly  referred  to  the  nose.  Persons  who  have  suffered  amputation  of 
a  lower  limb  often  complain  of  cramps  and  other  sensations  in  the  lost  toes. 
The  illusion  no  doubt  comes  from  irritation,  in  the  nerve-stump,  of  fibres 
which  previously  bore  irritations  from  the  toes. 

2.  Temperature  Sense. — The  skin  is  also  an  organ  for  the  detection  of 
changes  of  temperature  in  the  outep  world.  Such  temperature  differences  prob- 
ably make  themselves  manifest  by  raising  or  lowering  the  temperature  of  the 
skin  itself,  and  thus  in  some  way  irritating  the  terminal  parts  of  certain  sensory 
nerves,  the  temperature  nerves.  The  sensitiveness  of  the  skin  to  temperature 
variations  is  not  the  same  in  all  parts;  thus,  it  is  more  acute  in  the  skin  of  the 
face  than  in  that  of  the  hand ;  in  the  legs  and  the  trunk  the  sensibility  is  least. 
We  refer  temperature  sensations,  somewhat  like  those  of  touch,  to  the  periphery 
of  the  body,  and  localize  them  on  the  surface.  The  skin  over  various  parts 
of  the  body  may  have  different  temperatures  without  exciting  corresponding 
local  differences  of  sensation.  Thus,  the  forehead  and  the  hand  usually  seem 
to  be  of  the  same  temperature,  but  if  the  palm  be  laid  upon  the  temples, 
there  is  commonly  felt  a  decided  sensation  of  temperature  change  in  one  or 
both  surfaces.  As  in  other  sensations,  fatigue  and  contrast  play  an  important 
part  in  the  sense  perceptions  of  temperature,  and  stimuli  of  rapidly-changing 
intensity  provoke  the  strongest  sensations;  thus,  when  two  fingers  are  both 
dipped  into  hot  or  cold  water,  the  fluid  seems  hotter  or  colder  to  that  finger 
which  is  alternately  raised  and  lowered. 

In  changing  to  a  place  of  different  temperature  the  skin  for  a  time  seems 
warmer  or  cooler,  but  soon  the  temperature  sensation  declines,  and  on  return- 
ing to  the  original  temperature  the  reverse  feeling  of  cold  or  of  warmth  is 
experienced.  For  every  part  of  the  skin,  then,  there  is  a  degree  of  tempera- 
ture, elevation  above  or  depression  below  which  arouses  respectively  the 
feeling  of  warmth  or  of  cold,  and  the  temperature  of  the  skin  determining 
the  physiological  null-point  may  vary  within  wide  limits. 

The  smallest  differences  of  temperature  that  can  be  perceived  fall,  for  most 
parts  of  the  skin,  within  1°  C.  The  skin  of  the  temples  gives  perception  of 
differences  of  0.4°-0.3°  C.  The  surface  of  the  arm  discriminates  0.2° ;  the 
hollow  of  the  hand,  0.5°-0.4° ;  the  middle  of  the  back,  1.20.1 

The  size  of  the  sensory  surface  affected  modifies  the  intensity  of  temperature 
sensation  :  if  the  whole  of  one  hand  and  a  single  finger  of  the  other  hand  be 
dipped  into  warm  or  cold  water,  the  temperature  will  seem  higher  or  lower  to 
the  member  having  the  greatest  surface  immersed. 

1  Nothnagel :  Deutschcs  Arch /»•/»/,-  /•//// W-/K  M^lid,,,  1866,  ii.  S.  284. 


398 


AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


FIG.  201.— Cutaneous  "  cold  "  spots 
(vertical  shading)  and  "  hot "  spots 
(horizontal  shading),  anterior  sur- 
face of  the  thigh  (from  Waller,  after 
Goldscheider). 


Cold  and  Warm  Points. — The  skin  is  not  uniformly  sensitive  to  tem- 
perature changes,  but  its  appreciation  of  them  seems  to  be  limited  to  certain 

points  distributed  more  or  less  thickly  over  the 
surface.  These  spots  appear  to  be  the  places  of 
termination  of  the  temperature  nerves  in  the  epi- 
dermis (Fig.  201).  There  is  little  doubt  that  there 
are  two  distinct  varieties  of  temperature  nerves, 
one  of  which  appreciates  elevation  of  temperature, 
or  heat,  and  the  other  diminution  of  temperature, 
or  cold.  Thus,  if  a  blunt-pointed  metal  rod  be 
warmed  and  be  touched  in  succession  to  various 
parts  of  the  skin,  at  certain  spots  it  will  be  felt  as 
very  warm,  while  at  others  it  will  not  seem  warm 
at  all.  If,  on  the  contrary,  the  rod  be  cooled,  a 
series  of  cold  points  may  in  the  same  way  be  made 
out.  The  point  of  an  ordinary  lead-pencil  may 
be  used  with  some  success  to  pick  out  the  cold 
spots.  The  "cold  points"  are  more  numerous 
than  the  "hot,"  and  those  of  each  variety  are 
more  or  less  distinctly  grouped  round  centres,  as 
would  be  expected  from  the  manner  of  nerve-distribution,  though  the  groups 
overlap  to  some  extent  (Fig.  201).  Certain  substances  appear  to  act,  prob- 
ably by  chemical  means,  as  specific  excitants  of  the  two  sets  of  nerves. 
Thus,  menthol  applied  to  the  skin  gives  a  sensation  of  cold,  while  an  atmo- 
sphere of  carbon  dioxide  surrounding  an  area  of  skin  gives  a  sensation  of 
warmth.1 

The  specific  difference  of  the  two  sets  of  temperature  nerves  is  indicated  by 
the  fact  that  when  a  warm  and  a  cold  body  held  close  together  are  simulta- 
neously brought  near  the  skin,  the  sensation  is  either  one  of  both  warmth  and 
cold,  or  now  one  and  now  the  other  sensation  predominates.2  Any  stimulation, 
whether  mechanical  or  electrical,  applied  to  the  sensitive  points  thus  far  de- 
scribed in  the  skin,  for  the  appreciation  of  either  pressure,  heat,  or  coldj  pro- 
vokes, when  effective,  only  the  proper  sensation  of  that  point :  any  irritation 
of  a  cold,  hot,  or  pressure  point  gives  rise,  respectively,  to  the  sensation  of 
cold,  heat,  or  pressure  alone. 

As  in  other  organs  of  special  sense,  the  peripheral  terminations  of  the 
temperature  nerves  seem  modified  to  be  especially  irritable  toward  their  appro- 
priate form  of  physical  stimulus.  Cold  or  heat  directly  applied  to  the  nerve- 
trunk  excites  no  temperature  sensation.  Thus,  if  the  elbow  be  dipped  into  a 
freezing  mixture,  as  the  lowered  temperature  penetrates  to  the  ulnar  nerve  the 
sensation  will  be  one,  not  of  cold,  but  of  dull  pain,  and  it  will  be  referred  to 

1  Goldscheider:  Du  Bois-Reymond's  Archiv  fur  Physiologie,  1886, 1887  ;  Blix:  Zeifschrift  filr 
Biologic,  1884 ;  Donaldson  :  Mind,  1885,  vol.  xxxix. 

2  Czermak :  Sitzungsberichte  d.  Wiener  Akad.,  1855,  S.  500 ;  Klug :  Arb.  d.  physioL  Anstalt  zu 
Leipzig,  1876,  S.  168. 


Tin:  ,x7-:.v,s7-;  OF  PAIN.  399 

the  hand  and  the  fingers.  The  internal  mucous  surfaces  of  the  body,  from 
the  oesophagus  to  the  rectum,  inclusive,  have  no  power  of  discriminating 
temperature  sensations ;  a  clyster  of  water  cooled  to  from  7°  to  16°  C.,  if  not 
held  too  long,  is  only  perceived  as  cold  when  the  water  escapes  through  the 
skin  of  the  anus. 

The  doctrine  of  specific  nerve  energy,  enunciated  by  E.  H.  Weber,  was 
intended  to  convey  the  idea  elaborated  above,  that  each  nerve  of  special  sense, 
however  irritated,  gives  rise  to  its  own  peculiar  quality  of  sensation.  But  it 
seems  clear  that  the  existence  and  quality  of  the  sensation  are,  respectively, 
properties  of  the  activity,  not  of  the  nerve-fibre,  but  of  the  peripheral  end- 
organ  and  the  nerve-centres. 

3.  Common  Sensation  and  Pain. — The  sensations  thus  far  considered 
have  been  called  special  sensations,  because  each  affects  the  consciousness  in 
quite  a  different  way,  and  any  irritation  which  excites  the  sense  apparatus 
provokes  a  sensation  of  definite  quality  and  measurable  intensity. 

Pain  is  a  sensation  which,  according  to  a  common  but  unproved  belief,  is 
the  result  of  sufficiently  intensifying  any  of  the  simple  sensations. 

Pains  have  received  various  names  to  distinguish  their  quality,  according  to 
the  mode  in  which  experience  shows  they  may  have  been  produced,  as  cutting, 
tearing,  burning,  grinding,  etc.  One  peculiar  mark  that  distinguishes  painful 
sensations  is  the  lack  of  complete  localization.  While  lesser  pains  are  referred 
with  fair  exactness  to  different  parts  of  the  body,  and  even  to  those  internal 
parts  devoid  of  tactile  sensibility,  greater  pains  radiate  and  seem  diffused  over 
neighboring  parts.  Pain  also  differs  from  special  sensation  in  the  long  latent 
period  preceding  its  development.  The  evidence  of  physiological  experiment 
is  against  the  belief  that  any  irritation  of  the  nerves  of  so-called  "special 
senses  "  can  produce  pains,  but  it  teaches  that  this  sensation  is  the  result  of  the 
excessive  or  unnatural  stimulation  of  a  group  of  nerves  whose  function  is  to 
give  rise  to  what  is  indefinitely  called  "  common  sensation."  By  this  term  is 
designated  that  consciousness  which  we  more  or  less  definitely  have,  at  any 
moment,  of  the  condition  and  position  of  the  various  parts  of  our  bodies. 
When  tactile,  temperature,  and  visual  sensations  are  eliminated,  we  are  still 
able  to  designate  with  considerable  accuracy  the  position  of  our  limbs,  and  we 
become  aware  with  extraordinary  exactness  of  any  change  in  that  position, 
indicating  the  possession  of  a  posture  sense.  The  nerves  of  common  sensation 
must,  then,  be  continuously  active  in  carrying  to  the  sensorium  impulses 
which,  though  they  do  not  excite  distinct  consciousness,  probably  are  of  the 
utmost  importance  in  keeping  the  nerve-centres  informed  of  the  relative  posi- 
tions and  physiological  condition  of  the  various  parts  of  the  organism,  and  it 
is  not  improbable  that  they  are  the  afferent  channels  for  many  reflex  acts 
which  tend  to  preserve  the  equilibrium  of  the  body.  The  sudden  failure  of 
these  sensations  in  a  part  of  the  body  would  probably  be  felt  as  acutely  as  the 
silence  which  succeeds  a  loud  noise  to  which  the  ear  has  become  accustomed. 
Pain  is  thought  to  be  the  result  of  excessive  stimulation  of  the  nerves  of  com- 
mon sensation,  though  it  must  be  admitted  that  we  know  next  to  nothing 


400  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

of  the  anatomical  and  physiological  conditions  on  which  this  sensation  is 
dependent.  It  is  said  not  only  that  most  internal  organs  possess  no  def- 
inite tactile  or  thermal  sensibility,  but  that,  when  normal,  such  irritation  as 
is  caused  by  cutting,  burning,  and  pinching  seems  to  cause  no  pain;1 
let  them,  however,  become  inflamed,  and  their  sensitiveness  to  pain  is  suf- 
ficiently acute.  The  facts  of  labor-pains,  of  colic,  and  other  visceral  dis- 
turbances which  are  attended  by  no  inflammatory  condition  show,  however, 
that  the  factors  on  which  the  existence  of  pain  depends  are  not  as  yet  fully 
understood. 

The  physiological  facts  on  which  is  based  the  belief  in  "  common  sensa- 
tion "  are  indisputable,  but  the  evidence  for  a  special  nervous  apparatus  for 
such  sensibility  is  based  rather  on  exclusion  of  known  nerve-organs  than  on 
positive  demonstration.  In  the  category  of  common  sensations  have  been 
included  also  such  feelings  as  "  tickling,"  shivering,  hunger,  thirst,  and  sexual 
sensations.  The  feeling  of  fatigue  which  follows  either  muscular  or  mental 
exertion  may  be  placed  in  the  same  group. 

A  general  feature  of  common  sensations  is  their  subjective  character ;  they 
are  not  definitely  localized  within  the  body,  nor  are  they  projected  external  to 
it,  as  in  the  case  of  the  "  special  senses." 

Between  the  common  sensation  and  its  existing  cause  there  is  no  measurable 
proportion,  as  is  found,  for  instance,  in  the  study  of  the  pressure  sense.  It 
may  be  stated  that  pressure  and  temperature  sensations  were  within  a  recent 
period  grouped  among  common  sensations,  and  future  investigations  may  pos- 
sibly limit  each  of  the  feelings  now  classed  together  as  "  common  sensations  " 
to  definite  anatomical  structures. 

When  the  punctiform  distribution  of  various  sensations  in  the  skin  is  inves- 
tigated, some  points  are  found  in  which  no  other  sensation  than  that  of  pain 
can  be  excited,  and  it  has  been  thought  that  such  spots  mark  the  place  of 
ending  of  nerves  of  common  sensibility. 

According  to  v.  Frey,  the  pain-points  are  much  more  numerous  than  the 
pressure  points,  more  than  100  falling  within  a  square  centimeter  of  skin,  and 
their  nerves  are  probably  more  superficial.  They  require  about  1000  times 
as  great  an  intensity  of  stimulus  for  their  excitement  as  do  the  pressure- 
nerves  ;  they  have  a  long,  latent  period  of  stimulation  and  are  inert  to  rapid 
changes  in  the  stimulus.  This  author  believes  that  the  free  nerve-endings 
are  sense-organs  for  pain,  the  end-bulbs  for  cold,  the  terminal  coils,  or  net- 
works, for  heat,  and  the  tactile  corpuscles  for  pressure-sensations.2 

Transferred  or  "  Sympathetic  "  Pains ;  Allochiria. — It  has  long  been  a 
matter  of  clinical  observation  that  disease  seated  in  certain  internal  organs  is 
often  accompanied  by  superficial  pain  and  tenderness  in  widely  removed  parts 
of  the  body ;  for  example,  a  decayed  tooth  frequently  causes  intense  pain  in 
the  ear ;  disease  of  heart  or  of  aorta  may  cause  pain  between  the  shoulders, 

1  Foster's  Physiology,  1891,  p.  1420. 

2  Hermann's  Jahresbericht  ii.  Physiologic,  1897,  Bd.  v.  S.  115;  1896,  Bd.  iv.  S.  113  ;  1895, 
Bd.  iii.  S.  111. 


MUSCULAR   SENSATION.  401 

etc.  The  subject  has  received  most  accurate  investigation  from  Head,1  who 
has  shown  that  there  is  an  intimate  nervous  connection  between  the  internal 
organs  and  definite  areas  of  the  skin,  manifested  by  pain  and  tenderness 
appearing  in  sharply-localized  regions  on  the  surface  when  definite  organs 
become  disordered.  He  has  also  demonstrated  that  disorders  of  the  thoracic 
and  abdominal  viscera  not  only  produce  pain  and  tenderness  on  the  surface  of 
the  body,  but  also  cause  pain  and  tenderness  over  certain  areas  of  the  scalp. 
Head  is  inclined  to  explain  the  topographical  association  of  skin-tenderness 
with  visceral  disorders  by  the  assumption  that  the  nerve-supplies  of  the  parts 
so  related  find  their  origin  within  the  same  segment  of  the  spinal  cord.  The 
sensory  result  of  visceral  irritation  may  be  summarized  in  the  following  way : 
"  When  a  painful  stimulus  is  applied  to  a  part  of  low  sensibility  in  dose  cen- 
tral connection  with  a  part  of  much  greater  sensibility,  the  pain  produced  is 
felt  in  the  part  of  higher  sensibility  rather  than  in  the  part  of  lower  sensibility 
to  which  the  stimulus  was  actually  applied." 

Certain  transferred  pains  are  explained  by  Meltzer 2  in  the  following  man- 
ner :  An  inflamed  or  irritated  organ  originates  a  succession  of  sensory 
stimuli,  which  do  not  awake  consciousness  because  they  are  continuous. 
There  is,  nevertheless,  a  summation  of  such  irritations  within  the  central 
organ  which  elevates  its  plane  of  irritability  to  such  an  extent  that  sensory 
impulses  reaching  the  implicated  nerve-centre  from  any  part  of  the  body 
arouse  it  above  the  threshold  of  consciousness  and  give  rise  to  sensations 
which  are  referred  to  the  seat  of  peripheral  inflammation  or  constant  irrita- 
tion. For  example,  the  subject  of  a  mild  alveolitis  may  feel  in  the  teeth  a 
stronger  pain  than  is  felt  in  the  nose  when  a  concentrated  solution  is  thrown 
into  the  latter  organ. 

That  this  transferred  localization  may  characterize  other  sensations  than 
those  of  pain  has  been  definitely  observed  by  Obersteiner,3  who  found  that  in 
patients  suffering  from  certain  central  nervous  lesions  tactile  irritation  of  a  cer- 
tain point  on  the  skin  was  referred  by  them  to  some  other  part  of  the  body, 
usually  the  corresponding  point  on  the  other  side.  He  designated  this  trans- 
ference of  sensation  by  the  term  allochiria,  meaning  a  confusion  of  sides. 

4.  Muscular  Sensation. — Closely  allied  to  common  sensation,  if  not  a 
part  of  it,  is  muscular  sensation.  If  two  weights  are  to  be  compared,  we 
naturally  do  not  lay  them  on  the  skin  to  determine  their  pressure-difference, 
but  we  lift  and  weigh  them  in  the  hands,  and  experience  shows  that  a  much 
more  accurate  estimate  may  thus  be  made. 

We  undoubtedly  have  a  keen  perception  of  the  tension  of  a  muscle,  and 
therefore  of  the  amount  of  resistance  against  which  it  is  contracting.  This  per- 
ception may  be  the  outcome  of  a  direct  consciousness  of  the  amount  of  motor 
energy  sent  out  from  the  motor  cells,  or  it  may  be  due  to  the  inflow  of  sensory 
impulses  which  show  the  tension  to  which  the  muscles  have  been  subjected. 
The  latter  view  has  more  to  be  said  in  its  favor. 

1  Brain,  1893-4. 

J  S.  J.  Meltzer  :  Philadelphia.  Medical  Journal,  August  5,  1899,  p.  12.          9  Brain,  1881. 
VOL.  II.— 26 


402  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Recent  researches  have  demonstrated  the  existence  of  an  abundant  supply 
of  sensory  nerves,  whose  excitement  must  depend  upon  the  exercise  of 
jkej^iaLimiacle^.  Ciaccio  l  has  described  the  termination  of  sensory  nerves 
in  tendons  as  a  splitting  up  of  the  nerve-fibres  whose  axis-cylinders,  in  the 
form  of  varicose  threads,  end  freely  as  spirals  or  rings  around  the  tendon- 
bundles.  The  joints  seem  to  be  particularly  rich  in  sensory  nerve-supply. 
Golgi2  first  described  certain  special  modes  of  ending  of  sensory  nerves  just 
at  the  junction  of  the  voluntary  muscle  with  its  tendon.  This  terminal  organ 
is  a  fusiform  corpuscle  consisting  of  several  delicate  connective-tissue 
envelopes  with  nuclei,  and  is  situated  on  the  surface  of  the  tendon.  One  to 
several  nerve-fibres  enter  each  corpuscle,  and,  dividing  and  losing  their 
medullary  sheaths,  break  up  into  an  arborization  of  naked  axis-cylinders. 
The  skeletal  muscles  themselves  are  extraordinarily  rich  in  sensory  nerve- 
supply.  According  to  Sherrington,3  "  the  proportion  of  afferent-fibres  to  total 
myelinate  fibres  ranges  from  a  little  more  than  one-third  in  some  muscular 
nerves  to  a  full  half  in  others."  These  sensory  fibres  end,  for  the  most  part, 
in  the  so-called  "  muscle-spindles,"  which  are  fusiform  bodies,  usually  just 
visible  to  the  unaided  eye  (Fig.  199,  p.  390).  The  spindles  are  for  the  most 
part  scattered  between  the  ordinary  muscle-fibres,  though  many  abut  upon 
intramuscular  septa  or  are  in  the  immediate  vicinity  of  aponeuroses.  As 
many  as  thirteen  spindles  have  been  counted  in  one  cross-section  of  the  genio- 
glossus  muscle.  Sherrington 4  calculates  that  the  number  of  spindle-organs 
is  sufficient  to  account  for  nearly  or  quite  two-thirds  of  all  the  afferent  fibres 
demonstrated  to  exist  in  the  nerve-trunks  of  the  limb  muscles.  It  is  worth 
observing  that  the  spindle-organs  have  not  been  demonstrated  in  the  eye 
muscles  nor  in  the  intrinsic  muscles  of  the  tongue.  The  muscle-spindle  con- 
sists of  a  central  core  of  modified  muscle-fibres  inclosed  in  an  outer  capsule 
formed  of  several  layers  of  concentrically  disposed  membranous  lamellae 
composed  of  connective  tissue.  Between  the  capsule  and  the  central  muscle- 
bundle  is  a  wide  lymph-space  traversed  by  a  network  of  delicate  filaments. 
In  forming  the  spindle  two  or  three  ordinary  muscle-fibres  of  the  red  variety 
become  invested  at  the  proximal  end  of  the  organ  by  a  definite  sheath  of 
connective  tissue.  As  they  penetrate  further  into  this  envelope  the  muscle- 
fibres  tend  to  split  lengthways,  each  fibre  giving  rise  to  perhaps  three 
"  daughter  "-fibres,  which  are  proportionally  of  less  diameter.  The  striation 
and  fibrillation  are  frequently  confined  to  the  outer  portion  of  these  daughter- 
fibres,  some  of  which  are  devoid  of  sarcolemma.  For  the  middle  third  of* 
its  course  in  the  muscle-spindle  each  daughter-fibre  becomes  thickly  crusted 
with  a  sheet  of  nuclei.  Toward  the  distal  end  of  the  spindle  the  muscle- 
fibres  often  merge  in  tendon-bundles,  which  finally  combine  with  the  fibrous 
tissue  forming  the  capsule  of  the  spindle  ;  so  that  of  the  two  ends  of  the  axial 
bundle  within  the  spindle,  one  is  muscular  and  the  other  is  tendinous. 

According  to  Ruffini,5  sensory  nerves  may  end  upon  the  axial    muscle- 

1  Barker  :   The  Nervous  System,  1899,  p.  405.  2  Ibid. 

3  Sherrington  :  Journal  of  Physiology,  1895,  vol.  xvii.  p.  229.  4  Ibid. 

6  Ruffini :  Ibid.,  1898,  xxiii.  190. 


MUSCULAR   SENSATION.  403 

fibres  of  the  spindle  in  either  or  all  of  three  different  modes  (Fig.  199):  1. 
The  axis-cylinder  may  flatten  out  and  twine  in  rings  and  spirals  about  the 
muscle-fibre.  2.  The  axis-cylinder  may  break  up  into  a  number  of  leaflets 
applied  to  the  muscle-fibre  (secondary  mode).  3.  The  axis-cylinder  may 
end  in  a  plate  of  varicose  fibrils  resembling  the  motor  end-plate. 

When  we  consider  that  it  is  through  muscular  sensation  that  we  derive  our 
most  accurate  conceptions  of  the  form,  weight,  and  position  of  objects,  and  through 
which  we  explore  our  own  body-surface  and  distinguish  its  areas  of  localization ; 
that  this  is  the  fundamental  sense  by  which  the  sensations  arising  in  most 
other  organs  are  tested  and  verified ;  and  that  it  is  from  the  sense  of  muscular 
movement  that  we  can  form  ideas  of  time  and  space, — it  may  well  be  regarded 
as  the  mother  of  all  sense-perceptions.  Normal  muscles,  even  when  function- 
ally inactive,  are  still  in  a  state  of  tonic  contraction ;  it  is  not  improbable  that 
this  tone  is  a  reflex  action  whose  sensory  element  is  formed  by  the  impulses 
travelling  along  nerves  of  muscular  sensation.  Such  impulses  are  probably 
indispensable  to  the  preservation  of  the  equilibrium  of  the  body. 

Sherrington  found  that  if  he  separated  the  aponeurosis  belonging  to  the 
distal  portion  of  the  vastus  medialis  muscle,  under  which  the  muscle-spindles 
are  numerous,  the  knee-jerk  could  no  longer  be  excited  through  the  muscle. 

Our  appreciation  of  the  weight  of  bodies  is  determined  by  lifting  them. 
But  even  in  so  simple  an  exercise  of  the  muscular  sense  as  this  the  judgment 
is  subject  to  extraordinary  illusions  depending  on  the  preconception  of  the 
weight  of  a  body,  and  consequent  muscular  effort  put  forth  in  lifting  it. 
When  bodies  having  the  same  weight  and  size,  such  as  appropriately  loaded 
pieces  of  iron,  cork,  and  wood,  are  compared,  the  specifically  lighter  body  will 
seem  to  be  heavier.  "Before  lifting  an  object  we  normally  estimate  the 
approximate  weight  by  sight,  and  the  effort  to  be  exerted  in  lifting  is  adjusted 
semi-automatically  upon  the  basis  of  this  preliminary  estimate.  If  insufficient 
effort  is  put  forth  at  the  beginning  of  the  lifting,  the  weight  of  the  object  will 
be  overestimated.  If  too  great  effort  is  put  forth,  the  weight  of  the  object 
will  be  underestimated."1  In  comparing  the  weight  of  objects  having 
different  sizes  the  illusion  takes  another  direction.  Thus  an  inflated  paper 
bag  may  be  estimated  to  have  the  same  weight  as  a  piece  of  lead  weighing 
sixty  times  as  much.2 

The  clinical  study  of  disease  in  the  central  nervous  system  affords  strong 
evidence  of  the  functional  independence  of  the  sense  organs  involved  in  the 
appreciation  of  touch,  heat,  cold,  and  pain.  In  certain  diseases  of  the  spinal 
cord,  areas  of  skin  may  be  mapped  out  in  which  sensations  of  pressure  are 
lost,  but  those  of  temperature  remain,  and  vice  versd.  In  other  diseases  the 
patient  can  appreciate  warmth  applied  to  the  skin,  but  not  cold. 

The  sensations  of  cold  and  pressure  seem  to  be  usually  lost  or  retained 
together,  while  those  of  warmth  and  pain  have  a  similar  connection.  It  is  a 
peculiar  fact  that  sometimes  in  the  early  stages  of  ether  and  chloroform  narco- 
sis the  sense  of  touch  remains  while  that  of  pain  is  abolished.  Funke3  refers 

1  Seashore :   Op.  cit.  2  Wolfe  :  Psychological  Review,  1898,  p.  25. 

3  "  Der  Tastsinn,"  Hennann's  Handbuch  der  Physiologic,  Bd.  iii.  S.  2. 


404  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

to  two  cases  in  which,  while  the  tactile  sense  was  preserved,  muscular  sensation 
was  lost,  and  an  object  could  be  held  in  the  grasp  only  while  the  eyes  were 
turned  upon  it. 

Hunger  and  T/iirst. — Hunger  and  thirst  are  peculiar  sensations  which 
depend  partly  on  local  and  partly  on  general  causes.  Diminution  in  the  bulk 
of  water  and  of  circulating  aliment  in  the  body  no  doubt  causes  excitement 
of  sensory  nerves  on  which  depend  the  feelings  of  thirst  and  hunger,  but  in 
ordinary  life  these  feelings  are  dependent  on  the  physical  condition  of  certain 
mucous  surfaces.  Any  circumstance  which  causes  drying  of  the  lining  mem- 
brane of  the  mouth  provokes  thirst,  and  some  condition  of  the  empty  stomach 
arouses  hunger.  Thirst  may  be  assuaged  by  introducing  water  directly  into 
the  stomach  through  a  gastric  fistula,  though  to  effect  the  purpose  a  larger 
quantity  must  be  employed  in  this  way  than  by  the  mouth.  Hunger  in  a 
somewhat  similar  manner  may  be  appeased  by  rectal  alimentation.  It  seems 
probable,  however,  that  these  sensations  as  usually  felt  are  the  result  of  a 
sort  of  habit,  depending  on  the  physiological  condition  of  the  secreting  and 
absorbing  mechanisms  of  the  alimentary  canal. 

Clinical  observation  has  shown  that  "  bulimia,"  or  voracious  appetite,  is 
frequently  a  result  of  disease  in  certain  parts  of  the  central  nervous  system. 
We  are  therefore  justified  in  speaking  of  a  "hunger-centre."1 

E.    THE  EQUILIBRIUM  OF  THE  BODY;   THE  FUNCTION  OF  THE 
SEMICIRCULAR  OANALS. 

The  term  equilibrium,  as  applied  to  the  condition  of  the  body,  whether  at 
rest  or  in  motion,  indicates  a  state  in  which  all  the  skeletal  muscles  are  under 
control  of  nerve-centres,  so  that  they^  combine,  when  required,  to  resist  the 
effect  of  gravity  or  to  execute  some  co-ordinated  motion.  The  preservation 
of  equilibrium  is  manifestly  of  fundamental  importance  in  animal  life,  and  we 
find,  accordingly,  several  mechanisms  sharing  in  this  function.  That  the  motor 
co-ordinating  centres  may  act  properly,  they  must  receive  sensory  impres- 
sions conveying  information  of  the  relative  position  of  the  body  at  any  given 
moment.  The  sum-total  of  these  sensations  may  be  characterized  as  the  sense 
of  equilibrium,  and  it  is  probably  not  going  too  far  to  assume  that  every  known 
sensation  contributes  to  this  fund  of  information.  Thus,  in  ordinary  life  the 
position  of  objects  is  commonly  determined  by  the  sense  of  sight :  when  one 
tries  to  walk  while  looking  through  a  prism,  objects  are  not  properly  localized 
by  vision,  and  improper  co-ordination  results.  The  contact  of  the  soles  of  the 
feet  with  the  ground,  and  that  of  the  surface  of  the  body  with  various  objects, 
are  common  sources  of  information  as  to  our  relation  with  the  environment. 
Standing  upright,  and  still  more  when  in  motion,  the  muscular  sense  is  active 
in  appreciating  the  tension,  active  or  passive,  of  the  muscles.  In  the  erect 
position,  with  eyes  closed,  a  writing  point  attached  to  the  head  will  show  that 
the  body  sways  in  a  peculiar  manner  indicating  successive  contraction  of  differ- 
ent groups  of  muscles ;  and  a  person  with  failure  of  muscular  and  tactile  sen- 

\  Ewald  :  Diseases  of  the  Stomach,  p.  397. 


THE  SENSE    OF  EQUILIBRIUM.  405 

sibility,  as  in  locomotor  ataxy,  cannot  stand  with  eyes  closed,  and  his  move- 
ments, even  when  sight  is  employed,  are  exaggerated  and  unnatural.  Attention 
has  previously  been  called  to  the  fact  that  air-waves,  irrespective  of  those 
producing  sound-sensations,  exert  an  influence  upon  the  tympanic  membrane 
by  which  we  are  capable  of  appreciating  the  presence  and,  to  some  extent,  the 
physical  character  of  objects.  Whether  this  sensation  involves  the  nerves  of 
touch,  those  of  common  sensibility,  or  those  distributed  to  the  internal  ear,  is 
uncertain. 

In  the  absence  of  any  of  these  sensations  the  loss  may  be  made  up  by  more 
perfect  development  of  others.  Ordinarily,  the  sensory  information  from  all 
these  sources,  when  compared  in  consciousness,  harmonizes  and  gives  rise  to 
a  concrete  idea  of  position.  Frequently,  however,  one  of  the  sources  of  sense- 
impression  suddenly  fails  us  or  its  testimony  conflicts  with  that  of  other  sense 
organs ;  the  result  is  disturbance  of  equilibrium.  A  very  common  outcome 
of  this  conflict  of  sensations  is  dizziness  or  nausea.  The  distress  arising  from 
wearing  ill-fitting  glasses  and  the  sensations  experienced  when  one  looks  down 
from  a  high  eminence  are  examples  in  point.  Internal  disorders  exciting  nerves 
of  common  sensation  have  the  same  effect,  though  the  relation  borne  by  visceral 
sensations  to  equilibrium  is  very  ill  known.  A  false  idea  of  position  of  the 
body,  a  sense  of  falling  in  one  direction  or  another,  may  lead  to  sudden  effort 
of  recovery  by  which  the  person  is  precipitated  to  the  opposite  side.  Thus, 
when  looking  at  rapidly-moving  water  erroneous  ideas  of  equilibrium  are 
gained  through  the  visual  sense,  and  there  is  a  strong  tendency  for  the  body 
to  precipitate  itself  in  one  direction  or  another.  When,  in  going  up  a  stair- 
case, one  miscalculates  the  number  of  steps,  a  peculiar  sensation  of  want  of 
equilibrium  is  aroused  through  the  muscular  sense.  It  is  clear,  then,  that- 
the  sense  of  equilibrium  is  served  by  various  sense  organs,  and  a  complete 
discussion  of  this  function  would  entail  a  consideration  of  the  whole  field  of 
nerve-muscle  physiology.  There  is,  however,  good  reason  for  believing  that 
there  is  a  special  sense  organ  for  determining  the  position  and  direction  of 
movement  of  the  head  and,  by  inference,  of  the  whole  body.  The  terminal 
organ  of  this  sense  apparatus  of  equilibrium  is  found  in  the  system  of  semi- 
circular canals  of  the  internal  ear. 

Experiments  on  the  lower  animals,  chiefly  performed  on  birds,  show  a  con- 
stant motor  disturbance  to  follow  division  of  any  or  all  of  the  semicircular 
canals.  These  disturbances  are  of  two  'kinds.  When  the  animal  is  at  rest  it 
does  not  stand  in  a  natural  fashion,  but  sprawls  in  a  more  or  less  exaggerated 
degree.  It  holds  its  head  in  an  unnatural  position,  as  with  the  vertex  touch- 
ing the  back,  or  with  the  beak  turned  down  toward  the  legs  or  bent  over  to 
one  side.  Immediately  after  the  operation,  and  whenever  it  is  disturbed,  the 
animal  goes  through  peculiar  forced  movements,  together  with  rolling  or 
twitching  of  the  eyes,  of  various  kinds  and  degrees  of  violence,  depending  on 
the  position  and  number  of  canals  severed.  The  disturbance  varies  from 
simple  unsteadiness  in  gait,  with  swaying  motions  of  the  head,  to  complete 
lack  of  co-ordination  and  a  violence  of  movement  almost  comparable  to  that 


406  AN  AMERICAN   TEXT-BOOK   OF   PHYSIOLOGY. 

of  a  chicken  whose  head  has  been  cut  off.  Essentially  the  same  results  have 
been  determined  to  follow  injury  of  the  semicircular  canals  of  widely  different 
groups  of  animals. 

These  results  have  been  explained  by  the  assumption  that  the  hair-cells  on 
the  cristce  acusticce  of  the  ampullae  of  the  semicircular  canals  are  irritated  by 
increase  or  decrease  of  pressure  of  the  endolymph  upon  them,  and  thus  give 
rise  to  sensory  impressions  from  which  ideas  of  change  of  position  are  derived. 
Section  of  the  canal,  by  draining  off  the  endolymph,  would  cause  abnormal 
pressure-irritation.  The  anatomical  relations  of  the  semicircular  canals  afford 
an  obvious  basis  for  this  view,  for  the  canals  of  each  ear  are  almost  exactly  at 
right  angles  to  one  another,  occupying  the  three  planes  of  space ;  considering 
the  two  ears,  the  horizontal  canals  are  nearly  in  the  same  plane,  and  the  ante- 
rior vertical  canal  of  one  side  is  nearly  parallel  with  the  posterior  vertical  canal 
of  the  other  side.  Any  possible  movement  of  the  head  would  thus  produce 
an  increase  of  endolymph-pressure  upon  the  hair-cells  in  one  ampulla  and  a 
decrease  of  pressure  in  the  ampulla  of  the  parallel  canal,  and  every  change  of 
position  would  be  accompanied  by  the  irritation  of  definite  ampulla  with  defi- 
nite degrees  of  excitement  (Fig.  202).  Experiments  on  man  afford  considerable 

support  to  this  theory  of  the  function  of  the 
semicircular  canals.  A  person  with  eyes  closed 
and  with  muscular  and  tactile  sensations  elimi- 
nated, supported  on  a  table  which  can  be  rotated 
in  all  directions,  can  determine  with  consider- 
able accuracy  not  only  that  he  is  moved,  but 
in  what  direction  and,  to  some  extent,  through 
how  great  an  angle.  Further,  when  brought 

FIG.  202. —Diagrammatic  horizontal  &  " 

section  through  the  head  to  illustrate  to  rest  after  a  series  of  rotations  the  person 
the  planes  occupied  by  the  semicircu-  d  observation  feels  a  sensation  of  motion 

lar  canals  (after  Waller) :  s,  superior 

canal  ;p,  posterior  canal  ;H,  horizontal     in  the  opposite  direction.     Each  of  these  re- 
sults should  be  expected  to  follow  were  the 

theory  in  question  correct.  The  observations  of  James  have  shown  that 
with  deaf  mutes  in  whom  the  internal  ear  was  at  fault  rapid  rotation  in 
an  ordinary  "  swing "  failed  to  produce  the  dizziness  which  is  the  common 
effect  in  ordinary  individuals.  On  the  other  hand,  diseases  which  may  be  sup- 
posed to  alter  the  intra-labyrinthine  pressure  are  characterized  by  the  symp- 
toms of  vertigo  and  inco-ordination  of  movement.  The  presumable  effect  of 
cutting  the  semicircular  canals  is  that  the  escape  of  endolymph  changes  the 
pressure  upon  the  sensory  hair-cells  and  gives  the  animal  the  sensation  of 
falling  in  one  direction  or  another,  so  that  he  is  impelled  to  make  compensa- 
tory or  j 'weed  movements  to  counteract  this  imaginary  change  of  position.  In 
birds  and  in  fishes,  whose  life  is  passed  more  or  less  exclusively  in  a  medium 
in  which  tactile  and  muscular  sensation  can  contribute  little  to  the  sense  of 
equilibrium,  the  semicircular  canals  are  especially  well  developed.1  In  fishes, 
though  section  of  the  canals  themselves  produces  no  disturbance,  division  of 

1  Sewall :  Journal  of  Physiology,  1884,  iv.  p.  339. 


THE  SENSE    OF  EQUILIBRIUM.  407 

the  nerves  supplying  the  ampullae  usually  gives  rise  to  marked  forced  move- 
ments, as  shown  in  somersaults,  spiral  swimming,  etc.,  when  set  free  in  the 
water.  When,  however,  the  nerves  are  cut  with  great  care,  with  sharp  scis- 
sors, so  as  to  avoid  traction  on  or  crushing  of  the  nerves,  such  forced  move- 
ments do  not  follow. 

Lee l  found  that  when  a  fish  is  turned  in  different  positions  there  is  a 
compensatory  change  in  the  direction  of  the  fins  and  the  optic  axes  determined 
by  the  semicircular  canal  in  whose  plane  the  movement  is  made.  He  con- 
cludes that  "  Each  canal  has  a  principal  and  a  subordinate  function.  The 
former  is  the  appreciation  of  rotational  body  movements  in  its  own  plane  and 
toward  its  side  of  the  body  ;  the  latter  is  the  appreciation  of  similar  move- 
ments, but  in  the  opposite  direction."  Electric  stimulation  of  the  ampullary 
nerves  or  mechanical  pressure  upon  the  ampullae  excites  equally  definite 
movements  of  eyes  and  fins,  and  the  ocular  result  of  nerve-irritation  is  the 
exact  opposite  of  that  of  nerve-section. 

The  difference  in  function  between  the  divisions  of  the  internal  ear  is 
indicated  by  investigations  on  albinos.  White  animals  with  blue  eyes  are 
deaf,  but  possess  the  normal  power  of  equilibration.  Rawitz2  found  the 
cochlea  in  such  creatures  to  be  much  reduced  and  the  organ  of  Corti  atro- 
phied, while  the  semicircular  canals  were  normal. 

According  to  Lee 3  and  others,  the  equilibrium  of  rest  and  motion,  or  static 
and  dynamic  equilibrium,  depends  upon  the  irritation  of  different  nerve-ter- 
minals. The  manner  of  action  of  the  latter  has  been  considered.  As  to  the 
nervous  mechanism  on  which  static  equilibrium  depends,  Lee  is  of  the  opinion 
that  the  knowledge  of  the  position  of  the  head  while  at  rest  comes  from  the  rela- 
tion of  the  otoliths  in  the  vestibular  sacs  to  the  nerve-endings  on  the  maculoe 
acusticce.  These  otoliths  form  considerable  masses  in  the  ears  of  fishes,  and  the 
intensity  and  direction  of  their  pressure  upon  hair-cells  must  vary  with  the 
spatial  relations  of  the  head,  and  thus  be  comparable,  in  the  sense  of  posi- 
tion which  they  arouse,  to  the  tactile  sensations  derived  from  the  soles  of 
the  feet  in  man. 

The  opinion  may  be  ventured  that  in  the  semicircular  canals  we  have  a 
sense-organ  of  a  peculiar  kind.  The  evidence  is  satisfactory  that  impulses 
generated  in  the  nerves  of  the  ampulla,  and  probably  of  the  vestibular  sacs 
also,  give  rise  to  sensations  of  position  both  dynamic  and  static.  And  it  is 
highly  probable  that  such  sensations  form  a  constant  basis  for  our  notion  of 
the  spatial  relations  of  the  head.  But  the  preservation  of  equilibrium  does 
not  depend  wholly  upon  the  special  sense-organ,  as  does  sight  upon  the  eye. 
For  the  muscular  and  tactile,  not  to  speak  of  the  visual  and  other  senses, 
supply  information  in  the  same  direction,  and,  no  doubt,  these  may  to  a  cer- 
tain extent  vicariously  fill  the  function  of  the  semicircular  apparatus  when 
this  is  abolished. 

1  Lee:  Journal  of  Physiology,  xv.  p.  311  ;  xvi.  p.  192. 

2  Rawitz  :  Zoologucher  Jahresbericht,  1896. 

3  Journal  of  Physiology,  xv.  p.  311,  xvi.  p.  192. 


408 


AN   AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


P.  SMELL. 

The  complex  paired  cavity  of  the  nose  is  divisible  into  a  lower  respiratory 
and  an  upper  olfactory  tract,  the  mucous  membrane  over  each  of  which  is 
distinctive.  The  covering  of  the  respiratory  tract  is  known  as  the  Schneider- 
ian  or  pituitary  membrane;  its  surface  is  overlaid  with  cylindrical  ciliated 
epithelium,  the  ciliary  current  of  which  is  directed  posteriorly  toward  the 
pharynx. 

The  Schneiderian  membrane  lines  the  lower  two-thirds  of  the  septum,  the 
middle  and  inferior  turbinated  bodies,  and  the  bony  sinuses  which  communi- 
cate with  the  nasal  chamber.  The  mem- 
brane upon  the  turbinated  bodies  and 
the  lower  part  of  the  septum  is  composed 
largely  of  erectile  tissue. 

The  function  of  the  respiratory  tract 
is  threefold :  it  restrains  the  passage 
of  solid  particles  into  the  lungs;  it 
warms  the  air  inspired  to  approximately 


FIG.  203.— Section  of  olfactory  mucous  mem- 
brane (after  V.  Brunn) :  the  olfactory  cells  are  in 
black. 


FIG.  204.— Cells  of  the  olfactory  region  (after  V. 
Brunn) :  a,  olfactory  cells  ;  6,  epithelial  cells ;  n, 
central  process  prolonged  as  an  olfactory  nerve- 
fibril  ;  I,  nucleus ;  c,  knob-like  clear  termination 
of  peripheral  process  ;  h,  bunch  of  olfactory  hairs. 


the  temperature  of  the  body ;  and  it  gives  up  moisture  sufficient  nearly  to 
saturate  the  air. 

The  olfactory  mucous  membrane,  which  alone  is  the  peripheral  organ  for 
smell,  is  seated  in  the  upper  part  of  the  nasal  chamber,  away  from  the  line 
of  the  direct  current  of  inspired  air.  The  membrane  is  thick  and  is  covered 
by  an  epithelium  composed  of  two  kinds  of  cells,  columnar  and  rod  cells. 
The  latter  are  the  true  olfactory  cells  (Figs.  203,  204),  with  which  the  fibres 
of  the  olfactory  nerve  are  known  to  be  connected.  These  olfactory  cells,  in 
fact,  are  comparable  to  nerve-cells  in  that  the  fibres  connected  with  them,  the 
fibres  composing  the  olfactory  nerve,  are  direct  outgrowths  from  the  cells 
(Fig.  205),  essentially  similar  in  every  way  to  the  nerve-fibre  processes  springing 
from  nerve-cells  in  the  nerve-centres.  In  this  respect  the  olfactory  cells  differ 
from  the  sensory  cells  in  other  organs  of  special  sense.  The  membrane 


THE   SENSE    OF  SMELL. 


409 


appears  to  be  not  ciliated  except  near  its  juncture  with  the  Schneiderian 
membrane,  where  the  columnar  cells  acquire  cilia  and  gradually  pass  over 
into  the  cells  covering  the  respiratory  tract.  Substances  exciting  the  sense 
of  smell  exist  as  gases  or  in  a  fine  state  of  division  in  the  air  inspired. 
They  reach  the  olfactory  mucous  membrane  by  diffusion,  assisted  by  the 
modified  inspiratory  movements  of  "sniffing"  and  "smelling,"  and  are 
most  acutely  perceived  when  the  air  containing  them  is  warmed  to  the 
body-temperature.  The  amount  of  odoriferous  matter  that  may  thus  be 
recognized  is  extraordinarily  small ;  thus,  it  is  said  that  in  one  liter  of  air 
the  odor  of  0.000,005  gram  of  musk  and  of  0.000,000,005  gram  of  oil  of 
peppermint  can  be  perceived.1  The  odoriferous  particles  probably  excite  the 


olf.c 


FIG.  205.— Diagram  of  the  connections  of  cells  and  fibres  in  the  olfactory  bulb  (Schafer,  in  Quain's  Anat- 
omy) :  otf.c,  cells  of  the  olfactory  mucous  membrane ;  olf.n,  deepest  layer  of  the  bulb,  composed  of  the 
olfactory  nerve-fibres  which  are  prolonged  from  the  olfactory  cells ;  gl,  olfactory  gloineruli,  containing 
arborization  of  the  olfactory  nerve-fibres  and  of  the  dendrons  of  the  mitral  cells ;  m.c,  mitral  cells ; 
a,  thin  axis-cylinder  process  passing  toward  the  nerve-fibre  layer,  n.tr,  of  the  bulb  to  become  continuous 
with  fibres  of  the  olfactory  tract ;  these  axis-cylinder  processes  are  seen  to  give  off  collaterals,  some  of 
which  pass  again  into  the  deeper  layers  of  the  bulb ;  n',  a  nerve-fibre  from  the  olfactory  tract  ramifying 
in  the  gray  matter  of  the  bulb. 

sense  of  smell  by  coming  into  contact  with  the  olfactory  epithelium  after  solu- 
tion in  the  layer  of  moisture  covering  it.  This  epithelium  is  easily  thrown 
out  of  function,  as  the  com-mon  loss  of  smell  when  there  is  a  "  cold  in  the 
head "  testifies.  When  the  nostril  is  filled  with  water  in  which  an  odorous 
substance  is  dissolved,  no  sensation  of  smell  is  excited,  but  it  is  said  that  if 
normal  salt-solution,  which  injures  the  living  tissues  less  than  water,  be  used 
as  the  solvent,  the  odor  can  still  be  perceived.  In  many  lower  animals  the 
sense  of  smell  has  an  acuteness  and  an  importance  in  their  economy  unknown 
in  the  human  race.  It  is  probable  that  not  only  do  different  races  have  their 
distinctive  odors,  but  that  each  individual  exhales  an  odor  peculiar  to  himself, 
distinguishable  by  the  olfactory  organs  of  certain  animals.  The  classification 
1  Passy  :  Comptes-rendus  de  la  Societe  de  Biologie,  1892,  p.  84. 


410  AN  AMERICAN   TEXTBOOK   OF  PHYSIOLOGY. 

of  odors  is  not  very  definite,  and  the  relation  of  odors  to  one  another  in  the 
way  of  contrast  and  harmony  is  ill  understood.  No  limited  number  of  pri- 
mary sensations,  as  in  vision,  have  been  discovered  out  of  which  other  sen- 
sations can  be  composed.  Certain  sensations,  as  those  due  to  the  inhalation 
of  ammonia  and  other  irritant  gases,  are  thought  to  be  due  to  excitement  of 
the  nasal  filaments  of  the  fifth  nerve,  and  not  of  the  olfactory. 

Subjective  sensations  of  smell  are  sometimes  experienced,  the  result  of  some 
irritation  arising  in  the  olfactory  apparatus  itself. 

Finally,  in  man  sensations  of  smell  have  their  most  important  uses  in  con- 
nection with  taste ;  many  so-called  "  tastes "  owe  their  character  wholly  or 
partly  to  the  unconscious  excitement  of  the  sense  of  smell. 

G.  TASTE. 

The  peripheral  surfaces  concerned  in  taste  include,  in  variable  degree,  the 
upper  surface  and  sides  of  the  tongue  and  the  anterior  surfaces  of  the  soft 
palate  and  of  the  anterior  pillars  of  the  fauces.  Other  parts  of  the  buccal 
and  pharyngeal  cavities  are,  in  most  persons,  devoid  of  taste.1 

The  chief  peripheral  sensory  organs  of  taste  are  groups  of  modified  epi- 
thelial cells,  known  as  t(jiste-buds  (Fig.  206),  seated  in  certain  papilla?  of  the 
tasting  surfaces.  According  to  some  authors,  only  parts  provided  with  taste- 
buds  can  give  taste-sensations.2 

The  structure  of  taste-buds  is  most  easily  studied  in  the  papilla  foliata  of 
the  rabbit,  a  patch  of  fine,  parallel  wrinkles  found  on  each  side  of  the 
back  part  of  the  tongue  of  the  animal.  The  taste-bud  is  a  somewhat  globular 
body  seated  in  the  folds  of  mucous  membrane  between  the  furrows  of  the 
papilla.  It  is  made  up  of  a  sheath  of  flattened,  fusiform  cells  enclosing  a 
number  of  rod-like  cells  each  of  which  terminates  in  a  hair-like  process.  These 
cells  surround  a  central  pore  which  opens  into  a  furrow  of  the  papilla. 
The  hair-bearing  cells  recall  the  appearance  of  the  olfactory  rod-cells,  and 
are  probably  the  true  sensory  cells  of  taste,  since  between  them  terminate  the 
filaments  of  the  gustatory  nerve.  In  the  human  tongue  taste-buds  are  con- 
fined to  the  fungiform  papillae,  seen  often  as  red  dots  scattered  over  the  upper 
surface ;  to  the  circumvallate  papillas,  the  pores  of  the  buds  opening  into  the 
groove  around  the  papilla ;  and  to  an  area  just  in  front  of  the  anterior  pillar 
of  the  fauces,  which  somewhat  resembles  the  papilla  foliata  of  the  rabbit. 

The  sensory  nerves  distributed  to  the  tongue  include  filaments  from  the 
glosso-pharyngeal,  the  lingual  branch  of  the  fifth,  and  the  chorda  tympani. 
The  relation  of  these  nerves  to  the  sense  of  taste  has  been  the  occasion  of 
much  dispute.  The  weight  of  evidence  probably  favors  the  belief  that  the 
glosso-pharyngeal  is  the  nerve  of  taste  for  the  posterior  third  of  the  tongue, 
while  the  lingual  and,  to  some  extent,  the  chorda  carry  taste-impressions  from 
the  anterior  two-thirds.  Clinical  cases  have  been  cited  to  show  that  all  the 

1  V.  Vintschgau  :  "  Geruchsinn,"  Hermann's  Handbuch  der  Physiologie,  iii.  2,  1880. 
2Camerer:  Zeitschri/t  fiir  Biologic,  1870,  vi.  S.  440;  Wilczynsky :  Hofmann  und  Schwalbe's 
Jahresbericht  der  PhysioL,  1875. 


THE   HKNXE    OF    TAXTK. 


411 


FIG.  206.— Section  through  one  of  the  taste-buds 
of  the  papilla  foliata  of  the  rabbit  (from  Quain, 
after  Ranvier),  highly  magnified:  p,  gustatory 
pore ;  «,  gustatory  cell ;  r,  sustentacular  cell ;  m, 
leucocyte  containing  granules ;  e,  superficial  epi- 
thelial cells ;  n,  nerve-fibres. 


gustatory  fibres  arise  from  the  brain  as  part  of  the  glosso-pharyngeal  nerve, 
whatever  may  be  their  subsequent  course  to  the  tongue.  On  th<>  mntrary, 
other  cases  have  shown  a  marked  loss 
of  taste-sensation  following  upon  lesions 
of  the  fifth  nerve  at  or  near  its  origin 
from  the  brain,  while  still  others  indi- 
cate that  some  of  the  taste-fibres  may 
arise  in  the  seventh  nerve.  The  point 
is  of  practical  importance  in  diagnosis, 
in  the  interpretation  of  loss  of  taste 
over  any  given  part  of  the  tongue,  but 
the  contradiction  in  the  clinical  cases 
reported  has  led  to  the  general  belief 
that  the  origin  and  course  of  the  gusta- 
tory fibres  are  subject  to  considerable 
individual  variations. 

Our  taste-perceptions  are  ordinarily 
much  modified  by  simultaneous  olfac- 
tory sensations,  as  may  easily  be  dem- 
onstrated by  the  difficulty  experienced 
in  distinguishing  by  taste  an  apple,  an 
onion,  and  a  potato,  when  the  nostrils  are  closed.  In  the  condition  of  anosmia 
the  ability  to  discriminate  between  tastes  is  much  below  par.  Sight  has  also 
an  important  influence,  at  least  in  quickening  the  expectancy  for  individual 
flavors.  Every  smoker  knows  the  blunting  of  his  perception  for  burning 
tobacco  while  in  the  dark ;  various  dishes  having  distinctive  flavors  are  said 
to  lose  much  of  their  gustatory  characteristics  when  the  eyes  are  bandaged.1 

The  intensity  of  gustatory  sensation  increases  with  the  area  to  which  the 
tasted  substance  is  applied.  The  movements  of  mastication  are  peculiarly 
adapted  to  bring  out  the  full  taste-value  of  substances  taken  into  the  mouth, 
and  the  act  of  swallowing,  by  which  the  morsel  is  rubbed  between  the  tongue 
and  the  palate,  has  been  proved  to  develop  tastes  not  appreciable  by  simple 
contact  with  the  sensory  surface.  A  considerable  area  in  the  mid-dorsum  of 
the  tongue  is  said  to  be  devoid  of  all  taste-sensibility.2 

The  sensitiveness  of  taste-sensation  is  greatest  when  the  exciting  substance 
is  at  the  temperature  of  the  body.  Weber3  found  that  when  the  tongue  was 
dipped  during  one-half  to  one  minute  in  water  either  at  the  freezing  tempera- 
ture or  warmed  to  50°  C.,  the  sweet  taste  of  sugar  could  no  longer  be  appre- 
ciated by  it.  It  is  probable  that  sapid  substances  reach  the  sensory  endings 
of  the  nerves  of  taste  only  after  being  dissolved  in  the  natural  fluids  of  the 
mouth,  and  any  artificial  drying  of  the  buccal  surfaces  or  alteration  of  their 
secretion  must  affect  taste-perceptions. 

1  Cf.  Patrick :  "  Studies  in  Psychology,"  Univ.  Iowa,  1899,  vol.  ii. 

2  Shore:  Journal  of  Physiology,  1892,  vol.  xiii.  p.  191. 

3  Archivfiir  Anatomic  und  Physiologic,  1847,  S.  342. 


412 


AN   AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


The  excitement  of  the  taste-nerves  appears  to  depend  not  so  much  on  the 
absolute  amount  of  the  substance  to  be  detected  as  on  the  concentration  of  the 
solution  containing  it.  Thus,  when,  1  part  of  common  salt  to  213  of  water 
was  tasted  by  Valentin/  1^-  cubic  centimeters  of  the  fluid  was  sufficient  to  give 
a  saltish  taste ;  when  diluted  so  that  the  ratio  of  salt  to  water  was  1  to  426, 
12  cubic  centimeters  taken  in  the  mouth  scarcely  gave  the  salt  taste.  Sulphate 
of  quinine  dissolved  in  the  proportion  1  to  33,000  gave  a  decided  bitter  taste, 
but  a  solution  1  to  1,000,000  was  with  difficulty  perceived  as  bitter. 

It  has  generally  been  conceded  that  all  gustatory  sensations  may  be  built 
up  out  of  four  primary  taste-sensations — namely,  bitter,  sweet,  sour,  and  salt. 
Some  authors  even  limit  the  list  to  tastes  of  bitter  and  sweet  (V.  Vintschgau). 


Auditory. 


Gustatory, 


Tactile. 


FIG.  207.— Diagram  showing  the  mode  of  termination  of  sensory  nerve-fibres  in  the  auditory,  gustatory, 
and  tactile  structures  of  vertebrata  (from  Quain,  after  Retzius).  Each  sense  organ  may  be  considered  as 
essentially  constructed  of  a  nerve-cell  with  two  processes,  one  finding  its  way  centrally  to  cluster  round 
other  nerve-cells  or  their  processes,  and  the  other  to  terminate  in  the  periphery.  In  the  organ  of  smell 
the  peripheral  process  is  very  short  and  is  directly  irritated  by  foreign  particles,  the  original  nerve-cell 
being  represented  by  the  olfactory  cell  (Fig.  291).  In  the  organs  of  touch  the  nerve-cell  is  found  in  the 
ganglion  of  the  posterior  spinal  nerve-root ;  the  peripheral  process  is  very  long  and  is  acted  on  indirectly 
through  the  modified  epithelium  round  which  it  clusters.  The  same  may  be  said  of  the  other  sense 
organs.  See  Quain's  Anatomy,  10th  ed.,  vol.  iii.  pt.  3,  p.  152. 

There  is  strong  reason  to  believe  that  corresponding  to  the  four  primary  taste- 
sensations  there  are  separate  centres  and  nerve-fibres,  each  of  which,  when 
excited,  gives  rise  only  to  its  appropriate  taste-sensation.  Substances  which 
arouse  the  sense  of  taste  are  not  appreciated  in  uniform  degree  over  the  surface 
of  the  tongue.  Thus,  to  V.  Yintschgau,  at  the  tip  of  the  tongue  acids  were 
perceived  acutely,  sweets  somewhat  less  plainly,  and  bitter  substances  hardly 
at  all.  It  is  generally  admitted  that  sweet  and  sour  tastes  are  recognized 
chiefly  at  the  front,  and  bitter,  together  with  alkaline  tastes,  by  the  posterior 

1  Lehrbuch  qjer  Physiologic,  1848. 


THE  SENSE    OF    TASTE.  413 

part  of  the  tongue.  Strong  evidence  in  favor  of  the  specific  difference  between 
various  taste-nerves  is  found  in  the  fact  that  the  same  substance  may  excite  a 
different  gustatory  sensation  according  as  it  is  applied  to  the  front  or  the  back 
of  the  tongue.  Thus,  it  has  been  demonstrated  that  a  certain  compound  of 
saccharin  (para-brom -ben zoic  sulphimide)  appears  to  most  persons  to  be  sweet 
when  applied  to  the  tip  of  the  tongue,  but  bitter  in  the  region  of  the  circum- 
vallate  papilla.1 

Oehrwall 2  has  examined  the  different  fungiform  papillae  scattered  over  the 
tongue  with  reference  to  their  sensitiveness  to  taste-stimuli.  One  hundred  and 
twenty-five  separate  papillae  were  tested  with  succinic  acid,  quinine,  and  sugar. 
Twenty-seven  of  the  papilla?  gave  no  response  at  all,  indicating  that  they  were 
devoid  of  taste-fibres.  Of  the  remaining  ninety-eight,  twelve  reacted  to  suc- 
cinic acid  alone,  three  to  sugar  alone,  while  none  were  found  which  were  acted 
upon  by  quinine  alone.  The  fact  that  some  papillae  responded  with  only  one 
form  of  taste-sensation  is  again  evidence  in  favor  of  the  view  that  there  are 
separate  nerve-fibres  and  endings  for  each  fundamental  sensation;  but  the 
figures  given  in  the  experiments  show  that  the  majority  of  the  papilla?  are 
provided  with  more  than  one  variety  of  taste-fibre. 

An  extract  of  the  leaves  of  a  tropical  plant,  Gymnema  silvestre,  when 
applied  to  the  tongue,  renders  it  incapable  of  distinguishing  the  taste  of  sweet 
and  bitter  substances;  it  probably  paralyzes  the  nerves  of  sweet  and  bitter 
sensations.  When  a  solution  of  cocaine  in  sufficient  strength  is  painted  on 
the  tongue,  the  various  sensations  from  this  member  are  said  to  be  abolished 
in  the  following  order:  (1)  General  feeling  and  pain;  (2)  bitter  taste;  (3) 
sweet  taste ;  (4)  salt  taste ;  (5)  acid  taste ;  (6)  tactile  perception  (Shore). 

That  there  are  laws  of  contrast  in  taste-sensation  has  long  been  empirically 
known.  Thus,  the  taste  of  cheese  enhances  the  flavor  of  wine,  but  sweets 
impair  it  (Joh.  Miiller).  It  is  unfortunate,  from  a  hygienic  standpoint  at 
least,  that  in  this  most  important  department  of  the  physiology  of  sensation 
investigations  are  almost  wholly  wanting. 

Certain  tastes  may  disguise  others  without  physically  neutralizing  them; 
when,  for  example,  sugar  is  mixed  with  vinegar,  the  overcoming  of  the  acid 
taste  is  probably  effected  in  the  central  nerve-organ.3 

1  Howell  and  Kastle :  Studies  from  the  Biological  Laboratory  of  Johns  Hopkins  University. 
1887,  iv.  13. 

8  Sfcandinavisches  Archivfilr  Physwlogie,  1890,  vol.  ii.  S.  1. 
3  Briicke :   Vorlesungen  uber  Physiologic,  1876. 


IV.   PHYSIOLOGY  OF  SPECIAL  MUSCULAR 
MECHANISMS. 


A.  THE  ACTION  OP  LOCOMOTOB  MECHANISMS. 

The  Articulations. — The  form,  posture,  and  movements  of  vertebrates 
are  largely  determined  by  the  structure  of  the  skeleton  and  the  method  of 
union  of  the  bones  of  which  it  is  composed.  There  are  two  hundred  bones  in 
the  human  skeleton,  and  they  are  so  connected  together  as  to  be  immovable, 
or  to  allow  of  many  varieties  and  degrees  of  motion.  There  are  four  prin- 
cipal methods  of  articulation  : 

1.  Union  by  Bony  Substance  (Suture). — This  form  of  union   occurs 
between  the  bones  of  the  skull.     These  bones,  which  at  birth  are  independent 
structures  connected  by  fibrous  tissue,  gradually  grow  together  and  make 
a  continuous  whole,  only  a  more  or  less  distinct  seam  remaining  as  witness 
of  the  original  condition. 

2.  Union  by  Fibro- Cartilages  (Symphysis). — The  bodies  of  the  verte- 
brae and  the  sacro-iliac  and  pubic  bones  are  closely  bound  together  by  disks 
of  fibre-cartilage.      This  material,  which   is  very  strong,  but  yielding  and 
elastic,  acts  as  a  buffer  to  deaden  the  effect  of  jars,  permits  of  a  slight 
amount  of  movement  when  the   force  applied  is  considerable,  and  restores 
the  bones  to  their  original  position  on  the  removal  of  the  force.     The  spinal 
column  can  be  thought  of  as  an  elastic  staff;  the  capacity  for  movement 
differs  greatly  in  different  regions,  however,  partly  on  account  of  differences 
in  the  thickness  of  the  intervertebral  disks  as  compared  with  the  antero- 
posterior  and  lateral  diameters  of  the  bodies  of  the  vertebrae,  and  more  espe- 
cially on  account  of  the  method  of  contact  of  the  superior  and  inferior  verte- 
bral processes.     In  the  cervical  region  the  disks  are  thick  and  the  diameter 
of  the  vertebrae  is  small,  and  this  permits  of  considerable  bending  in  all 
directions  and  a  certain  amount  of  rotation.     In  the  dorsal  region  a  slight 
amount  of  bending  from  side  to  side  and  a  slight  amount  of  rotation  are  pos- 
sible ;  but  backward  bending  is  inhibited  by  contact  of  the  articular  processes, 
and  forward  bending  is  prevented  by  the  strong  articular  ligaments.     In  the 
lumbar  region  bending  in  all  directions  is  more  free,  but  rotation  is  made 
impossible  by  the  interlocking  of  the  articular  processes.1 

3.  Union  of  Fibrous  Bands  (Syndesmosis). — Some  of  the  bones,  as  those 
of  the  carpus  and  tarsus,  are  connected  by  interosseous  ligaments  which,  at  the 
same  time  that  they  bind  the  bones  together,  admit  of  a  certain  amount  of 

1  Fick  :  Compendium  der  Physiologic  des  Menschen,  Wien,  1891. 
414 


THE   ACTION   OF   LOCOMOTOR   MECHANISMS.  415 

play,  the  extent  of  the  movement  varying  with  the  character  of  the  surfaces 
and  the  length  of  the  ligaments, 

4.  Union  by  Joints  (Diarthrosis). — The  adjacent  surfaces  of  most  of  the 
bones  are  so  formed  as  to  permit  of  close  contact  and  freedom  of  movement  in 
special  directions.  The  parts  of  the  bones  entering  into  the  joint  are  clothed  with 
very  smooth  cartilage,  and  the  joint-surfaces  are  lubricated  by  synovial  fluid, 
a  viscid  liquid  secreted  by  a  delicate  membrane  which  lines  the  fibrous  capsule 
by  which  the  joint  is  surrounded.  The  joint-capsule  is  firmly  attached  to  the 
bones  at  the  margin  of  the  articular  cartilages,  and,  at  the  same  time  that  it 
completely  surrounds  and  isolates  the  joint-cavity,  it  helps  to  bind  the  bones 
together.  The  bones  are  further  united  by  strong  ligaments,  in  some  cases 
within  and  in  other  cases  without  the  capsule.  These  ligaments  are  so  placed 
that  they  are  relaxed  in  certain  positions  of  the  joints  and  tightened  in  others ; 
they  guide  and  limit  the  movements  of  the  joints.  The  joint-surfaces  always 
touch,  although  usually  the  parts  in  contact  change  with  the  position  of 
the  joint.  If  continuous  contact  of  the  joint-surfaces  is  to  be  maintained 
and  free  movement  is  to  take  place  in  special  directions,  it  is  evident  that  the 
opposing  surfaces  must  not  only  be  so  constructed  that  they  shall  fit  each 
other  with  great  accuracy,  but  also  have  forms  especially  adapted  to  the  move- 
ments peculiar  to  each  of  the  joints. 

The  different  joints  exhibit  a  great  variety  of  movements  and  may  be  clas- 
sified as  follows :  gliding  joints,  hinge  joints,  condyloid  joints,  saddle  joints, 
ball-and-socket  joints,  pivot  joints.  For  a  description  of  the  structure  and 
the  peculiarities  of  these  joints  the  student  is  referred  to  works  on  anatomy.1 
The  contact  of  the  surfaces  of  the  joint  is  secured  in  part  by  the  fibrous  capsule, 
in  part  by  the  joint  ligaments,  and  in  part  by  the  tension  of  the  muscles.  The 
elastic  muscles  are  attached  under  slight  tension,  and,  moreover,  during  wak- 
ing hours  are  kept  slightly  contracted  by  touus  impulses  of  reflex  origin. 
Another  less  evident  but  no  less  important  condition  is  the  atmospheric  pres- 
sure. The  capsule  fits  the  joint  closely  and  all  the  space  within  not  occupied 
by  the  bones  is  filled  by  cartilages,  fibrous  bands,  fatty  tissues  and  synovial 
fluid.  The  joint  is  air-tight,  and,  as  was  first  demonstrated  by  the  Weber 
brothers,  the  atmospheric  pressure  keeps  all  parts  in  close  apposition.  This 
force  is  sufficiently  great  in  the  case  of  the  hip-joint  to  support  the  whole 
weight  of  the  leg  even  after  all  the  surrounding  soft  parts  have  been  cut 
through.  The  proof  that  the  air-pressure  gives  this  support  is  found  in  the 
fact  that  the  head  of  the  femur  maintains  its  place  in  the  acetabulum  after 
all  the  soft  parts  which  surround  the  joint  have  been  divided,  but  falls  out 
of  its  socket  if  a  hole  be  bored  in  the  acetabulum  and  air  be  permitted  to 
enter  the  cavity  of  the  joint.  Though  the  air-pressure  keeps  the  bones  in 
constant  contact  it  offers  no  resistance  to  the  movements  peculiar  to  the  joints. 

The  movements  of  the  bones  are  effected  chiefly  by  muscular  contractions, 
but  the  direction  and  extent  of  the  movements  are  for  the  most  part  deter- 
mined by  the  form  of  the  joint-surfaces  and  the  limitations  to  movement 

1  Quain's  Anatomy,  vol.  ii.  pt.  1 ;   Gray's  Anatomy  ;  Morris's  Anatomy. 


416  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

which  result  from  the  method  of  attachment  of  the  ligaments.  The  follow- 
ing kinds  of  movement  are  possible  :  (a)  angular,  in  which  the  angle  formed 
by  the  longitudinal  axis  of  two  bones  changes,  as  in  flexion  and  extension  or 
abduction  and  adduction  ;  (b)  circumduction,  in  which  the  longitudinal  axis 
of  a  bone  describes  the  sides  of  a  cone,  the  apex  of  which  is  in  the  joint ; 
(c)  rotation,  in  which  a  bone  moves  about  its  longitudinal  axis ;  (d)  gliding, 
in  which  a  bone  so  moves  as  to  change  its  position  with  reference  to  its 
neighbor,  without  rotation  or  change  of  angle.  As  a  matter  of  fact,  most  of 
the  movements  that  are  made  are  the  resultant  of  two  or  of  all  of  these  simple 
motions.  In  the  gliding  joints,  in  which  the  articular  surfaces  are  nearly 
flat  (as  in  the  case  of  the  joints  between  the  articular  processes  of  the  verte- 
brae, and  the  carpal  and  tarsal  joints),  a  sliding  movement  may  occur  in  various 
directions,  and  a  rotation  movement  is  possible ;  but  the  extent  of  these 
movements  is  very  slight,  being  limited  by  the  strong  capsule  and  ligaments. 
Hinge  joints  have  but  a  single  axis  of  motion,  because  the  convex  and  some- 
what cylindrical  surface  of  one  bone  fits  quite  closely  the  concave  surface  of 
the  other,  and  because  of  tense  lateral  ligaments  which  permit  of  movements 
in  only  a  single  plane.  The  joint  between  the  humerus  and  the  ulna  at  the 
elbow  is  an  example.  In  this  case  only  flexion  and  extension  are  possible, 
although  a  slight  obliquity  of  the  surfaces  causes  the  head  to  move  in  flexion 
toward  the  middle  line  of  the  body,  which  is  interpreted  by  some  as  a  screw 
movement.  In  this  joint  the  limits  of  motion  are  determined  by  the  contact 
of  the  coronoid  and  olecranon  processes  of  the  ulna  with  the  bone  in  the  cor- 
responding fossae  of  the  humerus,  as  well  as  by  the  resistance  of  capsule  and 
ligaments.  The  knee-joint l  is  a  less  simple  form  of  hinge  joint.  The  pres- 
ence of  the  semilunar  cartilages  and  the  shape  of  the  joint-surfaces  cause 
flexion  to  be  produced  by  the  combined  action  of  sliding,  rolling,  and  rotation 
movements.  In  complete  extension  the  lateral  ligaments  and  the  posterior 
and  anterior  crucial  ligaments  are  put  on  the  stretch,  and  there  is  a  locking 
of  the  joint,  no  rotation  being  possible ;  in  complete  flexion,  on  the  other 
hand,  the  posterior  crucial  ligament  is  tight,  but  the  others  are  sufficiently 
loose  to  allow  of  a  considerable  amount  of  pronation  and  supination.  In  the 
condyloid  joint  the  articulating  surfaces  are  spheroidal,  as  in  the  case  of  the 
metacarpo-  and  metatarso-phalangeal  joints.  These  exhibit  all  forms  of 
angular  movement  and  circumduction.  In  the  saddle-joint  there  is  a  double 
axis  of  motion — e.  g.,  the  articulation  of  the  trapezium  with  the  first  meta- 
carpal  bone  of  the  thumb  permits  of  movement  about  an  axis  extending  from 
before  backward,  and  another,  at  nearly  right  angles  to  this,  extending  from 
side  to  side.  All  modes  of  angular  movement  are  possible  with  such  a  joint. 
The  ball-and-socket  joint,  of  which  the  shoulder-  and  hip-joints  are  exam- 
ples, permits  of  the  greatest  variety  of  movements,  any  diameter  of  the  head 

1 W.  Braunne  and  Fischer  have  studied  with  mathematical  accuracy  the  construction  and 
movements  of  many  of  the  joints  of  the  human  body.  Their  articles  are  published  in  the 
Abhandlungen  der  math.-phys.  Classe  der  konigl.  Sdchsischer  Gesellschaft  der  Wissenschaften,  Bd. 
xvii.,  and  others. 


THE  ACTION   OF  LOCOMOTOR   MECHANISMS.  417 

of  the  bone  serving  as  an  axis  of  rotation.  The  /th-of-jo/nf  allows  of  rotation 
only;  the  atlanto-uxial  and  nulio-ulnar  joints  inav  he  placed  in  this  class. 

Method  of  Action  of  Muscles  upon  the  Bones. — The  bones  can  be 
looked  upon  as  levers  actuated  by  the  forces  which  arc  applied  at  the  points 
of  attachment  of  the  muscles.  All  three  forms  of  levers  are  represented  in 
the  body ;  indeed,  they  may  be  illustrated  in  the  same  joint,  as  the  elbow. 

An  example  of  a  lever  of  the  first  class,  in  which  the  fulcrum  is  between 
the  power  and  the  resistance,  is  to  be  found  in  the  extension  of  the  forearm  in 
such  an  act  as  driving  a  nail :  the  inertia  of  the  hammer,  hand,  and  forearm 
offers  the  resistance,  the  triceps  muscle  acting  upon  the  olecranon  gives  the 
power,  and  the  trochlea,  upon  which  the  rotation  occurs,  is  the  fulcrum.  The 
balancing  of  the  head  upon  the  atlas  is  another  example :  the  front  part  of  the 
head  and  face  is  the  resistance,  the  occipito-atlantoid  joint  the  fulcrum,  and 
the  muscles  of  the  neck  the  power. 

In  the  case  of  a  lever  of  the  second  order,  the  resistance  is  between  the  ful- 
crum and  the  power;  for  example,  when  the  weight  of  the  body  is  being 
raised  from  the  floor  by  the  hands :  the  fulcrum  is  where  the  hand  rests  on  the 
floor,  the  weight  is  applied  at  the  elbow-joint,  and  the  power  is  the  pull  of  the 
triceps  on  the  olecrauon.  The  raising  of  the  body  on  the  toes  is  another  ex- 
ample :  the  fulcrum  is  at  the  place  where  the  toes  are  in  contact  with  the 
floor,  the  resistance  is  the  weight  of  the  body  transmitted  through  the  tibia  to 
the  astragalus,  and  the  power  is  applied  at  the  point  of  attachment  of  the 
tendo  Achillis  to  the  os  calcis.1 

The  raising  of  a  weight  in  the  hand  by  flexion  of  the  forearm  through 
contraction  of  the  biceps  gives  an  example  of  a  lever  of  the  third  order,  in 
which  the  power  is  applied  between  the  fulcrum  and  the  weight.  This  form 
of  lever,  because  of  the  great  length  of  the  resistance  arm,  as  compared  with 
the  power  arm,  is  favorable  to  extensive  and  rapid  movements,  and  is  the 
most  usual  form  of  lever  in  the  body. 

The  power  is  applied  to  best  advantage  when  it  is  exerted  at  right  angles 
to  the  direction  of  a  lever,  as  in  the  case  of  the  muscles  of  mastication  and  of 
the  calf  of  the  leg.  If  the  traction  be  exerted  obliquely,  the  effect  is  the  less 
the  more  acute  the  angle  between  the  tendon  of  the  muscle  and  the  bone ;  for 
example,  when  the  arm  is  extended  the  flexor  muscles  work  to  great  disad- 
vantage, for  a  large  part  of  the  force  is  expended  in  pulling  the  ulnar  and 
radius  against  the  humerus,  and  is  lost  for  movement,  but  as  the  elbow  is 
flexed  the  force  is  directed  more  and  more  nearly  at  right  angles  to  the  bones 
of  the  forearm,  and  there  is  a  gain  in  leverage,  which  is  of  course  again 
decreased  as  flexion  is  completed.  This  gain  in  leverage  which  accompanies 
the  shortening  of  the  muscles  is  the  more  important,  since  the  power  of  the 
muscle  is  greatest  when  the  muscle  has  its  normal  length,  and  continually 
lessens  as  the  muscle  shortens  in  contraction.  There  are  a  number  of  special 
arrangements  which  help  to  increase  the  leverage  of  the  muscles  by  lessening 
the  obliquity  of  attachment — viz.  the  enlarged  heads  of  the  bones,  and  in  some 

1  Certain  observers  would  class  this  movement  as  an  example  of  a  lever  of  the  first  class. 
(Ewald :  Pfliiger's  Archiv,  1896,  Bd.  Ixiv.  S.  53). 
VOL.  II.— 27 


418  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


special  processes  projecting  from  the  bones,  the  introduction  of  sesaraoid 
bones  into  the  tendons,  and  the  presence  of  pulley -like  mechanisms. 

The  contraction  of  a  muscle  causes  the  points  to  which  it  is  attached  to 
approach  one  another,  and  the  direction  of  the  movement  is  often  determined 
by  the  direction  in  which  the  force  of  the  contracting  muscle  is  applied  to  the 
bones.  In  the  case  of  certain  joints,  however,  the  form  of  the  joint-surfaces 
and  the  method  of  attachment  of  the  ligaments  limits  the  direction  of  move- 
ment to  special  lines;  and  when  this  is  not  the  case  the  movement  is  usually 
the  resultant  of  the  action  of  many  muscles  rather  than  the  effect  of  the  con- 
traction of  any  one  muscle.  This  question  has  been  made  the  subject  of 
careful  study  by  Fick.1 

In  the  case  of  many  muscles,  both  of  the  bones  to  which  they  are  attached 
are  movable,  and  the  result  of  contraction  depends  largely  on  which  of  the 
extremities  of  the  muscles  becomes  fixed  by  the  contraction  of  other  muscles. 
Though  most  muscles  have  direct  influence  over  only  one  joint,  there  are  certain 
muscles  which  include  two  joints  between  their  points  of  attachment,  and  pro- 
duce correspondingly  complex  effects.  The  accurate  adjustment  and  smooth 
graduation  of  most  co-ordinated  muscular  movements  is  due  to  the  fact  that  not 
only  the  muscles  directly  engaged  in  the  act,  but  the  antagonists  of  these  mus- 
cles take  part  in  the  movement.  It  would  appear  from  the  observations  of 
certain  writers 2  that  antagonistic  muscles  may  be  not  only  excited  to  contrac- 
tion, but  inhibited  to  relaxation,  and  that  the  tension  of  the  muscles  is  thereby 
accurately  adjusted  to  the  requirements  of  the  movement  to  be  performed. 
The  importance  of  the  elastic  tension  and  reflex  tonic  contractions  of  muscles 
to  ensure  quick  action,  to  protect  from  sudden  strains,  and  to  restore  the  parts 
to  the  normal  position  of  rest  has  been  referred  to  elsewhere. 

The  shape  of  the  muscle  has  an  important  relation  to  the  work  which  it 
has  to  perform.  A  muscle  consists  of  a  vast  number  of  fibres,  each  of  which 
can  be  regarded  as  a  chain  of  contractile  mechanisms.  The  longer  the  fibre, 
the  greater  the  number  of  these  mechanisms  in  series  and  the  greater  the  total 
shortening  effected  by  their  combined  action ;  consequently,  a  muscle  with 
long  fibres,  such  as  the  sartorius,  is  adapted  to  the  production  of  extensive 
movements.  In  order  that  a  muscle  shall  be  capable  of  making  powerful 
movements  it  is  necessary  that  many  fibres  shall  be  placed  side  by  side,  as  in 
the  case  of  the  gluteus :  "  Many  hands,  light  work." 

Standing-. — In  spite  of  the  ease  with  which  the  many  joints  of  the  body 
move,  the  erect  position  is  maintained  with  comparatively  little  muscular 
exertion.  It  is  an  act  of  balancing  in  which  the  centre  of  gravity  of  the 
body  is  kept  directly  over  the  base  of  support.  In  the  natural  erect  position 
of  the  body  the  centre  of  gravity  of  the  head  is  slightly  in  front  of  the  oc- 
cipito-atlantoid  articulation,  so  that  there  is  a  tendency  for  the  head  to  rock 
forward,  as  is  seen  from  the  nodding  of  the  head  of  one  falling  asleep.  The 
centre  of  gravity  of  the  head  and  trunk  together  is  such  that  the  line  of 
gravity  falls  slightly  behind  a  line  drawn  between  the  centres  of  the  hip- 

1  Hermann's  Handbuch  der  Physiologic,  1871,  Bd.  i.  pt.  2.  S.  241. 

2  Sherrington :  Proceedings  of  the  Royal  Society,  Feb.,  1893,  vol.  liii. 


THE  ACTION  OF  LOCOMOTOIl   MECHANISMS.  419 

joints,  which  would  incline  the  body  to  fall  backward.  The  line  of  gravity 
of  the  head,  trunk,  and  thighs  falls  slightly  behind  the  axis  of  the  knee- 
joints,  and  the  line  of  gravity  of  the  whole  body  slightly  in  front  of  a  line 
connecting  the  two  ankle-joints,  so  that  the  weight  of  the  body  would  tend  to 
flex  the  knee-  and  ankle-joints. 

We  cannot  here  consider  in  detail  the  mechanical  conditions  which  limit 
the  movements  possible  to  the  different  joints  in  the  erect  position  of  the  body. 
Although  these  conditions  help  to  support  the  body  in  the  upright  position, 
they  are  not  alone  sufficient  to  the  maintenance  of  this  posture,  as  is  shown  by 
the  fact  that  the  cadaver  cannot  be  balanced  upon  its  feet.  That  standing 
requires  the  action  of  the  muscles  is  further  proved  by  the  fatigue  which  is 
experienced  when  one  is  forced  to  stand  for  a  considerable  time.  The  body 
may  be  supported  in  the  standing  position  in  various  attitudes.  Thus,  the 
soldier  standing  at  "  attention  "  places  the  heels  together,  turns  the  toes  out, 
makes  the  legs  straight  and  parallel,  so  as  to  extend  the  knees  to  their  utmost, 
tilts  back  the  pelvis,  straightens  the  spine,  and  looks  directly  forward.  In 
this  position  many  of  the  muscles  are  relieved  from  action  by  the  locking 
of  the  hip-  and  knee-joints.  The  tilting  backward  of  the  pelvis  causes  the 
line  of  gravity  to  fall  slightly  behind  the  axis  of  rotation  of  the  hip-joint 
and  puts  the  strong  ilio-femoral  ligament  on  the  stretch,  which  balances 
the  tendency  of  the  weight  of  the  body  to  extend  the  hip.  The  line  of 
gravity  would  fall  slightly  behind  the  axis  of  rotation  of  the  knee,  and  tend 
to  cause  flexion ;  but  when  the  joint  is  extended,  the  thigh,  because  of  the 
horizontal  curvature  of  the  internal  condyle,  receives  a  slight  inward  rota- 
tion, and  the  knee  cannot  be  flexed  without  a  corresponding  outward  rota- 
tion. In  standing  with  the  feet  turned  out,  this  rotation  movement  is 
prevented  by  the  same  ilio-femoral  ligament  that  locks  the  hip-joint.  The 
ankle-joint  cannot  be  locked,  and  the  tendency  of  the  body  to  fall  forward  is 
resisted  by  the  strong  muscles  of  the  calf  of  the  leg.  The  erect  position  of 
the  spine  and  the  balancing  of  the  head  have  likewise  to  be  maintained  by 
the  action  of  muscles.  Although  this  position  gives  great  stability,  it  cannot 
be  long  maintained  with  comfort.  It  is  less  fatiguing  to  allow  the  joints  to 
be  a  little  more  flexed,  and  to  keep  the  balance  by  the  action  of  the  muscles, 
the  position  being  frequently  changed  so  as  to  bring  fresh  muscles  into  action. 
.Perhaps  the  most  restful  standing  position  is  found  in  letting  the  weight  of 
the  body  be  supported  on  one  leg,  the  pelvis  being  tilted  so  as  to  bring  the 
weight  of  the  body  over  the  femur,  and  the  other  being  used  as  a  prop  to  pre- 
serve the  balance.  Absolute  stability  in  standing  is  impossible  for  any  length 
of  time ;  the  body  is  continually  swaying,  and  a  pencil  resting  on  a  writing 
surface  placed  upon  the  head  is  found  to  write  a  very  complicated  curve. 
There  is  a  normal  sway  for  every  individual,  and  this  may  become  markedly 
exaggerated  under  pathological  conditions.  The  maintenance  of  equilibrium 
requires  that  afferent  impulses  shall  continually  pass  to  the  co-ordinating  cen- 
tres which  control  the  muscles  involved  in  this  act,  and  if  any  of  these  normal 
impulses  fail  the  sway  of  the  body  is  increased  ;  for  example,  it  is  more  diffi- 
cult to  stand  steadily  when  the  eyes  are  closed  than  when  they  are  open  ;  the 


420  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

absence  of  the  normal  sensory  impulses  from  the  skin  of  the  feet,  the  muscles, 
joints,  etc.,  also  makes  standing  more  difficult  and  tends  to  increase  the  sway. 
The  effect  of  the  normal  sway  of  the  body  is  to  shift  the  pressure  and  strain 
from  point  to  point  and  to  relieve  the  different  muscles  from  continuous  action. 

Locomotion.1 — The  movements  of  animals  were  first  studied  by  careful 
observation,  accompanied  by  more  or  less  accurate  direct  measurements,  and 
by  these  simple  methods  the  Weber  brothers 2  arrived  at  quite  accurate  con- 
clusions as  to  the  nature  of  the  processes,  walking,  running,  jumping,  etc. 
These  results  were  greatly  extended  by  Marey,3  who  employed  elaborate 
recording  methods,  and  exact  pictures  of  all  stages  of  these  processes  were 
later  obtained  through  the  remarkable  revelations  of  instantaneous  photog- 
raphy.4 

Walking. — During  the  act  of  walking,  at  the  same  time  that  the  body  is 
propelled  forward  it  is  continually  supported  by  the  feet,  one  or  the  other  of 
which  is  always  touching  the  ground.  Preparatory  to  beginning  the  move- 
ment the  weight  of  the  body  is  thrown  upon  one  leg,  while  the  other  leg  is 
placed  somewhat  behind  it,  the  knee  and  ankle  being  slightly  flexed.  At  the 
start  the  body  is  given  a  slight  forward  inclination,  then  the  back  leg  is  ex- 
tended and  impels  the  body  forward.  As  the  centre  of  gravity  progresses  so 
as  to  be  no  longer  over  the  supporting  leg,  it  would  fall  were  it  not  that  the 
back  leg  is  at  the  same  instant  swung  forward  to  sustain  it.  As  the  body 
moves  forward  and  its  weight  is  received  by  the  leg  which  has  just  been 
advanced,  the  leg  which  has  been  its  support  is  freed  from  the  weight 
and  becomes  inclined  behind  it.  This  leg  and  foot  are  next  extended,  the 
body  thereby  receiving  another  forward  impulse,  and  then  the  hip-,  knee-, 
and  ankle-joints  flexing  slightly,  the  leg  swings  forward  past  the  supporting 
leg  and  again  becomes  the  support  of  the  body.  The  forward  movement  of 
the  body  is  due  in  part  to  a  slight  inclination  which  tends  to  cause  it  to  fall 
forward,  and  in  part  to  a  push  given  it  by  each  leg  in  turn  as  it  leaves  the 
ground. 

The  amount  of  work  performed  by  the  legs  in  ordinary  walking  is  com- 
paratively slight,  since  the  swing  of  the  leg  is,  like  that  of  a  pendulum,  largely 
a  passive  act.  Speed  in  walking  is  attained  by  inclining  the  body  somewhat 
more,  by  which  it  is  better  able  to  oppose  the  resistance  offered  by  the  air,  and 
by  flexing  the  legs  somewhat  more,  which,  by  lessening  the  distance  between 
the  hip-joints  and  the  ground,  lengthens  the  step  at  the  same  time  that  it  per- 
mits the  propelling  limb  in  extending  to  push  the  body  forward  with  greater 
force.  The  more  rapid  movement  of  the  body  is  also  accompanied  by  a 
more  rapid  forward  swing  of  the  leg,  the  muscles  aiding  the  force  of  gravity. 

The  transfer  of  the  weight  of  the  body  from  one  leg  to  the  other  in  walk- 

1  Beannis :  Physiologic  humaine,  1888,  vol.  ii.  p.  269,  gives  many  references  to  the  litera- 
ture of  this  subject. 

2  W.  and  E.  Weber :  Mechanik  der  menschlichen  Gehewerkzeuge,  1836. 

3  La  Methode  graphique,  1885. 

4  Marey  :  Methode  graphique  (supplement),  1885 ;  Muybridge :  The  Horse  in  Motion,  as  Shown 
by  Instantaneous  Photography,  1882. 


VOICE  AND   SPEECH.  421 

ing  causes  an  up-and-down  and  a  lateral  sway  with  cadi  step.  Were  the  legs 
without  joints,  like  stilts,  these  oscillations  would  be  very  great,  especially 
when  the  step  was  long;  as  a  matter  of  fact,  they  arc  slight.  The  tendency 
for  the  centre  of  gravity  to  move  from  side  to  side  as  the  le-s  alternately 
push  the  body  forward  is  partly  balanced  by  the  swing  of  the  opposite  arm  ; 
and  the  vertical  oscillations  are  minimized  by  the  fact  that  the  leg  which  is 
about  to  receive  the  weight  flexes  as  the  centre  of  gravity  moves  forward  and 
comes  over  it,  and  extends  as  it  passes  on  to  be  received  by  the  other  leg. 
The  path  taken  by  the  centre  of  gravity  during  walking  is  a  complicated  one. 
If  referred  to  the  plane  in  which  the  body  is  moving,  it  describes  for  one 
double  step  an  oval ;  projected  on  the  horizontal  and  frontal  planes,  its  path 
has  the  form  of  the  sign  of  infinity,  oc .  The  rate  of  movement  influences  its 
position  in  special  parts  of  the  curve.1 

In  running,  the  body  is  inclined  more  than  in  walking,  and  the  legs  are 
more  flexed  in  order  that  the  extension  movement  of  the  back  leg,  which 
drives  the  body  forward,  may  be  more  effective.  In  running,  the  body  is  pro- 
pelled by  a  series  of  spring-like  movements  and  there  are  times  when  both 
feet  are  off  the  ground,  the  back  leg  leaving  the  ground  before  the  other 
touches  it. 

B.  VOICE  AND  SPEECH. 
1.  STRUCTURE  OF  THE  LARYNX. 

Voice-production. — The  human  vojee  is  produced  by  vibration  of  the 
true  vocal  cords*,  normally  brought  about  by  an  expiratory  blast  of  air  passing 
between  them  while  they  are  approximated  and  held  in  a  state  of  tension  by 
muscular  action.  Mere  vibration  of  the  cords  could  produce  but  a  feeble 
sound ;  the  voice  owes  its  intensity  both  to  the  energy  of  the  expiratory  blast 
(Helmholtz)2  and  to  the  reinforcement  of  the  vibrations  by  the  resonating 
cavities  above  and  below  the  cords. 

A  true  conception  of  the  action  of  the  larynx  can  only  be  gained  by  a  pre- 
liminary study  of  the  organ  in  situ,  in  its  relations  with  the  trachea,  pharynx, 
tongue,  extrinsic  muscles,  and  hyoidean  apparatus.  Removed  from  its  con- 
nections, the  larynx,  in  vertical  transverse  section,  is  seen  to  be  shaped  some- 
what like  an  hour-glass,  the  true  vocal  cords  forming  the  line  of  constriction 
half  way  between  the  top  of  the  epiglottis  and  the  lower  border  of  the  cri- 
coid  cartilage  (Fig.  208).  In  median  vertical  section  the  axis  of  the  larynx 
above  the  vocal  cords  extends  decidedly  backward,  and  below  the  cords  the 
axis  is  nearly  perpendicular  to  the  plane  in  which  they  lie.  The  epiglottis  is  an 
ovoid  lamella  of  elastic  cartilage,  shaped  like  a  shoe-horn,  that  leans  backward 
over  the  laryngeal  orifice  so  that  the  observer  must  look  down  obliquely  in 
order  to  inspect  the  cavity  of  the  larynx  (Fig.  212.)  The  mucous  membrane 
is  thickened  into  a  slight  prominence,  known  as  the  "  cushion,"  at  the  base  of 

1  Fischer:  Abhandl.  d.  math.-physik.  CL  d.  Sachs.  Gesellsch.  d.  Wissensch.,  xxv.  Nr.  i. 

2  Quoted  by  Grutzner:  Hermann's  Handb.  der  Physiologic,  1879,  J3d.  11,  Th.  2,  S.  14. 


422 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


the  epiglottis.  The  epiglottis,  which  is  extremely  movable  in  a  median  plane, 
may  be  tilted  backward  so  as  to  close  completely  the  entrance  into  the  larynx. 

Functions  of  the  Epiglottis. — One 
function  of  the  epiglottis  seems  obviously 
to  serve  as  a  cover  for  the  superior  entrance 
of  the  larynx,  over  which  it  is  said  to  shut 
in  the  act  of  swallowing.  But  it  is  found 
that  deglutition  occurs  in  a  normal  manner 
when  the  epiglottis  is  wanting  or  is  too  small 
to  cover  the  aperture,  the  sphincter  muscles 
surrounding  the  latter  being  capable  of  pro- 
tecting the  larynx  against  the  entrance  of 
foreign  substances.  It  is  held  by  some 
that  the  epiglottis  has  an  important  influ- 
ence in  modifying  the  voice  according  as  it 
more  or  less  completely  covers  the  exit  to 
the  column  of  vibrating  air.  It  is  also  held 
that  the  epiglottis  acts  as  a  sort  of  sounding- 
board,  taking  up  and  reinforcing  the  vibra- 
tions of  the  air-column  impinging  against  it.1 
Sweeping  downward  and  backward  from 
FIG.  208. -vertical  transverse  section  of  each  edge  of  the  epiglottis  is  a  sheet  of 

the  larvnx  (after  Testut) :  1,  posterior  face  of  -,  . ,  .    7   , . .      /.  7  7 

epiglottis,  with  i',  its  cushion;  2,  aryteno-  mucous  membrane,  the  ary-epiglottic  fold, 
epigiottic  fold;  3,  ventricular  band,  or  false  which  forms  the  lateral  rim  of  the  superior 

vocal  cord;  4,  true  vocal  cord;  5,  central  .  . 

fossa  of  Merkei ;  6,  ventricle  of  larynx,  with  aperture  oi  me  larynx  and  which  ends  in, 
6',  its  ascending  pouch;  7,  anterior  portion  and  covers  posteriorly,  the  arytenoid  carti- 

of  cricoid ;  8,  section  of  cricoid ;  9,  thyroid,  J  J  .        J 

cut  surface;  10,  thyro-hyoid  membrane;  ii,  lages.  The  rounded  prominence  on  the  pos- 
Sefls,  SSS^raFES  *»*»•  corner  of  this  fold  is  made  by  the  car- 
13',  its  inner  division,  contained  in  the  vocal  tilage  of  Santorini,  and  a  second,  less  marked, 

cord ;  14,  crico-thyroid  muscle  ;  15,  subglottic  n«  ,  •        i    j.      *j_    r      ji  ±>i  r 

portion  of  larynx;  1$,  cavity  of  the  trachea,   swelling  external  to  it,  by  the  cartilage  of 

Wrisberg  (Fig.  215).     Looking  down  into 

the  larynx,  it  is  seen  that  its  lateral  walls  approach  each  other  by  the  develop- 
ment on  each  side  of  a  permanent  ridge  of  mucous  membrane,  known  as  the 
ventricular  band  or  false  vocal  cord  (Fig.  208). 

Ventricular  Bands  and  Ventricles  of  Morgagni. — The  ventricular  bands 
or  false  vocal  cords  arise  from  the  thyroid  cartilage  near  the  median  line,  a 
short  distance  above  the  origin  of  the  true  cords.  They  are  inserted  into  the 
arytenoid  cartilages  somewhat  below  the  apices  of  the  latter.  Their  free  bor- 
der is  more  or  less  ligamentous  in  structure.  They  are  brought  into  contact 
by  the  sphincter  muscles  of  the  larynx,  and  thus  protect  the  glottis.  It  has 
even  been  stated  that,  in  paralysis  of  the  true  cords,  they  may  be  set  in  vibra- 
tion and  be  the  seat  of  voice-formation.  So-called  "  oedema  of  the  glottis  "  is 
chiefly  due  to  accumulation  of  fluid  in  the  wide  lymph-spaces  found  in  the 
false  cords. 

1  Mills:  Journ.  of  Physiology,  1883,  vol.  iv.  p.  135. 


VOICE   AND   SPEECH. 

The  ventricular  bands  are  parallel  with  and  just  above  the  true  vocal  cords, 
from  which  they  are  separated  by  a  narrow  slit.  They  do  not,  however,  reach 
so  near  the  middle  line  as  the  true  cords,  which  can  be  seen  between  and  below 
the  bands.  The  ventricular  bands  project  more  or  less  into  the  cavity  of  the 
larynx  like  overhanging  lips,  so  that  each  band  forms  the  inner  wall  of  a 
space  closed  by  the  true  vocal  cords  below,  and  communicating  with  the  cavity 
of  the  larynx  through  the  narrow  slit  above  mentioned.  The  spaces  thus 
bounded  internally  by  the  false  cords  are  known  as 

The  Ventricles  of  Morgagni  (Fig.  208). — No  complete  explanation  has  been 
offered  as  to  the  purposes  served  by  the  ventricles  of  Morgagni  and  the  false 
vocal  cords.  Numerous  mucous  and  serous  glands  seated  in  the  ventricular 
bands  pour  their  secretions  into  the  ventricles,  whence  the  fluid  may  be  trans- 
mitted by  the  overhanging  lips  of  the  ventricular  bands  to  the  true  vocal 
cords ;  hence,  an  important  function  of  the  former  structure,  probably,  is  to 
supply  to  the  vocal  cords  the  moisture  necessary  to  their  normal  action.  The 
secretion  contained  within  the  ventricle  is  protected  by  the  ventricular  band 
from  the  desiccating  influence  of  the  passing  air-currents.  The  existence  of 
the  ventricular  spaces  also  permits  free  upward  vibration  of  the  true  cords. 
The  ventricles  of  Morgagni  in  some  of  the  lower  animals,  as  the  higher  apes, 
communicate  with  extensiye  cavities  which  serve  an  obvious  purpose  as  reso- 
nating chambers  for  the  voice,  and  perhaps  the  preservation  of  this  function  in 
the  ventricles  themselves  is  still  of  importance  in  the  human  being.  It  is  not 
improbable  that  the  ventricular  bands  find  their  most  important  function  as 
sphincters  of  the  larynx,  the  superior  opening  of  which  may  be  firmly  occluded 
by  their  approximation.  The  well-known  fact  that  during  strong  muscular 
effort  the  breath  is  held  from  escaping  is,  according  to  Brunton  and  Cash,' 
due  to  the  meeting  of  the  false  cords  in  the  middle  line.  The  overhanging 
shape  of  the  cords  allows  them  to  be  readily  separated  by  an  inspiratory  blast, 
but  causes  them  to  be  more  firmly  approximated  by  an  expiratory  effort.  This 
mechanism  recalls  the  mode  of  action  of  the  semilunar  valves  of  the  heart. 

The  true  vocal  cords  arise  from  the  angle  formed  by  the  side;/ of  the  thyroid 
cartilage  where  they  meet  in  front,  a  little  below  its  middle  point,  and,  passing 
backward,  are  inserted  into  the  vocal  processes  of  the  arytenoid  cartilages. 
The  aperture  between  the  vocal  cords  and  between  the  vocal  processes  of  the 
arytenoids  is  known  as  the  glottis  or  rima  glottidis  (Figs.  214,  215).  Since,  as 
will  be  seen  later,  the  vocal  cords  may  be  brought  together  while  the  vocal  pro- 
cesses of  the  arytenoids  are  widely  separated  at  their  bases,  the  space  between 
the  cords  themselves  is  sometimes  called  the  rima  vocalis  and  that  between  the 
vocal  processes  the  rima  respiratoria. 

In  the  adult  male  the  vocal  cords  measure  about  15  millimeters  in  length 
and  the  vocal  processes  measure  8  millimeters  in  addition.  In  the  female  the 
cords  are  from  10  to  11  millimeters  in  length.  The  free  edges  of  the  cord  are 
thin  and  straight  and  are  directed  upward ;  their  median  surfaces  are  flattened. 
Each  cord  is  composed  of  a  dense  bundle  of  fibres  of  yellow  elastic  tissue, 
1  Brunton  and  Cash :  Journ.  Anat.  and  Phys.,  1883,  vol.  xvii. 


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AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


which  fibres,  though  having  a  general  longitudinal  course,  are  interwoven,  and 
send  off  shoots  laterally  into  the  subjacent  tissue.  The  compact  ligament, 
known  commonly  as  the  "  vocal  cord,"  forms  only  the  free  edge  of  a  reflexion 
from  the  side  wall  of  the  larynx.  This  reflexion  is  wedge-shaped  in  a  vertical, 
transverse  section  and  contains  much  elastic  tissue  and  the  internal  and  part 
of  the  external  thyro-arytenoid  muscle  (Fig.  208).  This  whole  structure 

properly  forms  the  vocal  cord,  and  by 
contraction  of  its  contained  muscle  its 
thickness  and  vibrating  qualities  may 
be  greatly  modified. 

Like  the  trachea,  the  larynx,  with  the 
exception  of  the  vocal  cords,  is  lined 


FIG.  209.— Cartilages  of  the  larynx,  separated 
(Stoerk) :  1,  epiglottis ;  2,  petiolus ;  3,  median 
notch  of  thyroid ;  4,  superior  cornu  of  thyroid ; 
5,  attachment  of  stylo-pharyngeus  muscle;  6, 
origin  of  thyro-epiglottic  ligament ;  7,  origin  of 
the  thyro-arytenoid  muscle ;  8,  origin  of  true 
vocal  cord  ;  9,  inferior  cornu  of  thyroid ;  10,  car- 
tilage of  Wrisberg ;  11,  cartilage  of  Santorini ;  12, 
12',  arytenoid  cartilages,  showing  attachments  of 
the  transverse  arytenoid  muscle;  13,  13',  pro- 
cessus  muscularis,  showing  attachments  of  the 
posterior  and  lateral  crico-arytenoid  muscles ; 
14,  base  of  the  arytenoid  cartilage ;  15,  vocal  pro- 
cesses of  the  arytenoids ;  16,  articular  surface  for 
the  base  of  the  arytenoid  cartilage ;  17,  posterior 
view  of  cricoid  cartilage,  with  outline  of  attach- 
ment of  the  posterior  crico-arytenoid  muscle ; 
18,  articular  surface  for  inferior  cornu  of  thyroid 
cartilage. 


FIG.  210.— Cartilages  and  ligaments  of  the 
larynx,  posterior  view  (after  Stoerk) :  1,  epiglot- 
tis; 2,  cushion  of  the  epiglottis;  3,  cartilage  of 
Wrisberg ;  4,  ary-epiglottic  ligament ;  5, 8,  mucous 
membrane  ;  6,  cartilage  of  Santorini ;  7,  arytenoid 
cartilage ;  9,  its  processus  muscularis ;  10,  crico- 
arytenoid  ligament;  11,  cricoid  cartilage;  12,  in- 
ferior cornu  of  thyroid  cartilage ;  13,  posterior 
superior  cerato-cricoid  ligament;  13',  posterior 
inferior  cerato-cricoid  ligament;  14,  cartilages 
of  the  trachea;  15,  membranous  portion  of 
trachea. 


by  columnar,  ciliated  epithelium,  the  direction  of  whose  movement  is  upward 
toward  the  pharynx.  The  vocal  cords  are  covered  by  thin,  flat,  stratified  epi- 
thelium. The  inner  surface  of  the  epiglottis,  the  walls  of  the  ventricles,  and 
the  ventricular  bands  contain  much  adenoid  tissue,  the  spaces  of  which  are  apt 
to  become  distended  with  fluid,  giving  rise  to  oedema  of  those  parts.  The 
whole  mucous  membrane  of  the  larynx,  except  over  the  vocal  cords,  is  richly 
supplied  with  glands  both  mucous  and  serous  in  character. 


VOICE  AND   SPEECH.  .425 

Cartilages  of  the  Larynx. — The  mechanism  of  the  larynx  is  supported 
by  a  skeleton  composed  of  several  pieces  of  cartilage.  The  lowermost  of  these 
cartilages  is  the  cricoid  cartilage,  so  called  from  its  resemblance  to  a  signet  ring 
(Fig.  209).  The  cricoid  cartilage  is  situated  above  the  topmost  ring  of  the 
trachea  to  which  it  is  attached  by  a  membrane.  The  vertical  measurement  of 
the  cricoid  cartilage  is  about  one  inch  on  its  posterior,  and  one-quarter  inch  on 
its  anterior  surface.  Superior  to,  and  partly  overlapping  the  cricoid,  is  the 
thyroid  cartilage,  which  forms  an  incomplete  ring,  being  deficient  posteriorly 
(Fig.  209).  The  free  corners  of  the  thyroid  behind  are  prolonged  upward  or 
downward  into  projections  known  as  the  cornua.  The  upper  pair  are  attached 
to  the  extremities  of  the  greater  cornua  of  the  hyoid  bone,  while  by  the  inner 
surface  of  the  ends  of  the  lower  cornua  the  thyroid  is  articulated  with  the 
cricoid  cartilage  and  rotates  upon  it  around  an  axis  drawn  through  the  points 
of  articulation.  The  lower  anterior  border  of  the  thyroid  cartilage  is  evenly 
concave,  but  its  upper  border  has  a  deep  narrow  notch  in  the  middle  line. 
The  upper  half  of  the  thyroid  in  front  projects  sharply  forward  in  an  elevation 
known  as  Adam's  apple  (pomum  Adami\  which  is  much  more  marked  in  adult 
males  than  in  females.  The  elliptical  space  between  the  cricoid  and  thyroid 
cartilages  in  front  is  covered  by  a  membrane.  Adam's  apple,  the  anterior  part 
of  the  cricoid  ring,  and  the  space  between  the  two,  can  easily  be  felt  in  the  liv- 
ing subject ;  they  rise  perceptibly  toward  the  head  with  each  swallowing  movement. 

The  arytenoid  cartilages  are  two  in  number  and  are  similar  in  shape  (Figs. 
209,  210).  Each  cartilage,  which  has  somewhat  the  form  of  a  triangular 
pyramid,  is  seated  on,  and  articulates  with,  the  highest  point  on  the  posterior 
part  of  the  cricoid  cartilage  some  distance  from  the  middle  line.  Of  the  free 
faces  of  the  pyramid,  one  looks  backward,  one  toward  the  middle  line,  and  the 
third  outward  and  forward.  Each  face  is  more  or  less  concave.  The  apex  of 
each  arytenoid  cartilage  is  capped  by  a  small  body  called  the  cartilage  of  San- 
torini  or,  from  its  bent  shape,  corniculum  laryngis  (Figs.  209,  210).  Outside 
and  in  front  of  the  latter  is  the  minute  cuneiform  cartilage  or  cartilage  of 
Wtisberg,  enclosed  in  the  ary-epiglottic  fold.  The  lateral  posterior  corner  of 
the  arytenoid  cartilage  forms  a  blunt  projection  which  serves  for  the  attach- 
ment of  muscles,  the  processus  muscularis.  The  anterior,  lower,  and  median 
part  of  each  cartilage  is  of  especial  interest,  since  it  serves  for  the  posterior 
attachment  of  the  vocal  cord ;  it  is  known  as  the  processus  vocalis. 

The  thyroid  and  cricoid  cartilages  and  the  body  of  the  aiytenoids  are  of 
hyaline  cartilage,  and  tend  to  become  ossified  in  middle  life.  The  other  carti- 
lages and  the  vocal  processes  of  the  arytenoids  are  composed  of  the  elastic 
variety. 

The  Muscles  of  the  Larynx  may  be  divided  into  two  classes — the  extrinsic 
and  the  intrinsic  ;  the  former  find  their  origin  outside  the  larynx,  and  the  latter 
both  arise  and  are  inserted  within  it. 

Extrinsic  Muscles. — To  this  group  belong  the  sterno-hyoid,  the  stemo-thy- 
roid,  and  the  omo-hyoid  muscles,  which  depress  the  larynx  or  hyoid  bone;  the 
thyro-hyoid  muscle,  which  depresses  the  hyoid  bone  or  elevates  the  thyroid 


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AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


cartilage.  To  the  elevators  of  the  larynx  belong  the  genio-hyoid,  the  mylo- 
hyoid,  the  digastric,  the  stylo-hyoid,  and  the  hyo-glossus.  The  muscles  of  the 
palate  and  the  constrictors  of  the  pharynx  enter  into  coordinated  action  with  the 
above.  When  food  is  passing  through  the  pharyx  in  the  act  of  swallowing, 
the  hyoid  bone  is  drawn  upward  and  forward,  raising  the  larynx  with  it ;  the 
tongue  is  thrown  backward  so  that  the  epiglottis  covers  the  entrance  into  the 
lairynx,  and  the  constrictors  of  the  larynx  contract,  completely  closing  the 
entrance  into  that  organ. 

The  intrinsic  muscles  of  the  larynx  are  the  crico-thyroids,  the  lateral  crico- 
arytenoids,  the  posterior  crico-arytenoids,  the  arytenoid,  the  aryteno-epiglot- 
tideans,  and  the  thyro-arytenoids ;  all  being  in  pairs  except  the  arytenoid, 
which  crosses  the  middle  line.  The  crico-thyroid  muscle  arises  from  the  front 
and  side  of  the  cricoid  cartilage  and,  passing  upward  and  backward,  is  inserted 
into  the  lower  edge  of  the  thyroid  cartilage  (Fig.  211).  The  action  of  the  crico- 
thyroid  muscle  is  to  diminish  the  distance  between  the  thyroid  and  cricoid  car- 
tilages in  front,  either  by  depressing  the  front  of  the  thyroid  or  by  elevating 
that  of  the  cricoid  cartilage,  or  both.  In  the  first  case  the  distance  between 
the  anterior  attachment  of  the  vocal  cords  and  the  vocal  processes  of  the 

arytenoid  cartilages  is  increased  by  movement  of 
the  thyroid,  and  in  the  second  case  the  same  effect 
is  produced  by  backward  rotation  of  the  edge  of 
the  cricoid  upon  which  the  arytenoid  cartilages  are 
seated  (Fig.  210).  The  muscle,  therefore,  is  a 
tensor  of  the  vocal  cords.  It  is,  probably,  the 
mechanism  we  ordinarily  use  in  raising  the  pitch 
of  the  voice  when  the  vocal  machinery  has  been 
"  set "  by  the  other  muscles  (see  below).  If  the 
fingers  be  placed  on  the  cricoid  ring  and  on  the 
pomum  Adami  while  the  ascending  scale  is  sung 
in  the  middle  chest  register,  both  descent  of  the 
front  of  the  thyroid  and  ascent  of  the  cricoid  can 
be  made  out.  The  lateral  crico-arytenoid  muscle 
arises  from  the  upper,  lateral  border  of  the  cricoid 

FIG.  211.— Lateral  view  of  the  ,  i          i    i       i  1^1 

cartilages  of  larynx  with  the  crico-  cartilage,  and  passes  upward  and  backward  to  be 
thyroid  muscle  (Quain's  Anatomy,  inserted  into  the  outer  edge  of  the  arytenoid  car- 
after  Willis) :  1,  crico-thyroid  mus-  .  /T  i  i  i 

cie;  2,  crico-thyroid  membrane;  3,    tilage,  on  and  in  front  oi  the  lateral  prominence 

(Fig.  212).  Its  main  action  is  to  wheel  the 
vocal  process  of  the  arytenoid  toward  the  middle 
line  and  thus  approximate  the  vocal  cords.  The  posterior  crico-arytenoid  is  a 
large  muscle,  which  rises  from  the  median  posterior  surface  of  the  cricoid  car- 
tilage and  passes  upward  and  outward  to  be  inserted  into  the  outer  surface  of  the 
arytenoid  cartilage,  behind  and  above  the  insertion  of  the  lateral  crico-arytenoid 
(Fig.  213).  Its  action  is  to  turn  the  vocal  processes  outward  and  thus  abduct  the 
vocal  cords.  The  posterior  crico-arytenoid  occupies  an  important  position  in  the 
group  of  respiratory  muscles;  during  vigorous  inspiration  it  is  brought  into  action 


cricoid  cartilage ;  4,  thyroid  carti- 
lage ;  5,  upper  rings  of  the  trachea. 


VOICE   AND    SPEECH. 


and  widens  the  glottis.  Paralysis  of  this  muscle  is  a  most  serious  condition,  since 
it  is  followed  by  approximation  of,  and  inability  to  separate,  the  vocal  cords. 
The  arytenoid,  or  transverse  or  posterior  aryteno'xl  muscle,  the  single  unpaired 


-11 


17  - 


-13 


FIG.  213. — Larynx  with  its  muscles,  posterior 
view  (Stoerk) :  1,  epiglottis ;  2,  cushion ;  3,  ary- 
epiglottic  ligament;  4,  cartilage  of  Wrisberg; 
5,  cartilage  of  Santorini ;  G,  oblique  arytenoid 
muscles;  7,  transverse  arytenoid  muscle;  8, 
posterior  crico-arytenoid  muscle ;  9,  inferior 
cornu  of  thyroid  cartilage;  10,  cricoid  car- 
tilage ;  11,  posterior  inferior  cerato-cricoid  lig- 
ament; 12,  cartilaginous  portion;  13,  mem- 
branous portion  of  trachea. 


FIG.  212.— Larynx  and  its  lateral  muscles  after 
removal  of  the  left  plate  of  the  thyroid  cartilage 
(Stoerk) :  1,  thyroid  cartilage ;  2,  thyro-epiglottic  mus- 
cle ;  3,  cartilage  of  Wrisberg ;  4,  ary-epiglottic  mus- 
cle ;  5,  cartilage  of  Santorini ;  6,  oblique  arytenoid 
muscles;  7,  thyro-arytenoid  muscle;  8,  transverse 
arytenoid  muscle ;  9,  processus  muscularis  of  aryte- 
noid cartilage ;  10,  lateral  crico-arytenoid  muscle ;  11, 
posterior  crico-arytenoid  muscle ;  12,  crico-thyroid 
membrane ;  13,  cricoid  cartilage  ;  14,  attachment  of 
crico-thyroid  muscle ;  15,  articular  surface  for  the 
inferior  cornu  of  the  thyroid  cartilage;  16,  crico- 
tracheal  ligament;  17,  cartilages  of  trachea;  18, 
membranous  part  of  trachea. 

muscle  of  the  larynx,  is  a  considerable  band  passing  across  the  middle  line  from 
the  posterior  surface  of  one  arytenoid  cartilage  to  that  of  the  other  (Fig.  213). 
Its  action  is  to  draw  the  arytenoid  cartilages  together  in  the  middle  line  and 
approximate  the  vocal  processes ;  its  action  is  essential  in  closing  the  glottis.  In 
the  resting  larynx  the  arytenoid  cartilages  are  kept  apart  by  the  elastic  tension 
of  the  parts.  The  aryfeno-epiglottidean,  sometimes  called  the  oblique  arytenoid, 
muscles  consist  of  two  bundles  of  fibres  seated  upon  the  surface  of  the  arytenoid 
muscle  (Fig.  213).  Each  muscle  arises  from  the  outer  posterior  angle  of  the 
arytenoid  cartilage,  and,  passing  upward  and  inward,  crosses  in  the  rnicMle  line 
partly  to  be  inserted  into  the  outer  and  upper  part  of  the  opposite  cartilage, 
partly  to  penetrate  the  ary-epiglottic  fold  as  far  as  the  epiglottis,  and  the 
remainder  to  join  some  fibres  of  the  thyro-arytenoid  muscle.  The  action  of 
the  aryteno-epiglottidean  muscles  is  to  close  the  glottis.  The  thyro-arytenoid 
is  a  muscle  of  complex  mechanism,  usually  described  as  formed  of  two  parts, 
an  external  and  an  internal.  The  external  thyro-arytenoid  arises  from  the  lower 


428  AN  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 

part  of  the  angle  of  the  thyroid  cartilage ;  its  fibres  pass,  for  the  most  part, 
backward  and  somewhat  upward  and  outward  to  be  inserted  into  the  outer 
edge  of  the  arytenoid  cartilage  and  its  lateral  processus  muscularis  (Figs.  208, 
214).  Some  of  its  bundles  of  fibres,  however,  have  different  directions,  and 
a  portion  of  them  pass  upward  into  the  ventricular  bands.  The  internal  thyro- 
arytenoid,  wedge-shaped  in  transverse  section,  lies  between  the  muscular  divis- 
ion just  described  and  the  vocal  ligament,  by  which  its  thin  median  edge  is 
covered.  The  internal  thyro-arytenoid  arises  from  the  anterior  angle  of  the 
thyroid  cartilage  and  is  inserted  into  the  processus  vocalis  and  the  outer  face  of 
the  arytenoid  cartilage.  Certain  fibre-bundles  of  this,  as  of  the  external 
division  of  the  muscle,  pass  in  various  directions,  some  of  them  being  inserted 
into  the  free  border  of  the  vocal  cord.  The  action  of  the  muscle  is,  on  the 
whole,  to  draw  the  arytenoids  forward  and  thus  relax  the  vocal  cords ;  but,  by 
its  contraction,  the  cords  may  also  be  approximated  and  their  thickness,  and 
probably  their  elasticity,  extensively  modified. 

Specific  Actions  of  the  Laryng-eal  Muscles. — To  sum  up  the  various 
effects  of  the  muscular  action  on  the  larynx :  A  sphincter  action  of  the  larynx 
is  brought  about  by  the  combined  contraction  of  all  the  muscles  with  the 
exception  of  the  crico-thyroids  and  the  posterior  crico-arytenoids ;  the  vocal 

cords  are  adducted  and  the  glottis  nar- 
rowed by  the  transverse  and  oblique  ary- 
tenoids, the  external  thyro-arytenoids, 
and   the   lateral    crico-arytenoids;    the 
i.thy.ar.e.  vocal  cords  are  abducted  and  the  glottis 
-\m.thy.ar.  widened  chiefly  or  wholly  by  the  poste- 
rior  crico-arytenoids ;   the   vocal  cords 
are  made  tense   by  contraction  of  the 
crico-thyroids ;  the  vocal  cords  are  slack- 
ened by  the   combined   action   of  the 
FIG.  214.— Diagram  to  illustrate  the  thyro-aryte-    sphincter  group  and  especially  by  the 

nold  muscles;  the  figure  represents  a  transverse  -.     ,  .  i 

section  of  the  larynx  through  the  bases  of  the    external  thyro-arytenoids. 


arytenoid  cartilages  (redrawn  from  Foster)  :  Ary,  J^   wiH    easily    be    Seen    that    in    the 

arytenoid  cartilage;  p.m,  processus  muscularis;  •       i         i     i                   i                u 

P.V,  processus  vocalis:  Th,  thyroid  cartilage;  c.v,  larynx,  as  in  the  skeleton  at  large,  the 

vocal  cords;  <E  is  placed  in  the  oesophagus;  efficiency  of  any  single  muscle  involves 

m.thy.ar.i,     internal     thyro-  arytenoid     muscle;  ' 

m.thy.ar.e,     external    thyro-arytenoid     muscle;  the   action    of  accessory   muscles  ;    thus, 


of  the  crico-thyroid  could 

verse  arytenoid  muscle.  have  little  effect  in  tightening  the  vocal 

cords  were  not  the  arytenoid  cartilages 

fixed  by  contraction  of  the  posterior  crico-arytenoid  and  arytenoid  muscles. 
Nerve-supply  of  the  Larynx.  —  The  larynx  receives  its  nerve-supply  from 
the  superior  and  the  inferior  or  recurrent  laryngeal  nerves.  The  extremely 
sensitive  surface  of  the  mucous  membrane  of  the  organ  above  the  vocal  cords 
is  supplied  by  sensory  filaments  of  the  superior  laryngeal  nerve.  The  superior 
laryngeal  also  supplies  motor  fibres  to  the  crico-thyroid  muscle,  whose  action 
as  a  tightener  of  the  vocal  cords  is  peculiar.  All  the  other  muscles  of  the 


VOICE   AND   SPEECH. 

larynx  receive  their  motor  impulses  from  the  inferior  larynirral  nerve.     Much 
of  the  nervous  mechanism  of  the  larynx  is  still  in  dispute. 

Laryngoscopic  Appearance  of  the  Larynx. — Much  may  be  learned  by 
inspection  of  the  larynx  during  life  by  means  of  the  laryngoscopic  mirror.  It 
is  not  difficult  for  an  observer  to  examine  his  own  larynx  by  placing  himself 
before  a  second  mirror  in  which  may  be  seen  the  image  reflected  from  the 
laryngoscope.  To  inspect  the  larynx  the  tongue  must  be  held  well  out  so 
as  to  pull  forward  the  epiglottis,  then  the  structures  below  appear  in  the 
laryngoscopic  mirror  in  reversed  position.  Beneath  the  middle  of  the  epiglottis 
the  cushion  may  be  seen  as  a  slight  swelling,  and  continuing  downward  and 
backward  from  the  edges  of  the  cartilage,  may  be  seen  the  ary-epiglottic  folds, 
each  marked  at  its  extremity  by  twro  rounded  nodules,  the  cartilages  of  Wris- 
bergand  Santorini  (Fig.  215).  In  quiet  breathing  the  glottis  is  nearly  stationary 
and  opened  to  the  extent  of  from  3  to  5  millimeters.  The  vocal  cords  bounding 
it  look  white  and  glistening  in  contrast  with  the  red  color  of  the  general  mucous 
membrane.  The  cartilages  of  Santorini  are  several  millimeters  apart,  and  a 
sheet  of  mucous  membrane  reaches  from  one  to  the  other.  The  ventricular 


15 


FIG.  215.— The  laryngoscopic  image  in  easy  breathing  (Stoerk) :  1,  base  of  the  tongue ;  2,  median 
glosso-epiglottic  ligament ;  3,  vallecula ;  4,  lateral  glosso-epiglottic  ligament ;  5,  epiglottis ;  6,  cushion  of 
epiglottis;  7,  cornu  major  of  hyoid  bone;  8,  ventricular  band,  or  false  vocal  cord;  9,  true  vocal  cord; 
opening  of  the  ventricle  of  Morgagni  seen  between  8  and  9 ;  10,  folds  of  mucous  membrane ;  11,  sinus 
pyriformis ;  12,  cartilage  of  Wrisberg ;  13,  aryteno-epiglottic  fold ;  14,  rima  glottidis ;  15,  arytenoid  carti- 
lage ;  16,  cartilage  of  Santorini ;  17,  posterior  wall  of  pharynx. 

bands  are  seen  as  red  shelves  reaching  to  the  outer  margin  of  the  shining 
cords  and  separated  from  the  latter  by  a  dark  line  which  is  the  entrance  into 
the  ventricles  of  Morgagni. 

When  a  deep  inspiration  is  taken  the  glottis  is  widely  opened,  even  to  the 
extent  of  half  an  inch ;  an  angle  is  formed  between  the  vocal  process  of  the 
arytenoid  and  the  vocal  cord,  the  space  between  the  cartilages  of  Santorini  is 
widened,  and  the  rings  of  the  trachea,  and  even  its  bifurcation  may  be  seen 
below.  With  the  succeeding  expiration  the  glottis  again  becomes  narrow. 
When  the  voice  is  sounded  the  picture  at  once  changes.  The  space  between 
the  cartilages  of  Santorini  is  obliterated,  the  vocal  processes  and  cords  are 


430  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

brought  together,  and  the  whole  rim  of  the  glottis  or  the  vocal  cords  alone, 
according  to  the  pitch  of  the  note,  may  be  seen  to  vibrate. 

2.  THE  VOICE. 

The  vocal  machinery  consists  of — (1)  the  motive  power  or  breath ;  (2) 
the  larynx,  which  forms  the  tone ;  (3)  the  chest,  the  pharynx,  the  mouth,  and 
the  nose,  which  color  the  tone ;  and  (4)  the  organs  of  articulation.1 

The  production  of  voice  is  undoubtedly  accomplished  by  the  vibration  of 
the  vocal  cords  which  have  previously  been  approximated  in  the  middle  line 
and  made  tense  through  action  of  the  nerve-muscular  apparatus  already  de- 
scribed. A  blast  of  air  from  below  pressing  against  the  cords  so  adjusted, 
causes  them  to  separate  and  fall  into  vibration.  We  have  to  distinguish  in 
voice  the  three  features  of  loudness,  pitch,  and  quality. 

The  loudness  of  the  tone  depends  on  two  factors:  (1)  the  strength  of  the 
tone-producing  blast  as  determining  not  only  the  amplitude  of  vibration  of 
the  vocal  cords,  but  also  the  energy  with  which  the  air  is  expelled;  (2) 
the  resonance  of  the  two  chambers  between  which  the  vocal  cords  are  sus- 
pended, the  chest  below  and  the  cavities  of  the  head  above,  whose  walls  and 
contained  air,  by  their  sympathetic  vibration,  powerfully  reinforce  the  oscilla- 
tions imparted  to  them. 

The  pitch  of  the  voice  is  determined  by  the  thickness,  tension,  and  length 
of  the  vocal  cords,  conditions  which  regulate  the  pitch  of  the  note  obtained 
from  any  vibrating  string.  The  thickness  and  the  elastic  quality  of  the  cords 
are  probably  largely  under  the  control  of  the  thyro-arytenoid  muscle.  The 
principal  tensor  of  the  cords  is  the  crico-thyroid  muscle.  Other  muscles,  as 
described  above,  may  so  fix  the  arytenoid  cartilages  that  their  vocal  processes 
may  be  prevented  from  taking  part  in  the  vibration  of  the  cords  throughout 
the  whole  and  also,  possibly,  throughout  part  only  of  their  length.  This 
dampening  of  the  vocal  processes  of  the  arytenoids  may  be  accomplished  either 
by  pressure  applied  to  them  throughout  their  whole  length,  in  which  case  the 
posterior  part  of  the  glottis  is  closed,  or  they  may  be  pressed  together  at  the 
tips  alone,  leaving  the  respiratory  glottis  open  as  a  triangular  aperture. 

Quality. — Variation  in  the  quality  of  the  voice  depends  on  the  fact  that 
vibrations  of  the  vocal  cords  are  composite  in  character,  giving  rise  to  notes 
made  up  of  a  fundamental  tone  combined  with  upper  partial  tones  (see  p.  383). 
By  reason  of  the  varied  adjustments  that  may  be  imparted  to  it,  the  larynx  is 
capable  of  producing  many  more  qualities  of  tone  than  is  any  artificial  instru- 
ment.2 Change  in  the  size  and  shape  of  the  resonance-chamber  above  and 
below  the  vocal  cords  produces  a  corresponding  change  in  their  fundamental 
notes  and,  therefore,  in  the  partial  tones  of  the  voice  which  they  reinforce  by 
sympathetic  vibration  (see  p.  385).  According  to  Helmholtz,3  the  difference 
in  quality  between  the  various  vowel  sounds  of  the  human  voice  depends  on 

1  C.  H.  Davis:  The  Voice,  1879. 

2  Helmholtz:  Sensations  of  Tone,  trans,  by  Ellis,  1885,  p.  98. 

3  Op.  tit.,  p.  104. 


VOICE  AND   SPEECH.  431 

the  number  and  relative  prominence  of  the  various  overtones  determined  by 
altering  the  shape  and  size  of  the  nasal  and  buccal  resonance-chambers. 

By  a  simple  experiment  the  production  of  voice  by  the  vocal  cords  can 
•easily  be  illustrated.  Take  a  glass  tube,  about  J  inch  in  diameter  and  of  con- 
venient length,  and  press  one  end  firmly  against  the  palmar  surfaces  of  the 
proximal  phalanges  of  two  fingers  at  their  line  of  division  when  thiy  are 
brought  together.  By  blowing  smartly  into  the  other  end  of  the  tube,  a 
musical  note  will  be  produced  by  the  vibration  of  the  folds  of  the  skin  be- 
tween which  the  air  is  forced.  By  relaxing  the  pressure  with  which  the 
fingers  are  held  together,  the  length  of  the  vibrating  segment  of  skin  is  in- 
creased and  its  tension  diminished ;  its  note  is  accordingly  lowered.  The 
reverse  conditions  are  produced  when  the  fingers  are  held  together  tightly  and 
the  tube  applied  firmly ;  the  pitch  of  the  note  is  then  raised.  In  these  ways 
the  pitch  of  the  note  may  be  varied  through  two  octaves,  which  is  the  range 
of  a  good  singing  voice.  Various  upper  partials  of  the  note  so  produced  may 
be  made  prominent  by  sympathetic  resonance,  if  the  vibrating  air-stream  is 
sent  across  the  opening  of  a  wide-mouthed  bottle,  of  about  a  pint  capacity. 
The  air  within  the  bottle  is  thrown  into  sympathetic  vibration  when  its  funda- 
mental tone  is  contained  in  the  note  emitted  through  the  fingers ;  when  the 
volume  of  the  air  is  diminished  by  slowly  pouring  water  into  the  bottle,  the 
fundamental  tone  of  the  resonator  is  changed,  and  it  responds  to  one  after 
another  of  the  partials  contained  in  the  musical  note. 

The  marvellous  adjustment  of  muscular  action  by  which,  at  will,  notes 
may  be  struck  of  definite  pitch  and  quality,  is  evidence  of  an  elaborate 
nervous  machinery  for  the  larynx,  not  only  on  the  efferent  side  but,  possibly 
through  a  muscular  sense,  on  the  afferent  side  as  well.  The  various  phe- 
nomena of  aphasia,  and  the  anatomical  importance  of  the  cerebral  areas 
devoted  to  the  elaboration  of  speech,  point  in  the  same  direction.  The 
relations  between  the  centres  for  speech  and  hearing  are  most  intimate.  The 
ear  plays  a  constant  part,  as  a  critical  medium,  in  the  tuition  of  the  vocal 
organs  in  either  speech  or  song.  So-called  "  dumbness  "  is  the  result,  usually, 
not  of  defects  in  the  vocal  organs,  but  of  lack  of  hearing  and,  hence,  of 
inability  to  control  by  the  ear  the  pitch  or  quality  of  the  vocal  notes. 

The  voice  and  the  larynx  of  the  child  fall  naturally  in  a  group  with  those 
of  the  female  as  contrasted  with  the  adult  male.  At  the  age  of  puberty  a 
boy's  larynx  becomes  congested  and  undergoes  rapid  development.  The  voice 
changes  rapidly  from  the  juvenile  to  the  adult  quality.  During  this  change, 
the  voice  frequently  "breaks"  or  rapidly  returns  from  the  newly-acquired 
chest  register  to  the  head  or  falsetto  notes  of  childhood  (see  p.  433).  In  boy!T\ 
who  are  castrated  a  good  while  before  the  age  of  puberty  is  reached,  the  larynx  ( 
does  not  undergo  its  characteristic  development,  and  the  voice  -remains  of  a 
peculiar  quality,  much  valued  in  some  countries  in  the  rendition  of  vocal 
music.  The  practice  of  castration  for  aesthetic  purposes  has,  accordingly,  in 
certain  districts,  long  been  in  vogue.  In  the  female  the  changes  in  the  larynx 
and  in  the  voice  at  puberty  are  much  less  marked  than  in  the  male. 


432  AN  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 

Arrangements  for  Chang-ing-  the  Pitch  of  the  Voice. — As  has  frequently 
been  mentioned,  the  vocal  cords  are  stretched,  and  the  pitch  of  their  note  is 
elevated,  by  contraction  of  the  crico-thyroid  muscle.  But  the  change  that  is 
thus  produced  in  the  tension  of  the  vocal  cords  is  by  no  means  capable  of 
accounting  for  the  full  range  of  pitch  which  falls  within  the  compass  of  the 
voice.  When  the  arytenoid  and  the  crico-arytenoid  muscles  sufficiently  con- 
tract, the  vocal  processes  are  brought  tightly  together  and  their  vibration  is 
prevented.  Voice-production  must  then  be  limited  to  the  vocal  cords  them- 
selves, and  the  stretching  action  of  the  crico-thyroids  may  begin  anew  and 
reach  its  maximum  with  the  glottis  so  set  that  only  its  ligamentous  borders  can 
vibrate.  It  can  also  be  seen  that  the  vocal  cords  themselves  may  be  shortened 
functionally,  or  even  be  broken  up  into  segments,  or  the  main  body  of  the  cord 
be  changed  in  thickness,  by  contraction  of  the  complex  thyro-arytenoid  muscles; 
each  such  condition  would  be  accompanied  by  a  change  in  the  rate  of  vibra- 
tion. We  are  probably  justified  in  assuming  that,  when  the  musical  scale  is 
sung,  the  lowest  notes  are  produced  by  vibration  of  the  glottic  borders  through- 
out their  full  length,  and  the  elevation  of  pitch  is  affected  by  the  gradually- 
increased  tension  of  the  vocal  ligaments  through  the  action  of  the  crico-thyroid 
muscle.  This  contraction  having  reached  its  maximum,  the  muscle  probably 
relaxes,  only  to  contract  again  after  the  vibrating  segments  of  the  glottis  are 
shortened  by  a  partial  or  complete  clamping  together  of  the  vocal  processes 
in  the  manner  described  above.  There  are  thus  two  or  three,  or  more, 
adjustments  which  may  be  imparted  to  the  vibrating  mechanism  of  the  lar- 
ynx, each  of  which  is  distinguished  by  giving  rise  to  a  note  of  different 
pitch  that  may  further  be  altered  by  action  of  the  crico-thyroid  muscle. 
It  might  be  anticipated  that  the  voice  whose  pitch  was  gradually  ele- 
vated in  the  manner  described  would  suffer  some  alteration  in  quality 
at  those  points  in  the  scale  where  there  is  a  change  in  the  set  of  the  lar- 
ynx producing  a  shortening  of  the,  vibrating  segment.  Such,  indeed,  is 
the  fact. 

Registers. — Long  before  the  invention  of  the  laryngoscope,  and  before  any- 
thing definite  was  known  of  the  method  of  voice-production,  it  was  recognized 
that  in  ascending  the  musical  scale  there  occur  certain  breaks,  as  it  were,  where 
the  voice  changes  in  quality  as  well  as  in  pitch.  It  is  an  object  in  musical 
education  to  render  these  breaks  as  little  prominent  as  possible.  The  kinds  of 
voice  included  between  these  breaks  were  distinguished  as  the  vocal  "  registers." 
There  is  no  general  agreement  among  musicians  as  to  how  many  registers  are 
compassed  by  the  voice,  and  the  nomenclatures  used  to  distinguish  them  differ 
in  the  most  confusing  fashion.  According  to  some  authors,  the  range  of  the 
voice  is  included  within  two  registers  only;  more  commonly  three  distinct 
registers  are  described,  to  which,  in  certain  cases,  a  fourth  is  said  to  be  prob- 
ably added.  The  most  common  designation  of  the  lowest  register  is  the  "  chest 
voice/'  though  it  has  also  been  called  " thick"1  as  distinguished  from  the 
"  thin  "  register ;  another  term  applied  to  it  is  the  "  long-reed  "  register  as  con- 
1  Browne  and  Behnke :  Voice,  Song,  and  Speech,  1890,  p.  135. 


VOICE  AND   SPEECH.  433 

trasted  with  the  "  short-reed  "  register.1  The  middle  register  of  all  voices  is 
by  some  authors  (Garcia,2  Mme.  Seiler3)  denominated  the  "falsetto,"  while 
other  writers  use  this  term  to  distinguish  certain  higher  notes  of  the  male 
voice  of  a  peculiar  quality  not  in  ordinary  use.  The  third  and  highest  series 
of  vocal  sounds  is  usually  known  as  the  "  head  "  register. 

The  lowest  or  chest  register  is  that  used  in  ordinary  life.  It  is  so  called 
from  the  strong  vibrations  of  the  chest-wall  which  may  be  felt  while  the  voice 
is  sounded.  In  passing  to  the  higher  register  the  chest  vibration  is  found  to 
diminish  and  that  of  the  head  bones  to  increase;  in  the  one  case  the  cavity  of 
the  head  acts  strongly  as  a  resonance  chamber,  and  in  the  other  that  *of  the 
thorax.  According  to  Madame  Seiler,  in  the  lowest  register  both  the  vocal 
ligaments  and  the  vocal  processes  of  the  arytenoids  vibrate.  In  the  middle 
register  the  vocal  processes  are  clamped  together  and  the  vibration  of  the  liga- 
ments seems  confined  chiefly  to  their  sharp  edges ;  while  in  the  highest  register 
the  ligaments  themselves  appear  to  be  damped  throughout  the  greater  part  of 
their  length,  the  vibrations  being  confined  to  the  edges  of  an  oval  slit  at  their 


FIG.  216. — The  voicing  (female)  larynx  (after  Browne  and  Behnke).  A,  Small  or  highest  register.  B, 
Upper  thin  or  middle  register.  C,  Lower  thin  or  middle  register:  T,T,  tongue;  F,F,  false  vocal  cords; 
S,S,  cartilages  of  Santorini ;  W,  W,  cartilages  of  Wrisberg;  V,  V,  vocal  cords. 

anterior  ends  (Fig.  216).  Within  any  definite  register  the  quality  of  individual 
voices  is  determined  by  the  size  and  elasticity  of  the  parts  of  the  larynx,  and 
probably  also  by  peculiarities  of  the  resonating  chambers ;  voices  are  accord- 
ingly classified  as  base,  tenor,  alto,  and  soprano. 

A  Whistling  Register. — A  friend  and  former  pupil  of  the  author's  has  the  remark- 
able power  of  emitting  from  the  larynx  notes  which  are  indistinguishable  in  quality  from 
an  ordinary  whistle.  He  writes,  "  The  whistle  cannot  be  made  to  '  slide  '  into  vocal  tones 
of  any  sort,  nor  can  any  other  tones  be  produced  simultaneously  with  it.  Its  range  is 
about  one  and  a  half  octaves,  or  half  an  octave  less  than  my  singing  voice. 

"The  lips  have  nothing  to  do  with  the  sound  except  as  their  position  changes  the  reso- 
nance-quality of  the  tone  by  '  reinforcement '  or  otherwise,  for  I  can  whistle  almost  as  read- 
ily with  the  teeth  closed  and  the  lips  wide  parted  as  with  the  jaws  and  lips  firmly  closed  as 
in  the  ordinary  position.  Any  other  movement  of  the  air-column  destroys  the  sound  at 
once."  Some  years  ago  the  author  made  a  laryngoscopic  examination  of  this  larynx  while 
it  was  in  the  act  of  whistling.  No  notes  were  written  at  the  time,  but  the  picture  remem- 
bered is  that  of  vocal  cords  closely  approximated,  except  for  an  oval  slit  between  their 
anterior  and  middle  portions,  as  in  singing  head  tones,  the  cords  vibrating  chiefly  along 
their  free  edges. 

Speech. — Language  consists,  in  general,  of  a  combination  of  short  musical 
sounds,  voweh  or  sonants,  which  are  produced  purely  by  vibration  of  the  vocal 

1  Mackenzie :  Hygiene  of  the  Vocal  Organs,  1891,  p.  55. 

2  Garcia:  Lond.,  Edin.,  and  Dub.  Mag.,  vol.  x.  1855,  p.  218.     (Quoted  by  Seiler.) 

3  Seiler :  op.  cit. 
VOL.  II.— 28 


434  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

cords,  together  with  superadded  noises  or  modes  of  obstruction,  con-sonants, 
produced  by  action  of  the  mouth-parts.  The  vowel  sounds  usually  carry  the 
accent  of  syllables,  and  the  consonants,  for  the  most  part,  are  sounded  only 
with,  or  represent  peculiar  modes  of  obstructing  the  former.  No  classification 
of  vocal  signs  can  be  made  in  which  exceptions  do  not  form  important  addenda 
to  general  rules. 

Articulation  is  the  modification  of  sound  in  speech,  usually  effected  by  action 
of  the  lips,  the  tongue,  the  palate,  or  the  jaws,  and  the  place  of  articulation 
depends,  in  any  definite  case,  on  the  mode  in  which  a  sound  is  formed.  Its  use 
as  an  expression  of  thought  is  the  chief  physiological  distinction  between  man 
and  the  lower  animals.  Distinctness  of  articulation,  so  essential  to  clearness 
of  language,  not  to  mention  its  aesthetic  value,  depends  on  the  accuracy  of  the 
muscular  adjustments  used  in  forming  sounds,  especially  consonantal  sounds. 

The  speaking  is  distinguished  from  the  singing  voice  partly  by  the  fact  that 
most  sounds  in  the  first  case  are  articulate  or  formed  in  the  mouth,  while  in 
the  latter  their  quality  is  only  there  modified.  In  singing  the  tone  is  sustained 
at  the  same  pitch  for  a  considerable  interval,  while  in  speaking  the  voice  is  con- 
tinually sliding  up  and  down  on  the  vowel  sounds.  In  speaking  the  conso- 
nantal noises  and  obstructions  are  more  prominent  because  of  their  more  abrupt 
formation.1- 2 

Vowel  sounds  owe  their  origin  to  vibration  of  the  vocal  cords,  and  their 
quality  to  the  selective  resonance  of  the  cavities  above  the  cords.  In  sounding 
the  series  of  vowels,  a,  e,  i,  o,  u  (pronounced  ah,  a,  e,  o,  oo),  it  is  found  that  the 


U 


FIG.  217.— Section  of  the  parts  concerned  in  phonation,  and  the  changes  in  their  relations  in  sound- 
ing the  vowels  AC*),  I  («),  V  (°°)  (after  Landois  and  Stirling) :  T,  tongue ;  p,  soft  palate ;  e,  epiglottis ;  g,  glot- 
tis ;  h,  hyoid  bone ;  1,  thyroid ;  2, 3,  cricoid ;  4,  arytenoid  cartilage. 

form  and  size  of  the  mouth-cavity,  the  position  of  the  tongue,  the  position  of 
the  soft  palate  separating  or  allowing  communication  between  the  nasal  and 
pharyngeal  cavities,  undergo  a  progressive  change  (Fig.  217).  Helmholtz  has 
shown  that  the  vowel  sounds  owe  their  differences  of  quality  to  the  varied 
resonance  of  the  mouth-cavity,  dependent  on  its  shape,  through  which  now  one, 
now  another,  of  the  overtones  in  the  note  produced  by  vibration  of  the'Yocal 
cords  is  reinforced.3  This  result  is  dependent  on  the  fact  that  when  the  mouth 
is  set  in  position  for  the  formation  of  the  various  vowel  sounds  the  pitch  of  its 

1  Browne  and  Behnke :  op.  cit.,  p.  28. 

2  Monroe:  Manual  of  Physical  and  Vocal  Training,  1869,  p.  51. 

3  Helmholtz :  loc.  cit. 


VOICE   AND   SPEECH.  435 

fundamental  note,  or  the  rate  of  vibration  to  which  it  sympathetically  responds, 
varies  accordingly.1  That  the  resonance  of  the  mouth  cavity  changes  with 
its  shape  is  illustrated  in  the  various  pitch  of  the  notes  produced  by  flipping 
the  edge  of  an  incisor  tooth,  the  cheek,  or  Adam's  apple  with  the  finger-nail, 
while  the  mouth  assumes  the  positions  for  production  of  the  different  vowels. 

Vowels  whose  normal  pitch  is  low,  as  o,  u,  cannot  be  sounded  easily  in  the 
higher  part  of  the  musical  scale ;  conversely,  high-pitched  vowels,  as  e  in  feet, 
lose  their  character  in  the  lower  part  of  the  scale.  Language  is,  therefore, 
much  less  distinct  in  song  than  in  speech.2 

It  has  already  been  stated  that  the  difference  in  quality  of  musical  notes 
depends  upon  the  number  and  relative  intensity  of  their  partial  tones,  each 
of  which  is  separated  from  the  fundamental  tone  by  a  fixed  interval.  Since 
the  mouth  parts  have  a  fairly  fixed  position  for  each  vowel  sound,  the  buccal 
cavity  reinforces  by  sympathetic  resonance  tones  of  definite  vibration  rates. 
When  a  given  vowel  is  sounded  in  different  parts  of  the  musical  scale,  now 
one,  now  another  partial  tone  is  reinforced,  according  as  its  pitch  harmonizes 
with  the  prime  tone  of  the  mouth  cavity,  so  that  the  interval  between  the 
resonated  partials  and  their  fundamental  tone  may  change,  with  corre- 
sponding change  in  the  quality  of  the  vowel  sound.  That  is,  the  resonated 
partial  depends  not  only  on  its  relation  to  the  fundamental,  but  also  on  its 
vibration  rate.3  This  feature  of  vocal  resonance  distinguishes  the  human 
larynx  from  most  musical  instruments.  That  the  ground  is  not  covered  by 
these  facts  was  shown  by  Auerbach,4  who  demonstrated  that  the  strength  of 
upper  partials  in  vowel  sounds  depends  also  on  the  strength  of  their  production 
by  the  vocal  cords  and,  therefore,  upon  their  relation  to  the  fundamental  tone. 
That  is  to  say,  the  quality  of  a  vowel  is  dependent  not  only  on  the  absolute 
vibration  numbers  of  its  upper  partials,  according  to  which  they  are  or  are  not 
reinforced  by  the  position  of  the  mouth,  but  also  on  the  relative  position  of  these 
upper  partials  as  compared  witli  the  fundamental  tone. 

The  peculiar  aesthetic  value  of  the  human  voice  is  dependent  on  the  fact 
that,  on  account  of  its  varied  powers  of  adjustment,  the  larynx  is  capable  of  pro- 
ducing many  more  kinds  of  tone-quality  than  any  artificial  instrument.  Helm- 
holtz5  found  no  less  than  sixteen  overtones  to  accompany  the  fundamental. 

The  posture  of  the  mouth-parts  differs  markedly  when  set  for  the  various 
principal  vowel  sounds ;  but  as  we  know  that  each  vowel  sound  has  several 
modifications  or  gradations  so  that  a  tone  may  pass  by  an  easy  glide  from  one 
to  another,  so  the  form  of  the  mouth  passes  by  insensible  steps  from  one  vowel 
position  to  another.  It  will  be  seen  later  that  several  articulate  sounds  play 
the  part  now  of  vowels,  now  of  consonants,  according  to  their  position  in  the 
syllable  or  mode  of  formation.  There  has  also  been  shown  reason  for  believ- 
ing that  the  form  of  the  chest  cavity  and  the  tension  of  its  walls  are  factors  in 
determining  the  pitch  of  its  fundamental  tone;  so  that  through  the  varied 

1  Helmholtz :  op.  tit.,  p.  108.  2  Op.  tit.,  p.  114.  3  Op.  cit.,  p.  118. 

4  Quoted  by  Grutzner:  op.  cit.,  p.  179.  6  Op.  cit.,  p.  103. 


436  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

sympathetic  resonance  of  the  thorax  the  reinforcement  of  laryngeal  tones  may 
here  be  altered  somewhat,  as  in  the  mouth  itself.1'2 

Whispering-  is  a  mode  of  speech  in  which  noise  largely  replaces  pendular 
musical  vibrations.  The  glottis  remains  more  or  less  widely  open  and  the 
vocal  cords  are  not  tense ;  the  vibrations  are  produced  both  in  the  larynx  and 
in  the  buccal-pharyngeal  chambers.  Vowel  sounds  may  be  produced  in  whis- 
pering as  well  as  in  true  voice  because,  from  the  multitude  of  irregular  vibra- 
tions, those  waves  are  reinforced  which  make  up  the  vowel  sounds  determined 
by  the  set  of  the  mouth.  Gentle  whispering  requires  much  less  effort  than  does 
speaking,  and  inspiratory  whispering  is  less  easily  distinguished  from  expiratory 
than  is  the  strained  voice  of  inspiration  from  the  natural  sound  of  expiration. 
Consonants,  as  already  indicated,  may  sometimes  play  the  part  of  vowels,  but 
pure  consonants  do  not  appear  in  syllables  except  in  combinations  with  vowels, 
which  combinations  always  carry  the  syllable  accent. 

Consonants. — The  distinction  between  consonants  and  vowels  lies  in  the 
fact  that  the  tones  of  the  latter  are  produced  by  vibration  of  the  vocal  cords, 
the  parts  above  which  act  only  as  resonance-boxes  and  modify  the  sound,  and 
never  offer  marked  obstruction  to  the  exit  of  air ;  whereas  in  the  formation  of 
consonants  there  is  some  adjustment  in  the  mouth-passage  either  in  the  nature 
of  a  local  narrowing,  by  which  a  peculiar  noise  is  added  to  the  vocal  sound,  or 
in  the  nature  of  a  sudden  closing  or  opening  of  the  air-channel  by  which  a 
characteristic  noise  is  likewise  added  to  the  vocal  sound.  In  other  words,  the 
parts  above  the  larynx  make  the  sounds  of  consonants  but  only  modify  those 
of  vowels.3  No  sharp  line  of  separation  can  be  drawn  between  vowels  and 
consonants,  since  certain  characters,  according  to  their  associations,  now  fall 
into  one,  now  into  another  class.  In  the  classification  of  consonantal  sounds 
much  confusion  exists,  dependent  chiefly  on  the  fact  that  several  letter  charac- 
ters change  their  modes  of  formation  and  expression  with  their  place  in  the 
syllable.  The  same  facts,  also,  are  expressed  by  different  authors  by  different 
nomenclatures,  and  sounds  occur  in  one  language  that  are  not  found  in  another. 
Adopting  the  general  classification  of  Grutzner,4  we  may  divide  consonants 
into  the  following  three  groups: 

1.  Semi-vowels  or  liquids,  which  can  be  used  either  as. vowels  or  consonants; 
this  group  includes  the  sounds  m,  n,  ng,  I,  and  r.  In  expressing  the  function 
of  a  consonant,  the  letter  is  not  to  be  sounded  as  if  it  stood  alone,  but  its  cha- 
racter given  as  actually  expressed  in  a  syllable ;  thus  the  sound  of  p  is  not  pee, 
but  is  the  abbreviated  labial  expression,  as  [unpack  or  piece  when  all  the  letters 
are  eliminated  after  the  first.  Of  the  liquids  the  n,  m,  and  ng  (sometimes 
called  "  resonants ")  have  the  nature  of  vowels  when  final  (as  in  him,  hen, 
being},  and  are  then  produced  by  vibration  of  the  vocal  cords,  the  lips  having 
previously  been  closed  for  the  m,  and  the  tongue  applied  to  the  roof  of  the 
mouth  to  cut  off  the  exit  of  air  for  n  and  ng ;  the  expelled  air  escapes  alto- 
gether through  the  nose,  which  acts  as  a  resonance-chamber.  Used  as  conso- 

1  Op.  tit.,  p.  93.          2  Sewall  and  Pollard :  Journal  of  Physiology,  1890,  vol.  xi.,  p.  159. 
3  Grutzner:  op.  tit.,  p.  196.  *  Op.  tit.,  p.  197. 


VOICE  AND   SPEECH. 


437 


nants,  as  in  make  and  no,  m  and  n  are  seen  to  have  the  characters  of  the  second 
group, — Explosives.  L  is  pronounced  somewhat  like  n,  but  air  is  allowed  t<> 
escape  through  the  mouth  on  each  side  of  the  tongue ;  it  may  be  produced 
either  with  voice  or  without  voice  (in  whispers).  It  may  have  vowel  charac- 
ters as  in  play.  R  is  characterized  as  a  vibrative  and  may  have  several  seats 
of  articulation,  as  by  the  thrill  of  the  tip  of  the  tongue  against  the  hard 
palate,  or  that  of  the  hind  part  of  the  tongue  against  the  soft  palate,  or  even 
by  the  coarse  vibration  of  the  vocal  cords  themselves.  In  the  first  two  cases 
it  may  be  sounded  either  with  or  without  voice.  Its  vowel  nature  is  shown  in 
such  words  as  pray. 

2.  Explosives,  which  are  produced  either  when  an  obstruction  is  suddenly 
offered  to  or  removed  from  the  exit  of  air  from  the  mouth ;  at  the  same  time 
a  characteristic  noise  is  produced.  They  may  be  subdivided  according  to  the 
place  of  articulation  into  labials  (p,  v)  ;  linguo-palatals  (t,  d)  ;  gutturals  (k,  g). 
The  similarity  in  the  method  of  formation  of  p  and  b,  t  and  d,  k  and  g,  is 
striking.  They  are  frequently  characterized  as  being  formed  with  or  without 
voice ;  that  is,  b,  d,  and  g  require  voice  for  their  distinct  recognition,  and  when 
whispered  they  are  easily  mistaken  for  p,  t,  k,  which  latter  do  not  require  voice 
(vibration  of  the  vocal  cords)  for  their  recognition.  A  consonant,  then,  is  said 
to  be  formed  with  voice  when  it  can  be  rendered  distinctly  only  by  an  accom- 
panying vibration  of  the  vocal  cords,  without  voiee  when  articulated  clearly 
without  laryngeal  aid.  The  former  are  sometimes  called  sonants,  the  latter 
surds.  This  classification  only  approximates  the  truth,  for  the  suddenness  and 
energy  with  which  the  obstruction  to  the  breath  is  removed  determines  our 
recognition  of  the  consonant  irrespective  of  voice.1 

Table  of  Consonantal  Elements? 


PLACE  OF  ARTICULATION. 

ORAL. 

NASAL. 

Momentary.                                  Continuous. 

Continuous. 

Surds 
(without  voice). 

Sonants 
(with  voice). 

Surds 
(without  voice). 

Sonants 
(with  voice). 

Sonants 
(with  voice). 

Lips 

P 

b 

w 
th(y) 
z,  r 
zh,  r 
7,1 

ra 
n 

ng 

Lips  and  teeth  

f 
th(in) 

s 
sh 

Tongue  and  teeth    .    .    . 

Tongue  and  hard  palate 
(forward)    
Tongue  and  hard  palate 
(back)     .       .    . 

t 
ch 

d 

j 

Tongue,  hard  palate,  and 
soft  palate  . 

Tongue  and  soft  palate   . 
Various  places  

k 
h 

g 

3.  Friction  sounds  or  frictionals,  often  called  aspirates,  are  all  noises  pro- 
duced by  the  expired  blast  passing  through  a  constriction  in  its  passage,  at 
which  point  a  vibration  is  set  up.  No  obstruction  being  offered  to  the  sound, 
they  are  known  as  continuous  as  distinguished  from  the  momentary  sounds  of 

1  Griitzner,  op.  cit,  pp.  211,  213. 

2  Webster  s  International  Dictionary,  1891,  p.  Ixvi. 


438  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

group  2.  They  may  be  divided  into  labio-dental  frictionals,  f  (without  voice)  ; 
v,  w  (with  voice) ;  the  lingual  frictionals  s,  th  (as  in  them) ;  sh,  ch  soft  (with- 
out voice) ;  2,  j  (with  voice).  The  sound  of  h  may  be  regarded  as  due  to  the 
vibration  of  the  separated  vocal  cords.  It  is  peculiar,  however,  in  appearing 
to  be  formed  in  any  part  of  the  vocal  chamber ;  when  it  is  formed  the  mouth 
parts  take  on  no  peculiar  position,  but  assume  that  of  the  vowel  following  the 
ht  as  hark9  hear,  etc. 


V.  REPRODUCTION. 


THE  principles  and  problems  of  Physiology  that  have  been  already  pre- 
sented in  this  work,  comprising  nutrition  and  the  functions  of  the  muscular 
and  the  nervous  systems,  have  reference  to  the  individual  man  or  woman. 
Through  the  normal  activity  of  those  functions  and  their  appropriate  co- 
ordination the  individual  lives  his  daily  life  and  performs  his  daily  tasks  as 
an  independent  organism.  But  man  is  something  more  than  an  independent 
organism ;  he  is  an  integral  part  of  a  race,  and  as  such  he  has  the  instincts  of 
racial  continuance.  The  continuance  of  the  race  is  assured  only  by  the  pro- 
duction of  new  individuals,  and  the  strength  of  the  human  reproductive 
instinct  is  indicated  in  some  measure  by  the  large  proportion  of  energy  that  is 
expended  by  woman  in  the  bearing  of  children  and  by  both  sexes  in  the  nur«- 
ture  and  education  of  the  young.  The  function  of  reproduction  is  not  limited 
to  the  daily  life  and  well-being  of  independent  organisms.  It  has  a  deeper 
significance  than  that.  Its  essence  lies  in  the  fact  that  it  has  reference  to  the 
species  or  race.  Many  of  its  problems  are,  therefore,  broad  ones ;  they  in- 
clude not  only  the  immediate  details  of  individual  reproduction,  but  larger 
ones  relative  to  the  nature  and  significance  of  reproduction  and  of  sex,  and  to 
heredity.  In  the  following  discussion,  while  attention  will  be  given  chiefly 
to  the  facts  of  individual  reproduction,  some  of  the  broader  applications  of 
the  facts  will  be  indicated. 

A.  REPRODUCTION  IN  GENERAL. 

In  all  forms  of  organic  reproduction  the  essential  act  is  the  separation  from 
the  body  of  an  individual,  called  the  parent,  of  a  portion  of  his  own  material 
living  substance,  which  under  suitable  conditions  is  able  to  grow  into  an  inde- 
pendent adult  organism. 

Among  living  beings  two  methods  of  reproduction  are  recognized,  the 
asexual  and  the  sexual  methods.  Both  are  widespread  among  animals  and 
plants,  but  the  asexual  method  is  the  more  primitive  of  the  two  and  is  rela- 
tively more  frequent  in  low  organisms.  The  sexual  method,  the  only  one 
present  in  the  production  of  new  individuals  among  the  higher  animals,  has 
evidently  been  acquired  gradually,  and  has  probably  been  developed  from  the 
asexual  method. 

Asexual  Reproduction. — Asexual  reproduction,  or  agamogenesis,  is  the 
chief  method  of  reproduction  among  unicellular  plants  and  animals,  and 
throughout  the  plants  und  in  the  lower  nr  7T  ;^ar  animals  it  is  important. 
Among  various  species  it  takes  various  foi  <  •  n  a->  fi— ion  or  division, 

gemmation  or  budding,  exogenous  cell-fun,  -tion  or  spore-formation  or  multi- 

439 


440  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

pie  fission ;  but  all  the  varieties  are  modifications  of  the  simplest  form,  fission 
or  division.  In  fission,  found  only  in  unicellular  organisms  and  typified  in 
Amoeba,  the  protoplasm  of  the  single  cell,  together  with  the  nucleus,  becomes 
divided  into  two  approximately  equal  portions  which  separate  from  one 
another.  In  the  process  no  material  is  lost,  and  two  independent  nucleated 
organisms  result,  each  approximately  half  the  size  of  the  original.  The 
parent  has  become  bodily  transformed  into  the  two  offspring,  which  have  only 
to  increase  in  size  by  the  usual  processes  of  assimilation  in  order  themselves 
to  become  parents.  In  higher  organisms,  even  where  sexual  processes  alone 
prevail  in  the  production  of  new  individuals,  the  asexual  method  has  per- 
sisted in  the  multiplication  of  the  individual  cells  that  constitute  the  body ; 
embryonic  growth  is  an  asexual  reproductive  process,  a  continued  fission,  dif- 
fering from  the  amoeboid  type  in  the  facts  that  the  resulting  cells  do  not  sepa- 
rate from  one  another  to  form  independent  organisms,  but  remain  closely 
associated,  undergo  morphological  differentiation  and  physiological  specializa- 
tion, and  together  constitute  the  individual.  Likewise  in  the  adult  the  pro- 
duction of  blood-corpuscles  and  of  epidermis,  the  regrowth  of  lost  tissues,  and 
the  healing  of  wounds  are  examples  of  asexual  cell-reproduction.  From  the 
standpoint  of  multicellular  growth  Spencer  and  jHaeckel  have  happily  termed 
the  process  of  asexual  reproduction  in  unicellular  organisms  "  discontinuous 
growth." 

Sexual  Reproduction. — Sexual  reproduction,  or  gamogenesis,  -occurs  in 
unicellular  organisms,  where  it  is  known  as  conjugation,  and  it  is  the  prevail- 
ing form  of  reproduction  in  most  of  the  multicellular  forms.  In  most  of  the 
invertebrate  and  vertebrate  animals  it  is  the  sole  form  of  reproduction  of 
individuals.  In  its  simple  form  of  conjugation,  typified  in  the  minute  monad, 
Hderomita,  it  consists  of  a  complete  fusion  of  the  bodies  of  two  similar  indi- 
viduals, protoplasm  and  nuclei,  followed  by  a  division  of  the  mass  into 
numerous  spore-like  particles,  each  of  which  grows  into  an  adult  Heteromita. 
In  the  higher  infusorian,  Paramceeium,  the  fusion  of  the  two  similar  individ- 
uals is  a  partial  and  temporary  one,  during  which  a  partial  exchange  ->f 
nuclear  material  takes  place;  this  is  followed  by  separation,  after  which  eai  h 
individual  proceeds  to  live  its  Ordinary  life  and  occasionally  to  multiply  by 
simple  fission. 

In  the  highly  specialized  sexual  reproduction  of  higher  animals,  including 
man,  the  individuals  of  the  species  are  of  two  kinds  or  sexes,  the  male  m  1 
the  female,  with  profound  morphological  and  physiological  differences  between 
them  ;  in  each  the  protoplasm  of  the  body  consists  of  two  kinds  of  cells,  somatic 
cells  and  germ-cells,  the  former  subserving  the  nutritive,  Min-riil  i  vous 

functions  of  daily  life,  the  latter  subserving  reproduction.     The  germ-cell.^  of 
the  male,  called    spermatozoa,  are   relatively  small  and  active,  thos    of  the 
female,  called  ova,  are  relatively  large  and  passive;  the  i**producti\ 
consists  of  a  fusion  of  a  male  and  a  female  germ-eel !  -ontiaJ  part  being 

a  fusion  of  their  nuclei ;  and  this  ik  followed  by  continued  asexual  c<:ll-r!ivision 
and  growth  into  a  new  individual.     Among  both  p;auts  aim! •  an-  niuls  it  is  not 


REPRODUCTION.  441 

difficult  to  find  a  series  of  forms  showing  progressively  greater  and  greater 
deviations  from  the  typical  asexual  toward  the  typical  sexual  method  of 
reproduction,  and  the  existence  of  such  a  series  is  indicative  of  the  derivation 
of  the  latter  from  the  former  type. 

Origin  of  Sex,  and  Theory  of  Reproduction. — It  is  obvious  that  the 
production  of  new  individuals  is  necessary  to  the  continued  existence  of  any 
species.  It  would  be  interesting  to  know  the  origin  and  significance  of  the  two 
existing  methods  of  reproduction.  Apropos  of  the  asexual  process,  Leuckart, 
and  especially  Herbert  Spencer,  have  pointed  out  that  during  the  growth  of 
a  cell  the  mass  increases  as  the  cube,  but  the  surface  only  as  the  square,  of 
the  diameter — i.  e.  the  quantity  of  protoplasm  increases  much  more  rapidly 
than  the  absorptive  surface.  It  follows  from  this  that  during  the  growth  of  a 
unicellular  organism  a  size  will  ultimately  be  reached  beyond  which  the  cell 
will  not  be  able  to  absorb  sufficient  food  for  the  maintenance  of  the  proto- 
plasm. In  order  that  growth  may  continue  beyond  this  point,  a  division  of 
the  cell,  which  ensures  a  relative  increase  of  surface  over  mass,  is  absolutely 
necessary.  Fission  is,  therefore,  a  necessary  corollary  of  growth,  and,  although 
we  are  ignorant  of  the  details  of  its  mechanism,  it  is  conceivable  that  the  method 
of  asexual  reproduction  arose  through  causes  connected  with  growth. 

The  explanation  of  sexual  reproduction  is  much  more  difficult,  for  here,  in 
addition  to  the  budding  off  of  the  germ-cells  from  the  parental  bodies,  which 
has  probably  the  same  fundamental  cause  as  fission  in  unicellular  forms,  we 
must  account  for  the  differentiation  into  sexes,  the  existence  of  special  sexual 
cells,  and  the  fusion  of  the  male  and  the  female  germinal  substance ;  in  short, 
we  must  account  for  the  conception  of  sexuality  itself  and  all  that  it  implies. 

Regarding  the  origin  of  sexuality  itself,  as  to  the  question  whether  sexuality 
is  an  original  and  fundamental  attribute  of  protoplasm  or  has  been  acquired, 
we  may  say  at  once  that  at  present  we  know  really  nothing.  Yet,  whatever 
view  is  held  as  to  the  origin  of  sexuality,  it  seems  entirely  probable  that  the 
method  of  reproduction  known  as  sexual  is  a  derivative  of  the  method  known 
as  asexual — the  latter  is  primitive,  the  former  has  arisen  from  it.  From  the 
wide  distribution  and  prominence  of  the  former  among  vital  phenomena  we 
must  believe,  with  biologists  generally,  that  sexual  differentiation  and  sexual 
processes  have  arisen  from  natural  causes,  and  for  the  reason  that  sexual  repro- 
duction is  of  advantage  to  living  beings  and  to  species.  In  what  way  it  is  of 
advantage,  however,  is  disputed.  Three  views,  all  of  which  have  evidence  in 
their  favor  and  which  are  not  mutually  exclusive,  are  at  present  engaging  the 
attention  of  scientific  men.  The  first  to  be  mentioned  is  the  theory  advocated 
by  Hensen,  Edouard  van  Beneden,  and  Butschli,  according  to  whom  the  fusion 
of  the  cells  in  sexual  reproduction  exists  for  the  purpose  of  rejuvenating  the 
living  substance.  The  power  possessed  by  cells  of  dividing  asexually  is 
limited;  in  time  the  protoplasm  grows  old  and  degenerates;  its  vital  powers  are 
weakened,  and  without  help  the  extinction  of  the  race  must  follow.  But  the 
mingling  of  another  strain  with  such  senescent  protoplasm  gives  it  renewed 
youth  and  vig^r,  restores  the  power  of  fission,  and  grants  a  new  lease  of  life  to 


442  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  species.  From  his  observations  upon  the  Infusoria,  Maupas 1  has  brought 
forward  valuable  evidence  which  has  been  quoted  in  favor  of  this  view.  Sty- 
lonychia  normally  produces  by  fission  130  to  180  generations  or  individuals, 
Onychodromus  140  to  230,  and  Leucophrys  patula  300  to  450,  after  which  con- 
jugation is  necessary  to  continued  division.  If  conjugation  be  prevented,  the 
individuals  become  small,  their  physiological  powers  become  weakened,  their 
nuclei  atrophy,  and  the  chromatin  disappears;  all  of  which  changes  are  evidence 
of  the  oncoming  of  senile  degeneration,  and  this  ultimately  results  in  death. 
Analogous  to  this  is  doubtless  the  fact,  pointed  out  by  Hertwig,2  that  in  sexual 
animals  an  unfertilized  ovum  within  the  oviduct  soon  becomes  over-mature 
and  enfeebled,  and  subsequent  fertilization,  even  though  possible,  is  abnormal. 
Even  if  the  idea  of  "  rejuvenescence"  be  regarded  as  fanciful  and  as  a  com- 
parison rather  than  an  explanation,  it  seems  to  be  a  principle  of  nature  that 
occasional  fusion  of  one  line  of  descent  with  another  is  necessary  to  continued 
reproduction  and  continued  life. 

A  second  theory,  defended  by  Hatschek  and  Hertwig,  argues  that  sexual 
reproduction  prevents  variation,  and  thus  preserves  the  uniformity  of  the  race. 
The  mingling  of  two  different  individuals  possessing  different  qualities  must 
give  rise  to  an  individual  intermediate  between  the  parents,  but  differing  from 
them.  Such  differences  between  parents  and  offspring  are  numerous,  but  in  a 
single  generation  are  minute,  and  they  are  easily  obliterated  by  a  subsequent 
union,  which  latter  in  turn  gives  rise  to  other  minute  differences.  Hence  sexual 
reproduction,  although  constantly  producing  variations,  as  constantly  eradicates 
them,  and,  by  striving  always  toward  the  mean  between  two  extremes,  tends 
toward  homogeneity  of  the  species.  The  essential  truth  of  such  a  view  seems 
obvious. 

A  third  theory,  advocated  by  Weismann  and  Brooks,  is  quite  the  opposite 
of  the  last,  and  maintains  that  the  meaning  of  sexual  reproduction  lies  in  the 
production  of  variations.  "  The  process  furnishes  an  inexhaustible  supply  of 
fresh  combinations  of  individual  variations."  These  minute  variations,  seized 
upon  by  natural  selection,  are  augmented  and  made  serviceable,  and  a  variety, 
better  able  to  cope  with  the  conditions  of  existence,  results.  The  transformation, 
not  the  homogeneity,  of  the  species  is  thereby  assured.  The  two  latter  views  are 
not  necessarily  mutually  exclusive.  Both  claim  that  fertilization  brings  into 
evidence  variations.  It  is  quite  conceivable  that  subsequent  fertilizations  may 
obliterate  some  and  augment  others,  the  result  of  union  being  the  algebraic  sum 
of  the  characteristics  contributed  by  the  two  sexes. 

Primary  and  Secondary  Characters. — In  the  human  species,  as  in  all 
the  higher  sexual  animals,  the  characters  of  sex,  anatomical,  physiological,  and 
psychological,  are  divisible  into  two  classes,  called  primary  and  secondary. 
Primary  sexual  characters  are  those  that  pertain  to  the  sexual  organs  them- 
selves and  to  their  functions.  They  are  naturally  the  most  pronounced  of  all 

1  E.  Maupas :  Archives  de  Zoologie  experimentale  et  generate,  2e  s£rie,  vii.,  1889. 

2  O.  und  K.  Hertwig :  Experimentelle  Studien   am  thierischen  Ei  vor,  wdhrend  und  nach  der 
Befruchtung,  L,  1890. 


REPRODUCTION.  443 

sexual  attributes.  Secondary  sexual  characters  comprise  those  attributes  that 
are  not  directly  connected  with  the  sexual  organs,  but  that,  nevertheless,  con- 
stitute marked  differences  between  the  sexes;  such  are  the  greater  size  and 
strength  of  man's  body  as  compared  with  woman's,  the  superior  grace  and 
delicacy  of  woman's  movements,  the  deeper,  rougher  voice  of  man,  and  the 
higher,  softer  voice  of  woman.  In  reality,  all  secondary  sexual  characters  are 
accessory  to  the  primary  ones,  and  the  greater  portion  of  the  present  article 
will  be  devoted  to  a  discussion  of  the  latter.  The  primary  sexual  characters 
of  the  male  centre  in  the  production  of  spermatozoa  and  the  process  of  impreg- 
nation, those  of  the  female  in  the  production  of  ova  and  the  care  of  the  devel- 
oping embryo. 

Sexual  Organs. — Sexual  organs  are  classified  into  essential  and  accessory 
organs.  The  essential  organs  are  the  two  testes  of  the  male  and  the  two 
ovaries  of  the  female.  The  accessory  organs  of  the  male  comprise  the  vasa 
deferentia,  the  seminal  vesicles,  the  urethra,  the  penis,  the  prostate  gland,  Cow- 
per's  glands,  and  the  scrotum  and  its  attached  parts.  The  accessory  organs  of 
the  female  comprise  the  oviducts  or  Fallopian  tubes,  the  uterus,  the  vagina,  the 
various  external  parts  included  in  the  vulva,  and  the  mammary  glands.  During 
the  greater  part  of  life  the  sexual  organs  perform  but  a  portion  of  their  duties ; 
only  at  intervals,  and  in  some  individuals  never,  do  they  complete  the  cycle 
of  their  functions  by  engaging  in  the  reproductive  process  itself.  In  the  fol- 
lowing account  we  shall  discuss  first  the  habitual  physiology  of  the  organs  of 
the  male  and  of  the  female,  and  later  their  special  activities  in  the  repro- 
ductive process. 

B.  THE  MALE  REPRODUCTIVE  ORGANS. 

The  male  reproductive  organs,  already  mentioned,  have  as  their  specific 
functions  the  production  of  the  essential  male  germ-cells,  the  spermatozoa,  the 
production  of  a  fluid  medium  in  which  the  spermatozoa  can  live  and  undergo 
transportation,  the  temporary  storing  of  this  seminal  fluid,  and  its  ultimate 
transference  to  the  outside  world  or  to  the  reproductive  passages  of  the  female. 

The  Spermatozoon. — Spermatozoa  were  first  discovered  by  Hamm,  a 
student  at  Leyden,  in  1677.  Hamrn's  teacher,  Leeuwenhoek,  first  studied 
them  carefully.  They  were  long  believed  to  be  parasites,  even  until  near  the 
middle  of  the  present  century,  when  their  origin  and  fertilizing  function  were 
established.  Spermatozoa  are  cells  modified  for  locomotion  and  entrance  into 
the  ovum.  Human  spermatozoa  are  slender,  delicate  cells,  averaging  0.055 
millimeter  (-^^  of  an  inch)  in  thickness,  and  consisting  of  a  head,  a  middle- 
piece,  and  a  tail  (Fig.  218).  The  head  (h)  is  flattened,  egg-shaped,  with  a  thin 
anterior  edge  and  often  slightly  depressed  sides.  It  terminates  anteriorly  in  a 
slender,  projecting,  and  sharply  pointed  thread  or  spear.  It  consists  of  a 
nucleus  composed  of  a  dense  mass  of  chromatin  and  covered  by  an  excessively 
thin  layer  of  cytoplasm,  von  Bardelebon  !  claims  the  number  of  chromo- 
somes in  the  chromatin  after  maturation  to  be  eight. 

1  K.  v.  Bardeleben :  Verhandlungen  der  anatomischen  Gesdlschaft ;  Anatomischer  Anzeiger  ; 
1892,  vii. 


444  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

The  middle-piece  (m)  is  a  short,  cytoplasmic  rod,  probably  containing  a  cen- 
trosome.     The  tail  (t)  is  a  delicate  filiform,  apparently  cytoplasmic  structure, 
and  analogous  to  a  single  cilium  of  a  ciliated  cell.     The  tail  is  tipped  by  an 
excessively  fine,  short  filament,  the  end-piece  (e).     The  most 
abundant  of  the  solid  chemical  constituents  of  the  spermato- 
zoon is  nuclein,  probably  in  the  form  of  nucleic  acid,  which 
is  found  in  the  head.     Other  constituents  are  proteids,  prota- 
mine,  lecithin,  cholesterin,  and  fat. 

The  structure  and  power  of  movement  of  the  spermatozoon 
plainly  show  it  to  be  adapted  to  activity.  It  is  not  burdened 
by  the  presence  of  food-substance  within  its  protoplasm.  It 
is  the  active  element  in  fertilization  ;  it  seeks  the  ovum,  and 
it  is  modified  from  the  form  of  the  typical  cell  for  the  special 
purpose  of  fertilization.  The  nucleus  is  the  fertilizing  agent. 
The  head  is  plainly  fitted  for  facilitating  entrance  into  the 
ovum.  The  tail  is  a  locomotor  organ  capable  of  spontaneous 
movements,  and,  after  expulsion  of  the  semen,  it  propels  the 
cell,  head  forward,  through  the  liquid  in  which  it  lies.  The 
J  movement  is  a  complex  one,  and  is  effected  by  the  lashing 

r         I          of  the  tail  from  side  to  side,  accompanied  by  a  rotary  move- 
ment about  the  longitudinal  axis.     The  rate  of  movement  has 
FIG.  2i8.-Human  been  variously  estimated  at  from  1.2  to  3.6  millimeters  in  the 
spermatozoa    (after  minute.      Spermatozoa  taken   directly   from    the   testis  are 

Retzius)  :  A,  seen  en 

face;  h,  head|;    m,  quiescent  ;   normally  they  begin  to  move  when  mixed  with 


^C  *^e  secretions  of  the  accessory  sexual  organs.1  Toward  heat", 
seen  from  the  side,  cold,  and  chemical  agents  spermatozoa  behave  like  ciliated 
cells. 

Ripe  spermatozoa  appear  to  be  capable  of  living  for  months  within  the  male 
genital  passages,  where  they  are  probably  quiescent.  Outside  of  the  body 
they  have  been  kept  alive  and  in  motion  for  forty-eight  hours.  It  is  not 
certain  how  long  they  may  remain  alive  within  the  genital  passages  of  the 
human  female.  Diihrssen  2  claims  to  have  found  motile  spermatozoa  in  the 
oviduct  at  least  three  and  one  half  weeks  after  coition.  It  seems  not  improb- 
able that  within  the  female  organs  their  environment  is  favorable  to  a  some- 
what prolonged  existence.  In  this  connection  it  is  of  interest  to  know  that 
spermatozoa  capable  of  fertilizing  have  been  known  to  live  within  the  recep- 
taculum  seminis  of  a  queen  bee  for  three  years. 

Spermatozoa  are  produced  in  large  numbers.  Upon  the  basis  of  observa- 
tions in  several  individuals,  Lode  3  computes  the  average  production  per  week 
as  226,257,000,  and  In  the  period  of  thirty  years  from  twenty-five  to  fifty- 
five  years  of  age  the  total  production  as  339,385,500,000.  This  excessive 
production  is  an  adaptation  by  nature  that  serves  as  a  compensation  for  the 

1  Cf.  Walker:  Archivfiir  Anatomie  und  Physiologie,  Anatomischer  Abtheilung,  1899,  S.  313. 
2Diihrssen:  Centralblatt  fur  Gynakologie,  1893,  xvii.  S.  592. 
8  A.  Lode:  Pftiige^s  Archivfiir  die  gesammte  Physiologic,  1891,  1. 


REPR  OD  UCTION.  445 

small  size  of  the  cells  and  the  small  chance  of  every  cell  finding  an  ovum. 
Without  large  numbers  fertilization  would  not  be  ensured  and  the  continu- 
ance of  the  species  would  be  endangered. 

Maturation  of  the  Spermatozoon. — Considerable  theoretical  inter,  st 
attaches  to  the  question  of  the  real  morphological  value  of  the  spermatozoon. 
It  is  undoubtedly  a  cell,  and  has  arisen  by  division  from  one  of  the  testicular 
cells,  called  the  spermatocyte  or  sometimes  the  mother-cell  of  the  spermato- 
zoon. l>ut  is  it  the  morphological  equivalent  of  one  of  the  mother-cells? 
In  most  animals,  and  probably  also  in  man,  each  spermatocyte  gives  rise  to 
four  spcrmatids,  which  grow  directly  into  four  spermatozoa.  The  process  of 
derivation  of  the  spermatozoa  may  be  called,  by  analogy  with  the  process  in 
the  ripening  of  the  ovum,  maturation.  The  details  and  essence  of  the  process 
have  been  much  discussed.  Van  Beneden  found  in  an  interesting  worm, 
Ascaris,  that  the  number  of  chromosomes  in  the  nucleus  of  a  single  sperma- 
tozoon is  only  half  that  in  the  original  testicular  cell ;  that  is,  the  process  of 
maturation  of  the  spermatozoon  consists  in  a  reduction  of  the  chromosomes 
by  one-half.  This  discovery  has  since  been  extended  to  many  other  forms, 
including  mammals  and  man,1  and  it  has  been  shown  further  that  the  mature 
spermatozoon  contains  only  one-half  of  the  number  of  chromosomes  charac- 
teristic of  the  tissue-cells  of  the  species  in  question.  In  the  light  of  the  sub- 
sequent process  of  fertilization  these  facts  are  interesting.  Following  Hert- 
wig  and  Strasburger,  who  regard  the  chromatic  substance  of  the  nucleus  as 
the  bearer  of  the  hereditary  qualities,  many  biologists  now  interpret  this 
halving  of  the  chromatin  as  a  provision  for  the  reduction  of  the  hered- 
itary mass,  which  later  will  he  restored  to  its  full  amount  by  union  with 
the  egg.  As  we  shall  see,  the  maturation  of  the  ovum  follows  a  some- 
what similar  course,  and,  since  the  process  has  been  more  fully  studied 
there,  we  shall  reserve  further  discussion  until  that  subject  is  reached 
(p.  451). 

Senien. — Semen  consists  of  spermatozoa,  together  with  liquid  and  dissolved 
solids,  coming  partly  from  the  testes  themselves,  but  secreted  chiefly  by  the 
accessory  sexual  glands — namely,  the  glands  within  the  vam  deferentia,  the 
seminal  vesicles,  the  prostate  gland,  and  Cowper's  glands.  It  is  a  whitish, 
viscid,  alkaline  fluid,  with  a  slight  characteristic  odor.  The  amount  passed  out 
at  any  one  time  has  been  estimated  at  between  0.5  and  6  cubic  centimeters.  Its 
chemical  composition  has  not  been  examined  exhaustively.  Besides  water,  it 
contains  approximately  18  per  cent,  of  solid  substances,  which  comprise  nuclein, 
protamine,  proteids,  xanthin,  lecithin,  cholesterin,  and  other  extractives,  fat,  and 
sodium  and  potassium  chlorides,  sulphates,  and  phosphates.  Under  proper  treat- 
ment colorless  crystals,  called  Bottcher's  crystals,  may  be  obtained  from  semen. 
They  appear  to  be  a  phosphate  of  a  nitrogenous  base,  which  has  been  called  sperm- 

1    •        •  :-    •' -•  in  it«  histolo^K"1!  i-Mthcr  tli.-m  its  chemical 

features.     The  fluid  ] .;>,   .i-n  s< :  i  the  trar  m  of  an<- 

sibly  also  for  the  nutrition  of  the  ripe  spermatozoa.     C-- 

1  v.  Burdeleben  :  U-<- 


446  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

seminal  granules,  exist  in  semen.  They  are  possibly  parts  of  nuclei  of  disin- 
tegrated cells.  Comparatively  little  is  known  of  the  composition  or  the  specific 
function  of  the  individual  secretions  contributed  by  the  various  organs.  The 
disintegration  of  the  nutritive  cells  of  the  testis  probably  furnishes  some  of  the 
nutritive  substance  of  the  liquid.  Prostatic  secretion  is  viscid,  opalescent,  and 
usually  alkaline,  and  contains  1.5  per  cent,  of  solids,  comprising  mainly  pro- 
teids  and  salts.  It  contributes  at  least  a  portion  of  the  substance  of  Bottcher's 
crystals  to  the  semen,  and  their  partial  decomposition  is  said  to  be  responsible 
for  the  characteristic  odor  of  the  seminal  fluid.  The  secretion  from  the 
seminal  vesicles  is  fairly  abundant,  is  albuminous,  .and  in  some  animals  at 
least,  such  as  the  rodents,  seems  to  contain  fibrinogen.  This  enables  the  fluid 
to  clot  after  its  reception  in  the  female  passages,  and  thus  to  prevent  loss  of 
spermatozoa.  Camus  and  Gley l  find  that  this  coagulation  is  caused  by  a 
specific  ferment  present  in  the  prostatic  fluid.  Cowper's  glands  secrete  a 
mucous  fluid.  By  careful  experiments  upon  white  rats  Steinach 2  has  shown 
that  removal  of  the  seminal  vesicles  and  the  prostate  gland,  while  not  dimin- 
ishing sexual  passion  and  the  ability  to  perform  the  sexual  act,  including 
the  actual  discharge  of  spermatozoa,  prevents  entirely  the  fertilization  of  the 
ova ;  removal  of  the  seminal  vesicles  alone  markedly  weakens  the  fertilizing 
power  of  the  semen.  Under  normal  circumstances  the  secretions  of  these 
accessory  glands  are  essential  to  the  motility  of  the  spermatozoa,3  and  they 
may  have  other  important  functions.  Ivanoff,4  however,  has  been  able  to 
impregnate  dogs,  rabbits,  and  guinea-pigs  artificially  by  injecting  into  the 
vagina  spermatozoa  taken  directly  from  the  epi<l  lymis  and  mixed  with  a  0.5 
per  cent,  solution  of  sodium  carbonate. 

The  Testis. — The  testes  (Fig.  219,  t)  are  com  pound  tubular  glands  with 
a  unique  structure.  Formed  early  in  embryonic  life  as  solid  structures,  with 
th  "5  seminiferous  tubules  (ts)  represented  by  solid  co'rdjS  of  cells,  they  remain 
in  the  embryonic  condition  until  the  time  of  puberty.  Some  of  the  cells, 
the  mother-cells  of  the  spermatozoa,  then  begin  actively  to  divide,  and  the 
result  of  division  with  differentiation  is  the  mature  spermatozoa.  These 
latter  accumulate  at  the  centre  of  the  tubules,  the  walls  being  formed  largely 
of  the  dividing  cells  or  immature  spermatozoa.  Other  cells  do  not  produce 
spermatozoa.,  but  seem  to  disintegrate  and  give  rise  to  the  nutritive  fluid  .and 
nuclear  particles  that  are  found  mixed  with  the  sperm-cells.  From  the  time 
of  puberty  on,  usually  throughout  life,  this  cellular  activity  proceeds,  the 
rate  and  regularity  probably  varying  greatly  with  individuals  m  and  Depend- 
ing largely  on  the  frequency  of  discharge  of  the  semen.  Spermatozoa  may 
be  wanting  in  old  men,  but  they  have  been  found  in  individuals  at  eighty 
or  ninety  years  of  age.  The  spermatozoa  accumulate  within  the  seminal 

1  damns  and  Gley  :  Comptes  rendus  de  la  Societe  de  -Biologic,  1896,  p.  787,  and  1897,  p.  787. 

2E.  Steinach:  Pfluget's  Archiv  fur  die  gesammle  Physiologit,  1894,  Ivi.  Cf.  also  Kehfisch : 
Deutsche  medicinische  Wochenschri/t,  1896,'xxii.  S.  24?  ;  and  Lode:  Siteu-ngsber.  d.  Kais.  .  I 

Wiss.  Wien.  Math,  naturw.  CL,  1895,  civ.,  Abth.  iii. 

*df.  Walker:  Archiv  fur  Anatomie  und  Physiologic,  Anatomisclier  Abtheilung,  1899,  S.  313. 

*  Ivanoff:  Journal  de  Physiologie  et  de  Pathologic  generate,  1900,  ii.  p.  95. 


REPRODUCTION.  449 

and  is  covered  by  a  layer  of  muscular  fibres  constituting  a  distinct  muscle — the 
bulbs  of  the  corpora  cavernosa  by  the  ischio-cavemosi  (erectores  penis),  that  <>f 
the  corpus  spongiosum  (called  bulbus  urethras)  by  the  bulbo-cavemosus  (accel- 
erator urince).  At  its  distal  end  each  corpus  cavernosum  terminates  bluntly, 
while  the  corpus  spongiosum  projects  farther  and  enlarges  to  form  the  extrem- 
ity of  the  organ,  the  glans  penis.  Each  corpus  is  spongy  in  consistence,  being 
formed  of  a  trabecular  framework  of  white  and  elastic  connective  tissue  and 
plain  muscular  fibres,  with  cavernous  venous  spaces,  and  it  is  covered  by  a 
tough  fibrous  tunic.  When  the  spaces  are  distended  with  blood  the  whole 
organ  becomes  hard,  rigid,  and  erect  in  position.  The  mechanism  of  erection 
will  be  studied  more  in  detail  later  (p.  463).  The  penis,  especially  toward 
its  termination,  is  beset  with  end-bulbs,  Pacinian  bodies,  and  other  nerve-ter- 
minations, which  make  it  particularly  sensitive  to  external  stimulation. 

0.  THE  FEMALE  REPRODUCTIVE  ORGANS. 

The  female  reproductive  organs,  already  mentioned,  have  as  their  specific 
functions  the  production  of  the  essential  female  germ-cells,  the  ova,  and  their 
transference  to  the  uterus,  and,  if  unfertilized,  to  the  outside  world ;  if  the 
ova  are  fertilized,  other  specific  functions  are  the  protection  and  nutrition  of 
the  developing  embryo,  its  ultimate  transference  to  the  outside  world,  and 
the  nutrition  of  the  child  during  early  infancy. 

The  Ovum. — The  human  ovum  was  discovered  in  1827  by  von  Baer,  and 
it  was  he  who  first  completely  traced  the  connection  between  ova  in  the  gene- 
rative passages  and  ova  in  the  Graafian 
follicles  of  the  ovary.  The  conception 
of  ova  as  the  essential  female  element 

had,  however,  long  been  held,  and  Har-  _ 

vey's  dictum  of  the  seventeenth  century,       f--  \ 

that  everything  living  is  derived  from      /  — -<l 

an  egg  (omne  vivum  ex  ovo),  is  well 
known.  The  human  ovum,  as  it  comes 
from  the  ovary,  is  a  spherical,  proto- 
plasmic cell  (Fig.  220),  averaging  with 
the  zona  radiata,  approximately  0.2  milli- 
meter (y^-y  inch)  in  diameter.  As  in 
other  cells,  the  cell-body  may  be  distin- 
gnished  from  the  nucleus,  the  proto- 

plasm   of  the   former   being   Called    Cyto-     the  amoeboid  nucleolus  (germinal  spot) ;  d,  deu- 
7  -r       .,      ft  ,i  toplasmic  zone;  p,  protoplasmic  zone;  z,  zona 

plasm.     In  its  finer  structure  the  cyto-    radiata ; «,  peri^iteiiL  s^ce. 
plasm  consists  of  an  excessively  delicate 

network  of  protoplasmic  substance.  As  in  other  mammalian  eggs,  it  proba- 
bly contains,  adjoining  the  nucleus,  a  minute,  specially  differentiated  portion, 
consisting  of  a  single  or  double  centrosome  surrounded  by  an  attraction  sphere 
(Fig.  221,  A).  For  some  distance  inward  from  the  border  the  cytoplasm  is 
pure  and  transparent,  and  this  portion  is  often  called  the  protoplasmic  zone 

Vol..  II.— 29 


450  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

(Fig.  220,  p).  Throughout  the  centre  of  the  cell,  however,  it  is  obscured  by 
the  presence  of  an  abundance  of  yolk-substance,  or  deutoplasm,  from  which 
the  corresponding  part  of  the  ovum  is  sometimes  called  the  deutoplasmic 
zone  (d).  Deutoplasm  is  non-living  substance;  it  consists  of  granules  of 
yolk  imbedded  in  the  meshes  of  the  cytoplasmic  network,  and,  like  its  ana- 
logue, the  yolk  of  the  hen's  egg,  it  serves  as  food  for  the  future  cells  of  the 
embryo. 

A  comparison  of  the  respective  amounts  of  food  in  the  human  and  the 
fowl's  egg,  with  the  manner  of  embryonic  development,  is  suggestive.  The 
chick  develops  outside  the  body  of  the  hen,  and,  therefore,  requires  a  large 
supply  of  nutriment,  which  it  finds  in  the  yolk  and  the  white  of  the  egg.  The 
child  develops  within  the  mother's  body  and  receives  its  nourishment  from  the 
maternal  blood ;  hence  the  supply  of  food  within  the  egg  is  only  enough  to 
ensure  the  beginning  of  growth,  special  blood-vessels  being  formed  to  facilitate 
its  continuance. 

The  nucleus  (n),  frequently  called  by  its  early  name,  the  germinal  vesicle,  is 
spherical,  and  usually  occupies  a  slightly  eccentric  position.  Its  protoplasm 
consists  of  a  network  composed  of  two  kinds  of  material :  the  more  delicate, 
slightly  staining  threads  are  the  achromatic  substance,  the  coarser,  deeply 
staining  portion,  the  chromatic  substance  or  chromatin.  The  former  is  con- 
tinuous with,  and  probably  of  exactly  the  same  nature  as,  the  cytoplasm. 
The  chromatin  is  peculiar  to  the  nucleus,  and  at  certain  stages  in  the  nuclear 
history  is  resolved  into  distinct  granules  or  filaments,  the  chromosomes  (Fig. 
221,  A),  the  number  of  which  in  the  human  ovum  before  maturation  is 
thought  to  be  sixteen.  There  is  every  reason  for  believing  that  the  chromatin 
is  the  bearer  of  whatever  is  inherited  from  the  mother.  The  nucleus  is 
limited  by  a  nuclear  membrane,  and  contains  a  strongly  marked  nucleolus, 
which  has  likeAvise  retained  its  original  name  of  germinal  spot.  There  is 
probably  no  proper  cell-wall,  or  vitelline  membrane,  such  as  is  said  to  exist  in 
many  mammalian  and  other  eggs.  The  ovum  is,  however,  surrounded  by  a 
thick,  tough,  transparent  membrane  of  ovarian  origin,  about  0.02  millimeter 
(TTTO  incn)  in  thickness,  and  called  the  zona  radiata  or  zona  pellucida  (Fig. 
220,  z).  It  is  pierced  by  a  multitude  of  fine  lines  radiating  from  the  surface 
of  the  zona  to  the  ovum ;  these  are  thought  to  represent  pores,  to  contain  fine 
protoplasmic  processes  of  the  surrounding  ovarian  cells,  and  thus  to  serve  as 
channels  for  the  passage  of  nutriment  to  the  egg.  Between  the  zona  radiata 
and  the  ovum  a  narrow  space,  the  perivitelline  space  (s),  exists.  Attached  to 
the  outside  of  the  zona  radiata  are  usually  patches  of  cells  derived  from  the 
discus  proligerus  of  the  Graafian  follicle  of  the  ovary,  which  may  form  a  com- 
plete covering  and  constitute  the  corona  radiata.  They  disappear  soon  after 
the  egg  is  discharged  from  the  ovary. 

Regarding  the  chemistry  of  the  mammalian  ovum  little  is  known  definitely, 
and  of  the  human  ovum  nothing  whatever  except  by  inference  from  the  eggs 
of  lower  animals.  The  protoplasmic  basis  undoubtedly  resembles  other  undif- 
ferentiated  protoplasm  in  its  general  composition,  with  an  abundance  of  proteid 


REPR  OD  UCTION.  45 1 

among  its  solid  constituents.  Dentoplasm  is  a  rich  mixture  of  food-substance 
in  concentrated  form,  and  contains  among  its  solids  probably  vitellin,  nuclein, 
albumin,  lecithin,  fats,  carbohydrates,  and  inorganic  salts. 

The  form  and  the  structure  of  the  egg  suggest  the  part  that  it  plays  in 
reproduction.  It  is  not  locomotor ;  in  fertilization  it  is  the  passive  element ; 
it  remains  in  its  place  and  is  sought  by  the  spermatozoon.  Its  nucleus  is  the 
equivalent  of  that  of  the  spermatozoon.  Its  form  renders  easy  the  entrance  of 
the  male  element.  Its  bulk  consists  largely  of  food  in  a  very  concentrated 
form,  and,  as  development  proceeds,  it  supplies  this  food  to  the  growing  cells. 

In  lower  forms  of  animal  life,  where  eggs  are  fertilized  outside  the  body 
of  the  parent  in  the  water  into  which  they  are  set  free,  they  are  usually  pro- 
duced in  enormous  numbers.  Some  fail  of  fertilization,  while  others  are 
destroyed  by  enemies,  and  the  large  number  is  a  compensatory  adaptation  by 
nature  for  their  poor  chance  of  survival.  In  mammals  and  man,  however, 
ova  have  a  much  better  opportunity  of  being  fertilized  and  of  developing  into 
adults,  and  their  number  is  correspondingly  reduced.  Their  relative  fewness, 
as  compared  with  the  spermatozoa,  is  in  harmony  with  their  larger  size  and 
the  fact  that,  while  awaiting  fertilization,  they  are  carefully  protected  within 
the  body  of  the  mother. 

Maturation  of  the  Ovum. — Attention  has  been  called  to  the  maturation 
of  the  spermatozoon.  The  ovum  undergoes  an  analogous  process  of  ripening, 
which  has  been  studied  very  carefully,  and  from  its  theoretical  interest  has 
given  rise  to  a  large  amount  of  discussion.  Maturation  begins  approximately 
as  the  ovum  is  leaving  the  ovary,  and  is  not  completed  until  after  the  ovum 
has  received  the  spermatozoon,  although  the  exact  time- relations  in  the 
human  species  are  not  yet  determined.  It  consists  of  a  mitotic  division  of 
the  nucleus,  essentially  like  mitosis  (karyokinesis)  in  ordinary  cell-division,  and 
an  expulsion  of  one  portion  from  the  cell.  This  occurs  twice  in  succession. 
The  cast-off  bits  of  protoplasm  are  known  as  polar  bodies.  The  details  of  the 
process  of  maturation  are  as  follows  (Fig.  221) :  In  all  animals  the  nucleus 
of  the  ovarian  ovum,  or  oocyte,  at  the  time  of  its  formation  receives  from  its 
mother-cell  the  same  number  of  chromosomes  as  the  ordinary  tissue-cells  con- 
tain. These  constitute  its  chromatic  reticulum.  As  the  oocyte  prepares  for 
maturation  the  chromatin  is  resolved  into  masses,  the  number  of  which  is  one- 
half  that  of  the  somatic  chromosomes.  In  a  large  number  of  species,  such  as 
many  of  the  worms,  the  insects,  and  the  crustaceans,  each  chromatic  mass 
constitutes  a  group  of  chromosomes,  usually  four  in  each  group,  which  is 
called  a  "quadruple-group"  or  "tetrad"  (E).  The  number  of  tetrads  is 
hence  one-half  the  number  of  original  chromosomes,  while  the  total  number 
of  chromosomes  in  the  nucleus  at  this  stage  is  double  the  original  number. 
The  nucleus  moves  from  its  position  in  the  interior  of  the  egg  toward  the  sur- 
face, and  the  nuclear  membrane  begins  to  disappear.  At  the  same  time  the 
two  minute  cytoplasmic  structures,  the  centrosomes,  which  lie  close  beside 
the  nucleus,  separate  and  take  up  positions  at  a  considerable  distance  apart 
from  each  other,  in  some  cases  even  upon  opposite  sides  of  the  nucleus.  The 


452  AN  AMERICAN   TEXT-BOOK  OF  PHYSIOLOGY. 


FIG.  221.— Stages  in  the  maturation  of  the  ovum ;  diagrammatic  (mainly  from  Wilson) :  A,  the  orig- 
inal ovarian  ovum ;  n,  its  nucleus,  containing  four  chromosomes ;  c,  its  double  centrosome,  surrounded 
by  the  attraction  sphere ;  in  if  much  of  the  chromatin  has  begun  to  degenerate ;  the  rest  has  become 
arranged  into  two  quadruple  groups  of  chromosomes,  or  tetrads ;  the  formation  of  the  spindle  and  the 
asters  has  begun;  in  Cthe  first  polar  amphiaster,  bearing  the  chromosomes,  is  completed ;  in  D  the  am- 
phiaster  has  become  rotated  and  has  travelled  toward  the  surface  of  the  ovum ;  g.  v,  the  degenerated 
remains  of  the  nucleus ;  in  E  the  division  of  the  tetrads  into  double  groups  of  chromosomes,  or  dyads, 
has  begun,  and  the  first  polar  body,  p.  6»,  is  indicated  ;  in  JPthe  first  polar  body,  containing  two  dyads,  has 
been  extruded ;  the  formation  of  the  second  polar  amphiaster  has  begun ;  in  G  the  first  polar  body  is  pre- 
paring to  divide ;  the  second  poter  amphiaster  is  fully  formed ;  in  Hthe  division  of  the  dyads  into  single 
chromosomes  in  both  the  first  polar  body  and  the  egg  has  begun,  and  the  second  polar  body,  p.  63,  is  in- 
dicated; in  I  the  formation  of  the  polar  bodies  is  completed ;  9,  the  egg-nucleus,  containing  two  small 
chromosomes,  one-half  the  original  number.  In  fertilization  the  spermatozoon  will  bring  in  two  addi- 
tional chromosomes,  thus  restoring  the  total  number  of  four. 


REPROD  UCTION.  \  53 

substance  lying  between  them — either  the  cytoplasm!*-  network  or  the  achro- 
matic substance  of  the  nucleus — loses  its  reticular  appearance,  becomes  fila- 
mentous, and  arranges  itself  in  the  form  of  a  spindle  with  the  threads  extend- 
ing from  pole  to  pole  (C,  I>).  The  groups  of  chromosomes  become  attached  to 
the  spindle  threads  midway  between  the  poles.  At  each  pole  there  mav  lie  a 
centrosome,  and  about  it  the  cytoplasm  may  become  arranged  in  the  form  of 
a  star,  the  «,s/<r,  though  these  structures  are  not  universal  among  species.  The 
spindle  with  the  two  asters  is  known  as  the  polar  ampkiaxta-,  and  the  com- 
plicated structure  seems  to  be  formed,  as  in  ordinary  cell-division,  for  the 
sole  purpose  of  dividing  the  nucleus  into  two  portions.  This  is  now  per- 
formed (E)  :  each  quadruple-group  of  chromosomes  splits  into  two,  and  these, 
known  as  "double-groups,"  or  "dyads,"  separate  from  each  other  and  pass 
toward  the  poles  of  the  spindle.  The  nucleus  is  thus  divided  into  halves. 
While  the  division  has  been  proceeding,  the  spindle  has  wandered  halfway 
outside  the  egg,  and,  when  it  is  completed,  one  of  the  resulting  nuclear  halves, 
comprising  one-half  of  the  full  number  of  dyads,  together  with  the  centro- 
some and  the  aster,  finds  itself  entirely  extruded  from  the  egg  and  lying 
within  the  perivitelline  space.  It  is  known  as  the  first  polar  body  (F,  p.  61). 
The  diminished  nucleus  within  the  ovum  proceeds  at  once  to  undergo  a  second 
rnitotic  division  (G,  H,  /) ;  each  of  the  remaining  dyads  divides  into  two 
single  chromosomes,  which  are  separated  from  each  other ;  and  a  second  polar 
body  (p.  62),  containing  one-half  the  number  of  single  chromosomes  charac- 
teristic of  the  tissue-cells,  is  extruded.  Apparently  the  two 'polar  bodies  are 
of  no  further  use.  In  many  animals  the  first  divides  into  two,  but  sooner  or 
later  both  degenerate  and  disappear.  The  remnant  of  the  nucleus  left  within 
the  egg,  much  reduced  in  size,  wanders  back  to  the  interior.  In  the  mam- 
mals no  true  tetrads  are  formed,  and  a  considerable  interval  of  time  elapses 
between  the  formation  of  the  two  polar  bodies,  during  which  the  sperma- 
tozoon enters  the  egg.  But  in  them  the  process  of  maturation  is  the  same  in 
essence  as  in  the  lower  animals.  In  all  species  the  chromosomes  are  reduced 
to  one-half  the  number  belonging  to  the  ovarian  ovum  ;  in  many  species  they 
are  then  resolved  again  into  scattered  chromatic  substance.  The  nucleus 
develops  a  membrane  and  again  enters  the  resting  stage.  It  is  known  hence- 
forth as  the  egg-nucleus,  or  female  pronucleus,  and  it  awaits  the  coming  of  the 
spermatozoon.  According  to  most  observers,  its  centrosome  gradually  degen- 
erates and  disappears. 

Thus  the  curious  process  of  maturation  of  the  ovum  is  different  in  detail 
from  that  of  maturation  of  the  spermatozoon.  In  the  latter  the  spermatoey  t«- 
divides  into  four  functional  spermatozoa;  in  the  former  the  oocyte  divides 
into  two  functionless  polar  bodies  (or,  by  subdivision  of  the  first,  three,  which 
have  been  called  abortive  eggs)  and  one  functional  ovum.  It  is  entirely 
probable,  however,  that  the  essence  of  the  process  is  exactly  the  same  in  the 
two  eases,  and  lies  in  the  reduction  of  the  number  of  the  chromosomes.  The 
three  import.  LOW  been  demonstrated  in  a  large  number  of 

species,  viz. --that   in  tin'  maturation  of  both  the  ovum  and  the  spermatozoon 


454  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  number  of  chromosomes  is  halved,  that  the  number  in  the  two  mature 
germ-cells  is  the  same,  and  that  this  number  is  one-half  that  of  the  chromo- 
somes of  the  somatic  cells.  It  is  wholly  probable  that  these  facts  are  uni- 
versal in  sexual  reproduction.  Each  mature  germ-cell,  therefore,  while  in 
reality  a  cell,  is,  when  compared  with  the  somatic  cells,  incomplete.  The 
subsequent  union  of  the  two  in  fertilization  restores  the  chromosomes  to  their 
normal  number.  Inasmuch  as  the  chromatin  is  probably  the  all-important 
constituent  of  the  germ-cells,  the  bearer  of  the  paternal  and  the  maternal 
inherited  characteristics,  the  phenomena  of  maturation  are  of  great  interest. 
Many  biologists  follow  Hertwig  and  Strasburger  in  regarding  maturation  as 
an  adaptation  for  the  prevention  of  the  constant  increase  in  quantity  of  the 
hereditary  substance  that  would  otherwise  take  place  with  ev^ry  union  of 
ovum  and  spermatozoon.  Without  a  reducing  process  the  quantity  of  chro- 
matin in  cells  would  become  in  a  very  few  generations  inconveniently  great. 
The  most  striking  feature  of  maturation,  however,  is  the  halving  of  the  num- 
ber of  chromosomes.  The  significance  of  this  is  not  clear.  Nevertheless  it 
is  evident  that  maturation  is  a  preparation  of  each  germ-cell  for  union  with 
its  mate.1 

The  Ovary ;  Ovulation. — The  ovaries  (Fig.  222,  o)  are  often  spoken  of 
as  glands,  but  they  are  not  glands  according  to  the  ordinary  histological  and 
physiological  use  of  the  term.  They  are  solid  organs  with  a  structure 
peculiar  to  themselves,  and  their  function  is  the  production  of  ova.  Their 
stroma  consists  of  fine  connective  tissue  with  numerous  connective-tissue 

^x  cells.  The  ova  are  developed  in  the  interior  within  cavities  called,  from 
their  discoverer,  Graafian  follicles  (Gf\  from  primitive  ova  that  are  modified 
cells  of  the  germinal  epithelium  of  the  embryo.  It  has  been  calculated 
that  a  single  human  ovary  at  the  age  of  seventeen  years  contains  17,600 
primitive  ova,2  but  that  not  more  than  400  of  these  arrive  at  maturity.3 
Each  Graafian  follicle  is  lined  by  an  epithelial  layer  several  cells  thick, 
the  membrana  granulosa,  and  is  filled  with  a  clear,  serous,  viscid  liquid,  the 
liquor  folliculi.  Imbedded  in  the  epithelium  upon  one  side  is  usually  a 
single  ovum,  completely  surrounded  by  the  cells  and  forming  a  prominent 
hillock  which  projects  well  into  the  cavity  of  the  follicle.  The  epithelium 
immediately  surrounding  the  ovum  is  the  discus  proligerus.  Within  the 
discus  the  ovum  grows  and  becomes  surrounded  by  the  zona  pellucida.  In 
the  process  of  growth  the  Graafian  follicle  approaches  the  surface  of  the  ovary, 
and  finally  comes  to  form  a  minute  rounded  vesicular  projection  covered  only 

-— -Jjy  the  ovarian  epithelium.  When  fully  ready  for  discharge,  the  wall  of  the 
follicle  becomes  ruptured,  probably  by  the  increasing  pressure  of  the  contained 
liquid,  and  the  ovum  with  its  zona  pellucida  and  a  portion  or  all  of  the  discus 
proligerus,  now  called  the  corona  radiata,  is  cast  out  upon  the  surface  of  the 

1  For  a  critical  discussion  of  maturation,  see  Wilson  :  The  Cell  in  Development  and  Inheiii- 
ance,  1900,  2d  ed.,  New  York. 

2  Heyse  :  Archiv  filr  Gynakologie,  1897,  liii.  S.  321. 

3  Henle :  Handbuch  der  Anatomie,  1873. 


REPRODUCTION. 


455 


ovary  to  be  taken  up  by  the  Fallopian  tube.  The  empty  follicle  undergoes 
changes  and  becomes  the  corpus  luteum  (c.l).  Usually  the  corpus  luteum  de- 
generates within  a  few  days  and  ultimately  disappears.  If,  however,  pregnancy 
follows  ovulation,  it  grows  very  large,  perhaps  because  of  the  congested  state 
of  the  reproductive  organs,  and  remains  for  months  before  the  retrograde 
metamorphosis  sets  in.  Not  all  Graafian  follicles  reach  maturity  and  burst, 
for  many,  after  developing  to  a  considerable  size,  undergo  degenerative 
changes,  characterized  by  liquefaction  and  disappearance  of  their  contents. 

The  discharge  of  the  ovum  is  known  technically  as  ovulation.  In  most 
animals  ovulatiou  is  a  periodic  phenomenon  accompanying  certain  seasons,  and 
is  marked  by  general  sexual  activity.  In  woman  and  many  domesticated  ani- 
mals the  relation  to  the  seasons  no  longer  exists,  but  too  little  is  known  of  the 
causes  and  time-relations  of  the  phenomenon  and  its  general  bearings  upon 
other  physiological  processes,  notably  upon  menstruation  in  woman.  A  large 


FIG.  222.— Diagram  of  the  female  reproductive  organs  (modified  from  Henle  and  Symington) :  o,  ovary; 
G.f,  Graafian  follicle  containing  an  ovum;  c.l,  corpus  luteum;  p,  parovarium ;  /,  fimbriated  end  of  F.t* 
Fallopian  tube ;  u,  body,  and  c,  cervix  of  uterus ;  o.e,  os  uteri  externum ;  vg,  vagina;  h,  hymen ;  u,  open- 
ing of  urethra ;  v,  vulval  cleft ;  n,  labia  minora,  or  nymphse ;  l.m,  labia  majora. 

but  not  wholly  decisive  literature  upon  the  subject  in  the  human  being  has 
been  written.     It  is  a  common  belief,  originating  in  the  seventeenth  century, 
that  ovulation  in  woman  is  a  periodic  phenomenon  occurring  regularly  every 
month  and  contemporaneous  with  the  occurrence  of  the  menstrual  flow,  and 
numerous  post-mortem  observations  of  the  preseiv 
discharged  Graafian  follicles  at  the  menstrual  ]><• 
frequent  coincidence  of  the  two  phenomena.     Bu 
menstruation,  though  probably  usual,  i« 


456  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

times,  for  inter  menstrual  observations  of  fresh  ovarian  scars  are  not  rare,  and 
prove  without  doubt  that  discharge  of  an  ovum  may  occur  at  any  time  between 
two  successive  periods  (see  under  Menstruation,  p.  457).  Graafian  follicles 
develop  even  during  infancy ;  most  of  them,  and  perhaps  all,  retrograde  with- 
out discharging  their  ova,  but  the  occasional  instances  of  pregnancy  at  the  ages 
of  seven,  eight,  or  nine,  prove  that  ovulation  may  occur  during  childhood. 
Ovulation  usually  begins  at  puberty,  its  commencement  thus  coinciding  with  that 
of  menstruation,  and  continues  until  the  climacteric.  After -the  climacteric  it 
may  occur  in  exceptional  cases,  although  here,  as  before  puberty,  retrogressive 
degeneration  of  the  Graafian  follicles  is  the  rule.  It  is  commonly  believed  that 
ovulation  is  at  a  standstill  during  both  pregnancy  and  lactation.  The  un- 
doubted possibility  of  a  pregnancy  originating  during  lactation  would,  how- 
ever, seem  to  prove  the  possibility  of  ovulation  during  the  latter  period.  It  is 
not  decided  whether  removal  of  the  uterus  does  away  wholly  with  ovulation. 

The  Fallopian  Tube.— Each  of  the  Fallopian  tubes  (Fig.  222,  F.  f),  or 
oviducts,  opens  into  the  peritoneal  cavity  about  one  inch  from  the  correspond- 
ing ovary.  Around  the  opening  is  an  expanded  fringe  of  irregular  processes, 
thefimbrice  (/),  one  of  which  is  attached  to  the  ovary.  The  length  of  the  tube  is 
between  three  and  four  inches,  and  the  opening  into  the  uterus  is  extremely 
small.  The  chief  structures  in  the  walls  of  the  oviducts  that  are  of  physio- 
logical interest  are  the  double  layer  of  plain  muscle,  an  outer  longitudinal 
and  an  inner  circular  coat,  longitudinal  fibres  from  which  pass  also  into  the 
fimbrise ;  and  the  cilia  with  which  the  tube  is  lined  throughout,  and  which  are 
present  also  upon  the  inner  side  of  the  fimbriaB.  The  direction  of  the  ciliary 
movement  is  from  the  ovary  toward  the  uterus.  The  primary  function  of  the 
Fallopian  tubes  is  to  convey  ova  from  the  ovary  to  the  uterus ;  they  also  con- 
vey spermatozoa  in  the  reverse  direction ;  and  within  them  the  union  of  ovum 
and  spermatozoon  usually  takes  place. 

The  mechanism  of  the  receipt  of  the  ovum  by  the  tube  is  not  fully  under- 
stood. After  ovulation  the  ovum  is  slightly  adherent  to  the  surface  of  the 
ovary  by  the  agency  of  the  viscid  liquor  folliculi.  It  is  possible,  but  it  has 
not  been  proved,  that  in  the  human  being,  as  has  been  seen  in  some  animals, 
the  expanded,  fimbriated  end  of  the  Fallopian  tube  clasps  the  ovary  when 
the  egg  is  discharged.  The  passage  of  the  ovum  into  the  tube  is  probably 
brought  about  by  the  cilia  lining  the  fimbriae.  Once  within  the  tube,  the 
ciliary  action,  assisted  perhaps  by  contraction  of  the  muscular  fibres  in  the 
walls,  carrias  the  ovum  slowly  along  toward  and  finally  into  the  uterus.  In 
some  mammals  the  passage  occupies  three  to  five  days  ;  the  time  in  woman  is 
not  definitely  known,  but  is  thought  to  be  from  four  to  eight  days. 

The  Uterus. — The  uterus  (Fig.  222,  u),  or  womb,  receives  the  ovum  from 
the  Fallopian  tube  and  passes  it  on,  if  unimpregnated,  to  the  vagina;  on  the 
r  hand,  it  receives  from  the  vagina  spermatozoa  and  transmits  them  to  the 
Ira  tubes;  it  is  the  seat  of  the  function  of  menstruation ;  when  impreg- 
ken  place,  it  retains  and  nourishes  the  growing  embryo,  and  ulti- 
he  child  from  tJik  bocryy  Its  structure  accords  with  these  func- 


REPRODUCTION.  i:,7 

tions.  Its  thick  walls  consist  largely  of  plain  muscular  tissue  arranged 
roughly  in  the  form  of  three  indistinctly  marked  layers.  Of  these,  the  exter- 
nal and  the  middle  coats  are  thin  ;  the  fibres  of  the  former  are  arranged  in 
general  longitudinally,  those  of  the  latter  more  circularly  and  obliquely. 
The  third,  most  internal  layer,  which  is  regarded  by  some  as  a  greatly  hyper- 
trophied  muvcularis  mucosce,  forms  the  greater  part  of  the  uterine  wall.  Its 
fibres  are  arranged  chiefly  circularly  ;  toward  the  upper  part  they  become  trans- 
verse to  the  Fallopian  tubes,  and  at  the  cervix  longitudinal  fibres  lie  within 
the  circular  ones.  The  individuality  of  the  muscular  layers  and  uniformity 
in  the  course  of  the  fibres  is  largely  interfered  with  by  the  numerous  blood- 
vessels of  the  uterine  walls.  The  uterus  is  lined  by  an  epithelium  composed 
of  columnar  ciliated  cells,  except  in  the  lower  half  of  the  cervix,  where  a  stratified 
non-ciliated  epithelium  exists.  The  direction  of  the  ciliary  movement  in  woman, 
as  in  other  mammals,  is  toward  the  os  uteri.1  The  mucous  membrane  is  thick, 
and  contains  very  numerous,  branching,  tubular  glands,  which  are  lined  by 
ciliated  epithelium  and  have  a  tortuous  course,  terminating  in  the  edge  of  the 
muscular  layer.  They  secrete  a  viscid,  mucous  liquid.  Between  the  glands 
are  branched  connective-tissue  cells,  which  are  not  unlike  the  connective-tissue 
cells  of  embryonic  structures,  and  wandering  cells.  Lymph-spaces  and  blood- 
capillaries  exist.  The  development  of  the  tissue  goes  on  slowly  up  to  the 
time  of  puberty,  and,  as  we  shall  see,  after  puberty  the  mucous  membrane  is 
subject  to  constant  change. 

Menstruation. — Except  during  pregnancy  the  most  striking  activities  of 
the  uterus  are  associated  with  that  peculiar  female  function  which,  from  its 
monthly  periodicity,  is  called  menstruation.  The  most  obvious  external  fact 
of  this  phenomenon  is  the  discharge  every  month  of  a  bloody,  mucous  liquid 
through  the  vagina  ;  the  most  obvious  internal  facts  are  the  bleeding  and  the 
degeneration  and  disappearance  of  a  portion  of  the  mucous  membrane  of  the 
body  of  the  uterus.  This  curious  process,  though  having  analogies  in  lower 
animals,  occurs  most  markedly  in  the  human  female,  and  from  before  the  time 
of  Aristotle  to  the  present,  among  both  primitive  and  civilized  races,  its  signifi- 
cance has  been  the  cause  of  much  speculation.  The  detailed  phenomena  of 
menstruation  are  not  as  well  known  as  they  should  be.  Experimentation  is 
practically  out  of  the  question,  and  the  opportunities  of  careful  post-mortem 
study  of  normal  healthy  uteri  at  different  stages  are  rare.  The  main  facts  are 
as  follows : 

Some  days  before  the  flow  occurs  the  mucous  membrane  of  the  body  of  the 
uterus  begins  to  thicken,  partly  by  an  active  growth  of  its  connective  tissue 
elements  and  partly  by  an  excessive  filling  of  its  capillaries  and  veins  with 
blood.  The  cause  of  this  swelling  is  not  known.  It  continues  until  the 
membrane  has  doubled  or  trebled  in  thickness,  and,  according  to  some  authori- 
ties, the  uterine  cavity  becomes  a  mere  slit  between  the  walls.  Then  occurs  an 
infiltration  of  blood-corpuscles  and  plasma,  probably  largely  by  diapedesis, 
although  possibly  assisted  by  rupture,  through  the  walls  of  the  swollen  capil- 
1Hofmeier:  CenlrcUblatt  fiir  Gyndkolugie,  1893,  xvii.  S.  764. 


458  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

laries  into  the  connective-tissue  spaces  beneath  the  epithelial  lining  of  the 
uterine  wall.  The  epithelium  is  thus  pressed  up  from  beneath,  and  begins 
rapidly  to  undergo  fatty  degeneration  in  places,  and  to  disappear.  The 
immediate  cause  of  the  degeneration  is  not  definitely  known.  The  connective- 
tissue  elements  and  the  upper  portion  of  the  glands  are  involved  to  some  extent 
in  the  degenerative  change.  The  capillaries,  thus  laid  bare,  burst,  and  the  dark 
blood  oozes  forth  and,  mixed  with  disintegrated  remains  of  the  uterine  tissues, 
with  the  mucous  secretion  of  the  uterus  and  the  vagina,  and  with  the  escaped 
lymph,  passes  away,  drop  by  drop,  from  the  body.  There  is  great  difference  of 
opinion  as  to  the  extent  of  the  destruction  of  uterine  tissue.  On  the  one  extreme 
side  are  those  who  claim  that  the  loss  of  tissue  is  normally  wholly  trivial  and 
secondary,  the  hypersemia  and  the  bloody  glandular  discharge  being  the 
important  events.  Other  authorities,  equally  extreme,  have  observed  a  disap- 
pearance of  the  whole  mucous  membrane  except  the  deepest  layers  containing 
the  bases  of  the  glands ;  this  is  probably  pathological.  From  all  the  evidence  an 
opinion  inclining  toward  the  former  view  seems  most  reasonable — namely, 
that  usually  and  physiologically  only  the  superficial  portion  of  the  mucous 
membrane  disintegrates,  and  this  only  in  spots.1  Differences  in  the  amount 
undoubtedly  occur.  Occasionally  it  happens  that  the  membrane,  instead  of 
disintegrating,  comes  away  in  pieces  of  considerable  size.  The  term  decidua 
menstrualis  is  applied  to  the  lost  coat.  The  flow  continues  upon  an  average 
four  days  or  more.  From  observations  upon  2080  American  women  Emmet 2 
finds  the  average  duration  of  the  flow  at  puberty  to  be  4.82  days,  the  average 
in  later  life  4.66  days.  The  amount  of  blood  discharged  can  be  determined 
only  with  great  difficulty.  It  probably  varies  greatly,  but  is  commonly 
estimated  at  from  1 00  to  200  cubic  centimeters  (4  to  5  ounces).  The  blood  is 
slimy,  with  abundant  mucus ;  usually  it  does  not  coagulate.  Epithelium 
cells,  red  corpuscles,  leucocytes,  and  detritus  from  the  disintegrated  tissues, 
occur  in  it,  and  it  has  a  characteristic  odor.  As  the  flow  ceases  a  new  growth 
of  connective-tissue  cells,  capillaries,  glands,  and  from  the  glands  superficial 
epithelium,  begins,  and  the  mucous  membrane  is  restored  to  its  original 
amount.  Whether  a  resting  period  follows  before  the  succeeding  tumefaction 
occurs  is  not  definitely  known,  but  it  seems  probable.  The  durations  of  the 
various  steps  in  the  uterine  changes  are  not  well  known,  and  probably  vary 
in  individual  cases.  Minot 3  suggests  the  following  approximate  times  : 

Tumefaction  of  the  mucosa,  with  accompanying  structural  changes 5  days. 

Menstruation  proper 4 

Eestoration  of  the  resting  mucosa 7 

Besting  period 12 

Total  ../..:. 28  days. 

The  menstrual  changes  in  the  uterus  are  accompanied  by  characteristic 
phenomena  in  other  parts  of  the  body.     The  Fallopian  tubes  are  congested, 
^ee  Westphalen  :  Archivfilr  Gynakologie,  1896,  lii.  S.  35  ;  and  Mandl:  Ibid..  S.  557. 
*T.  A.  Emmet  •   The  Principles  and  Practice  of  Gynaecology;  1880,  2d  ed. 
*C.  S.  Minot:  Human  Embryology,  1892. 


REPRODUCTION.  459 

and,  according  to  some  authorities,  their  mucous  membrane  degenerates  and 
bleeds  like  that  of  the  uterus.  The  ovaries  are  likewise  congested.  As  has 
been  stated,  it  is  commonly  believed,  but  not  definitely  proved,  that  ovulation 
accompanies  each  period.  Frequent  accompaniments  are  turgescence  of  the 
breasts,  swelling  of  the  thyroid  and  the  parotid  glands  and  the  tonsils,  cortges- 
tion  of  the  skin,  dull  complexion,  tendency  toward  the  development  of  pigment, 
and  dark  rings  about  the  eyes.  The  skin  and  the  breath  may  have  a  character- 
istic odor.  In  singers  the  voice  is  often  impaired,  which  is  one  instance  of 
a  general  nervous  and  muscular  enervation.  Mental  depression  often  exists. 
Pain  is  a  frequent  accompaniment,  and  nervous  and  congestive  pathological 
phenomena  may,  at  times,  become  very  pronounced.  Recent  work  has  shown 
that  the  various  phenomena  accompanying  menstruation  are  evidences  of  a 
profound  physiological  change,  with  a  monthly  periodicity,  that  the  female 
human  organism  undergoes,  and  of  which  the  uterine  changes  are  only  a  part. 
Thus,  during  the  intermenstrual  period  there  is  a  gradual  increase  of  nervous 
tension  and  general  mobility,  of  vascular  tension  manifested  by  turgescence  of 
the  blood-vessels,  a  gradual  increase  of  nutritive  activity  manifested  by 
increased  production  and  excretion  of  urea  and  increased  temperature,  and 
a  gradual  increase  of  the  heart's  action  in  strength  and  rate.1  These  various 
activities  of  the  organism  usually  attain  a  maximum  a  few  days  before  the 
menstrual  flow  begins  and  then  undergo  a  rapid  fall,  which  reaches  a  minimum 
toward  the  close  of  the  flow ;  a  second  lesser  maximum  may  occur  a  few  days 
after  the  flow  ceases.  All  organic  activities  that  have  been  carefully  investi- 
gated show  evidences  of  such  a  monthly  rhythm.  It  is  not  known  that  the 
male  possesses  such  a  period. 

The  first  menstruation  is  usually  regarded  as  the  index  of  the  oncoming  of 
puberty  or  sexual  maturity,  and  in  temperate  climates  occurs  usually  at  the  age 
of  fourteen  to  seventeen.  Its  onset  is  earlier  in  warm  than  in  cold  climates,  in 
city  than  in  country  girls,  and  varies  in  time  with  food,  growth,  and  environ- 
ment. Exceptionally  menstruation  may  begin  in  infancy  or  later  than  puberty, 
and  it  has  even  been  known  to  be  wholly  wanting  in  otherwise  normal  women. 
Normally,  it  ceases  during  pregnancy,  and  probably  usually  during  lactation, 
although  there  are  frequent  exceptions  to  the  latter  rule.  In  nearly  all  cases 
complete  removal  of  the  ovaries  puts  an  end  to  menstruation.  Removal  of  the 
ovaries  and  Fallopian  tubes  diminishes  the  number  of  exceptional  cases.  The 
•  final  cessation  of  menstruation,  which  is  a  gradual  process  extending  over 
several  months,  usually  marks  the  climacteric  (menopause),  or  end  of  the 
sexual  life,  and  occurs  usually  at  the  age  of  forty-five  to  forty-eight.  Excep- 
tionally the  flow  may  cease  early  in  life  or  extend  to  extreme  old  age. 

•:n.jKirative  Physiology  of  Menstruation. — The  comparative  physiology  of 
menstruation,  although  it  has  been  studied  only  incompletely  in  a  few  domesti- 

1  Cf.  Mary  Putnam  Jacob! :  "The  Question  of  Rest  for  Women  during  Menstruation," 
Boylston  Prize  Essay,  1876 ;  C.  Keinl :  Sammlung  klinische  Vortrage,  1884,  No.  243 ;  O.  Ott : 
JVbttrv.  ea  <f  obstetrique  et  de  gynecologic,  1890,  v. ;  and  A.  E.  Giles:  Transactions  of  the 

Obsh*  '(y  of  London,  1897,  xxxix.  p.  115. 


460  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

cated  animals  and  some  monkeys,1  sheds  some  valuable  light  upon  the  phe- 
nomenon in  woman.  In  animals  lower  than  man,  in  a  wild  state,  the  desire 
and  power  of  reproduction  are  usually  limited  to  seasonal  periods.  At  such 
times  conception  is  possible,  and  probably  usually  takes  place.  Such  periods 
are  known  as  "rut,"  "heat,"  and  "oestrus."  During  the  rest  of  the  year 
sexual  activities  are  in  abeyance.  Domestication,  with  its  artificial  condi- 
tions of  regular  food-supply,  warmth,  and  care,  has  increased  productiveness 
(Darwin)  and  rendered  the  reproductive  periods  more  frequent.  If  impregna- 
tion be  prevented,  as  is  often  the  case  in  domesticated  animals,  the  periods  of 
"  heat "  appear  for  a  while  with  great  frequency  and  regularity  (monkey, 
mare,  buffalo,  zebra,  hippopotamus,  at  intervals  of  four  weeks ;  cow,  three 
weeks ;  sow,  fifteen  to  eighteen  days ;  sheep,  two  weeks ;  bitch,  twelve  to 
sixteen  weeks).  They  are  characterized  by  general  nervous  excitement,  desire 
and  power  of  conception,  congestion  and  swelling  of  the  external  genital 
organs,  and  a  uterine  discharge.  The  latter  is  scanty,  mucous,  and  bloody, 
the  amount  of  blood  increasing  in  ascending  the  evolutionary  scale.  The 
histological  processes  occurring  in  the  uterus  have  been  studied  carefully  by 
Retterer  in  the  dog  and  by  Heape  in  the  monkey.  In  the  latter  the  proc- 
esses seem  to  be  nearly  identical  with  those  of  man.  In  the  dog,  growth 
and  congestion  of  the  mucosa  occur,  and  are  followed  by  rupture  of  the  capil- 
laries, extravasation  of  blood,  and  degeneration  of  the  tissues ;  but  it  is  doubt- 
ful whether  the  epithelium  is  actually  shed.  It  is  generally  believed  that 
"  heat "  in  the  lower  mammals  is  accompanied  by  ovulation.  It  is  not  neces- 
sarily so  in  monkeys.  The  phenomena  of  "  heat "  are  thus  closely  similar  to 
those  of  human  menstruation,  the  similarity  being  most  marked  in  the 
monkeys.  In  addition  to  these  more  hidden  phenomena  there  is  present 
sexual  desire,  which  in  the  human  female  is  largely  absent  at  such  periods, 
although  it  may  be  pronounced  just  before  and  just  after  the  actual  flow. 

Theory  of  Menstruation. — The  significance  of  menstruation  is  in  great  dis- 
pute. All  modern  theories  agree  in  regarding  it  as  associated  in  some  way 
with  the  function  of  childbearing.  The  flow  was  early  believed  to  be  a  means 
employed  by  the  body  to  get  rid  of  a  plethora  of  nutriment.  This  was 
followed  by  the  well-known  hypothesis,  put  forward  especially  by  Pfliiger 
(1865).  According  to  this  hypothesis,2  the  menstrual  bleeding  and  the 
uterine  denudation  occur  for  the  purpose  of  providing  a  fresh  uterine  surface 
to  which  the  egg,  if  impregnated,  can  readily  attach  itself,  just  as,  in  graft- . 
ing,  the  gardener  provides  a  wounded  surface  upon  which  the  young  scion  is 
set,  or,  in  uniting  two  membrane-covered  tissues,  the  surgeon  first  wounds  or 
freshens  their  surfaces.  This  conception  of  menstruation  is  not  now  commonly 
accepted.  Pfliiger  regards  the  mechanism  of  the  uterine  process  to  be  as  fol- 
lows :  The  constant  growth  of  the  ovarian  cells  and  the  consequent  swelling  of 

1  Cf.  A.  Wiltshire:  British  Medical  Journal,  March,  1883;  E.  Eetterer:  Comptes  rendus  des 
stances  et  memoires  de  la  societe  de  biologie,  1892 ;  W.  Heape :    Philosophical  Transactions  of  the 
Royal  Society,  1894.  (B),  vol.  385,  pt.  i.  ;  and  Proceedings  of  the  Royal  Society,  1897,  Ix  p.  202. 

2  E.  F.  W.  Pfliiger  :   Untersuchungen  cms  dem  physiologischcn  Laboratorium  zu  Bonn,  1865, 


REPRODUCTION.  I'll 

the  ovary  subject  the  ovarian  nerve-fibres,  and  through  them  the  spinal  cord, 
to  a  constant  slight  stimulation.  Through  the  summation  of  the  stimuli  within 
the  cord  a  reflex  dilatation  of  the  vessels  in  the  genital  organs  is  produced. 
The  excessive  blood-supply  leads  in  turn  to  the  tumefaction  of  the  uterus,  and 
frequently  to  the  ripening  of  a  Graafian  follicle.  The  bleeding  follows,  and 
at  the  same  time  or  slightly  later  the  rupture  of  the  follicle  occurs,  provided 
the  latter  be  sufficiently  advanced  in  growth.  The  menstrual  flow  and  ovulation 
are,  therefore,  two  phenomena  conditioned  usually  by  the  same  cause,  namely, 
the  menstrual  congestion,  yet  either  may  occur  without  the  other.  Pfliiger's 
hypothesis  accounts  clearly  for  the  absence  of  menstruation  after  removal  of 
the  ovaries.  Numerous  other  theories  have  been  proposed,  no  one  of  which 
can  be  said  to  be  widely  and  generally  accepted.  The  present  tendency  in 
belief  is  as  follows  :  Ovulation  and  menstruation  are  in  great  part  independent 
phenomena  ;  they  may  or  they  may  not  coexist ;  the  uterine  growth  is  a  prep- 
aration for  the  future  embryo ;  the  tissue  of  the  deddua  menstrualis  is  the  fore- 
runner of  the  deddua  graviditatis  (p.  471) ;  if  an  ovum,  whenever  it  is  discharged, 
be  fertilized,  it  attaches  itself  to  the  thickened  uterine  wall,  the  tissues  become 
the  deddua  graviditatis,  pregnancy  follows,  and  the  deddua  is  not  discharged 
until  the  time  of  parturition ;  if,  however,  fertilization  does  not  take  place, 
there  is  no  attachment,  the  tissues  degenerate  and  become  the  deddua  men- 
strualis,  and  the  flow  occurs.  The  suggestion  of  Jacobi l  is  not  an  extreme 
one :  "  The  menstrual  crisis  is  the  physiological  homologue  of  parturition." 
Its  periodicity,  which  is  approximately  that  of  a  tropical  month  (27.32  days), 
has  been  the  subject  of  much  hypothesis.  In  a  suggestive  paper  based  upon 
much  careful  statistical  study  Arrhenius2  ascribes  it  to  the  influence  of 
atmospheric  electricity,  which  he  finds  to  undergo  a  periodic  variation  of 
similar  length.  Regarding  the  mechanism  of  menstruation  the  above  hy- 
pothesis of  Pfluger  seems  not  unreasonable,  and,  moreover,  seems  to  be  sup- 
ported by  the  experiments  of  Strassmann,3  who  by  pressure  artificially  pro- 
duced in  the  ovary  by  means  of  injections  into  it  of  salt  solution,  produced 
hypersemia  and  swelling  of  the  uterine  mucous  membrane,  congestion  of  the 
external  genitals,  and  mucous  and  bloody  discharges. 

The  mystery  of  menstruation  largely  ceases  when  we  recognize  what  is  un- 
doubtedly a  fact,  that  the  phenomenon  is  a  highly  developed  inheritance  from 
our  mammalian  ancestors,  and  that,  although  in  the  human  race  under  the 
influence  of  civilization  and  social  life  it  has  largely  lost  its  technical  sexual 
significance,  it  is,  nevertheless,  primarily  a  reproductive  phenomenon  derived 
directly  from  the  lower  females.  Nature  has  endowed  the  latter,  in  a  manner 
yet  unknown,  with  reproductive  periods  that  are  pronounced  in  the  wild  state 
and  are  coincident  with  certain  of  the  seasons.  A  primitive  seasonal  period 
may  perhaps  still  be  shown  in  woman  by  the  greater  proportion  of  births  that 
take  place  during  the  winter  months  than  at  other  times  of  the  year :  this  sig- 

1  Mary  Putnam  Jacobi:  American  Journal  of  Obstetrics,  1885,  xviii. 
a  Ar^ienius :  Skandinavisches  Archivfur  Physiologic,  1898,  viii.  S.  367. 
3  dtrassmann :  Archivfur  Gynakologie,  1896,  lii.  S.  134. 


462  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

nifies  greater  sexual  activity  during  the  months  of  spring,  as  is  the  case  in 
most  animals.1 

Domestication  has,  however,  interfered  with  the  original  plan  of  nature. 
It  has  rendered  the  lower  forms  more  prolific  and  has  made  more  frequent 
their  reproductive  periods.  Civilization  has  done  exactly  the  same  for  woman. 
It  has  rendered  her  more  prolific  and  has  made  more  frequent  her  reproduc- 
tive periods.  It  is  wholly  probable  that  the  menstrual  periods  of  woman  are 
the  homologues  of  the  frequent  reproductive  periods  of  the  lower  forms.  It 
has  been  seen  that  the  latter  are  characterized  by  the  same  kind  of  phenomena 
that  exist  in  the  former ;  the  characteristic  human  menstrual  phenomena  are 
least  developed  in  the  lower  mammals,  much  more  so  in  the  monkey,  and  are 
most  pronounced  in  the  human  female.  For  what  purpose  this  evolution  of 
function  has  taken  place  we  do  not  know.  Below  the  human  species  concep- 
tion is  confined  to  these  times  of  "  heat ; "  in  woman  it  is  possible  at  other 
than  her  menstrual  periods.  In  this  respect  woman  is  more  highly  endowed 
than  her  mammalian  ancestors. 

The  Vagina. — The  vagina  (Fig.  222,  vg)  is  the  broad  passage  from  the 
uterus  to  the  external  organs.  Its  walls  consist  of  smooth  muscle  fibres, 
arranged  both  circularly  and  longitudinally.  It  is  lined  by  stratified  scaly 
epithelium  and  is  surrounded  by  erectile  tissue.  Its  walls  contain  few  glands. 
Its  specific  functions  are  connected  solely  with  the  reproductive  process ;  in 
copulation  it  receives  the  penis  and  the  semen.  Its  cavity  is  the  pathway  out- 
ward for  the  products  of  menstruation  and,  in  parturition,  for  the  child. 

The  Vulva  and  its  Parts. — The  vulva  (Fig.  222)  comprises  the  genital 
organs  that  are  visible  externally — viz.  the  mons  Veneris,  the  labia  majora  (l.m), 
the  labia  minora  or  nymphce  (n),  the  clitoris,  which  is  the  diminutive  homologue 
of  the  penis  of  the  male,  and  the  hymen  (h),  or  perforated  curtain  that  guards  the 
entrance  to  the  vagina  and  is  usually  ruptured  at  the  time  of  the  first  coition. 
The  vulva  receives  the  openings  of  the  vagina,  the  urethra  (u),  and  the  ducts  of 
Bartholini's  glands.  Its  parts  are  capable  of  turgidity  through  its  rich  vas- 
cular supply,  and  perform  minor  ill-defined,  adaptive,  and  stimulating  func- 
tions in  copulation.  Their  surface  is  covered  by  mucous  membrane  which  is 
moistened  and  lubricated  by  a  secretion  from  numerous  mucous  follicles,  seba- 
ceous glands,  and  the  glands  of  Bartholini.  The  latter  are  comparable  to 
Cowper's  glands  of  the  male  and  secrete  a  viscid  liquid. 

The  Mammary  Glands. — The  mammary  glands,  being  active  only  during 
the  period  of  lactation,  may  best  be  studied  in  connection  with  that  function 
(see  Vol.  I.,  p.  261). 

Internal  Secretion. — A  priori,  the  reproductive  organs  can  scarcely  be 
regarded  as  organs  that  are  quiescent  during  the  greater  part  of  life  and  pas- 

1 "  The  largest  number  [of  human  births]  almost  always  falls  in  the  month  of  February, 

....  corresponding  to  conceptions  in  May  and  June Observations  tend  to  show  the  largest 

number  of  conceptions  in  Sweden  falling  in  June ;  in  Holland  and  France,  in  May-June  ;  in 
Spain,  Austria,  and  Italy,  in  May;  in  Greece,  in  April.  That  is,  the  farther  south  the  earlier 
the  spring  and  the  earlier  the  conceptions." — Mayo- Smith  :  Statistics  and  Sociology,  1895. 


REPRODUCTION.  463 

sively  await  the  reproductive  act.  The  view  that  they  are  more  than  this  is 
supported  by  some,  although  slight,  experimental  evidence.  Notwithstanding 
the  fact  that  removal  of  the  testis  or  the  ovary  in  adult  life  is  often  unaccom- 
panied by  great  somatic  changes,  the  profound  effects  of  early  castration  upon 
development,  in  both  the  male  and  female,  show  that  upon  the  presence  of  the 
sexual  organs  depends  the  appearance  of  many  of  the  secondary  sexual  cha- 
racters— characters  which  apparently  are  independent  of  those  organs,  and  yet 
of  themselves  distinguish  the  individual  as  specifically  masculine  or  feminine. 
The  mode  of  dynamic  reaction  of  the  sexual  organs  upon  the  other  organs  can 
at  present  be  little  more  than  hinted  at.  It  is  entirely  probable  that  such 
reaction  is  either  nervous  or  chemical,  or  perhaps  it  is  both  combined. 
Regarding  the  former  little  is  known.  Regarding  the  latter  certain  facts 
point  to  a  possible  normal  and  constant  contribution  of  specific  material  by 
the  reproductive  glands  to  the  blood  or  lymph,  and  thus  to  the  whole  body. 
Such  a  process  is  spoken  of  as  internal  secretion.  This  subject  is  discussed 
more  fully  in  Vol.  I.  p.  273. 

D.   THE  REPRODUCTIVE  PROCESS. 

Thus  far  attention  has  been  given  to  the  general  functions  of  the'  repro- 
ductive organs.  We  come  now  to  the  special  phenomena  connected  with  the 
reproductive  process  itself,  and  have  to  trace  the  history  of  the  spermatozoon, 
the  ovum,  and  the  embryo.  It  should  be  borne  clearly  in  mind  that  the 
essential  part  of  the  reproductive  process  is  the  fusion  of  the  nuclei  cf  the  two 
germ-cells.  Investigation  is  making  it  more  and  more  probable  that  the 
spermatozoon  and  the  ovum,  although  so  different  in  appearance  a  id  general 
behavior,  are  fundamentally  and  in  origin  both  morphologically  aid  physi- 
ologically equivalent  cells.  In  the  processes  of  their  growth  and  maturation 
they  are  secondarily  modified,  the  one  into  an  active  locomotive  body,  the  other 
into  a  passive  nutritive  body.  The  modifications  in  both  are  confined,  how- 
ever, to  the  cell-protoplasm  (cytoplasm  and  centrosome) ;  the  essential  parts, 
the  nuclei,  remain  unmodified  and  both  morphologically  and  physiologically 
equivalent  down  to  the  time  of  their  fusion  in  the  process  of  fertilization. 
The  many  and  complex  details  of  the  reproductive  process  exist  for  the  sole 
purpose  of  bringing  together  these  two  minute  masses  of  chromatin.1 

Copulation. — Copulation  is  the  act  of  sexual  union,  and  has  for  its  object 
the  transference  of  the  semen  from  the  genital  passages  of  the  male  to  those  of 
the  female.  It  is  preceded  by  erection  of  the  penis  and  turgidity  of  the  organs 
of  the  vulva.  These  latter  occurrences  are  in  the  main  vascular  phenomena, 
and  are  brought  aboui  by  a  distention  of  the  cavernous  spaces  of  the  erectile 
tissues  with  blood.  The  vascular  phenomena  are,  however,  accompanied  by 
complex  nervous  and  muscular  activities.  As  regards  the  penis,  the  arteries 
supplying  the  organ  relax  and  allow  blood  to  flow  in  quantity  to  the  corpora 
cavemosa  and  the  cjrpus  spongiosum.  Simultaneous  relaxation  of  the  smopth 

1  Compare  Th.  B_>veri :  "  Befruchtung,"  Merkel  und  Bonnet's  Ergebnisse  der  Anatomic  und 
Entwickelungsgeschifhte,  1892,  i. 


464  AN  AMERICAN  TEXT-BOOK   OF   PHYSIOLOGY. 

muscle  fibres  scattered  throughout  the  trabecular  framework  of  the  corpora 
increases  the  capacity  of  the  blood-spaces.  Furthermore,  the  ischio-cavernosus 
(erector  penis)  and  bulbo-cavernosus  muscles  contract  and  compress  the 
proximal  or  bulbous  ends  of  the  corpora  and  the  outgoing  veins.  The  result 
of  this  combined  muscular  relaxation  and  contraction  is  a  free  entrance  of 
blood  into  and  a  difficult  exit  from  the  vascular  spaces ;  this  leads  to  a  swelling 
and  distention  which  aid  further  in  compressing  the  venous  outlets  and,  being 
limited  by  the  tough,  fibrous  tunics  of  the  corpora,  result  in  making  the  organ 
stiff",  hard,  erect  in  position,  and  well  adapted  to  its  specific  function.  During 
the  process  of  erection  the  cresta  of  the  urethra  or  caput  gallinaginis,  which  is 
an  elevation  extending  from  the  cavity  of  the  bladder  into  the  prostatic  por- 
tion of  the  urethra  and  containing  erectile  tissue,  becomes  turgid  and,  by  the 
aid  of  the  contraction  of  the  sphincter  urethra;,  effectually  closes  the  passage 
into  the  bladder.  Erection  is  a  complex  reflex  act,  the  centre  of  which  lies 
in  the  lumbar  spinal  cord  and  may  be  aroused  to  activity  by  nervous  impulses 

om  different  directions.     Impulses  may  originate  in  the  walls  of  the 

ducts  of  the  testis  from  the  pressure  of  the  contained  semen  or  in  the  penis 

from  external  stimulation  of  the  nerve-endings  in  the  skin,  in  both  cases 

-  the  sensory  nerves  of  the  organs  to  the  spinal  centre ;  or  they 

mite  in  the  brain  and  pass  downward  through  the  cord,  the  impulses 

•orrespoiiding  to  sexual  emotions.     The  centrifugal  paths  for  the 

plHp  V-i  along  th:    nervi  erigente ,< ?,  which  are  true  vaso-dilator  nerves,  and 

in  tin  where  experiment  has  proven!  their  existence,  pass  from  the 

spinal  <     a  along  the  posterior  lumbar  (monkey)  or  anterior  sacral  (monkey, 

. >erves  to  their  arterial  distribution.  The  ischio-  and  bulbo-cavwno- 
su$  muscles  are  under  the  control  of  their  motor  nerve  supply,  consisting  of 
branches  of  the  perineal  nerve. 

In  the  female,  anatomists  recognize  the  hornologues  of  the  male  erectile 

as  follows :  the  clitoris  with  its  corpora  cavemosa  and  glans  as  the  homo- 

^^B  of  the  penis,  the  two  bulbi  vestibuli  as  that  of  the  bulb  of  the  corpus 

iKngiosum,  the  pars  intermedia  perhaps  as  that  of  the  corpus  spongiosum 

\  the  erector  clitoridis  muscle  as  the  homologue  of  the  erector  penis 

rcrnosus).  The  mechanism  of  erection  is  similar  to  that  in  the  male, 
and  the  result  is  a  considerable  degree  of  firmness  in  the  external  genital 
organs. 

The  sexual  excitement  attendant  upon  copulation  is  usually  much  greater 
in  man  than  in  woman,  and  culminates  in  the  sexua>  orgasm,  when  the  emis- 
sion of  semen  from  the  penis  into  the  vagina  c  't  will  be  remembered 
that  the  prepared  semen  is  stored  in  the  ducts  of  the  testes.     The  discharge 
of  the  fluid  is  a  muscular  act  which  begins  probably  in  the  vasa  efferentia 
•  he  canal  of  the  epididymis,  and  sweeps  along  the  powerful  muscular 
walls  of  the  vasa  defer  entia  in  the  form  of  a  series  of  peristaltic  waves.     The 
'^>  contract,  and  the  mixed  liquid  and  spermatozoa  are  poured 
through  the  ejaculatory  ducts  into  the  prostatic  portion  of  the  urethra.     The 
he  prostate  expel  the  prostatic  iiuid  and  help  to  *»ass  the  semen 


RE  PR  OD  UCTION.  1 1 J  5 

onward.  The  glands  of  Cowper  possibly  add  their  contribution.  But  the 
final  urothral  discharge  is  effected  especially  by  powerful  rhythmic  contractions 
of  the  already  partially  contracted  striped  muscles,  viz.  the  ischio-  and  hulho- 
cavernosi,  the  constrictor  urethras,  and  probably  the  anal  muscles,  the  result  of 
the  complex  series  of  actions  being  to  expel  the  semen  with  some  force  into 
the  upper  part  of  the  vagina  close  to  the  os  utei*i.  Ejaculation  is  a  reflex  net. 
The  centre  lies  in  the  lumbar  spinal  cord  ;  the  centripetal  nerves  are  the  sen- 
sory nerves  of  the  penis,  stimulation  of  the  glans  being  especially  effective ; 
the  centrifugal  nerves  are  the  nerves  to  the  various  muscles.  In  the  female 
during  ejaculation  the  glands  of  Bartholini  pour  out  a  mucous  liquid  upon  the 
vulva.  There  is  possibly  a  downward  movement  of  the  uterus,  brought  about 
by  contraction  of  its  round  ligaments  and  accompanied  perhaps  by  a  contrac- 
tion of  the  uterine  walls  themselves.  But  all  muscular  and  erectile  activity, 
as  well  as  sexual  passion,  is  usually  less  pronounced  in  woman  than  in  man. 
Locomotion  of  the  Spermatozoa. — The  union  of  the  spermatozoon  and 
the  ovum  probably  takes  place  usually  in  the  Fallopian  tube  not  far  from  its 
ovarian  end,  and  to  this  place  the  spermatozoa  at  once  proceed.  Their  mode 
of  entrance  into  the  uterus  is  not  wholly  clear ;  it  is  quite  generally  believed, 
but  without  conclusive  experimental  proof,  that  relaxation  of  the  uterus  im- 
mediately after  copulation  exerts  a  suction  upon  the  liquid  which  aids  in  its 
passage  through  the  os  and  the  cervix.  It  is  possible  that  active  contraction 
of  the  vaginal  walls  assists.  Spermatozoa  have  been  found  in  the  uterus  a 
half  hour  after  coition.1  The  main  agency  in  the  locomotion  of  the  sper- 
matozoa through  the  body  of  the  uterus  and  the  Fallopian  tubes,  and  prob- 
ably also  from  the  vagina  into  the  uterus,  is  the  spontaneous  movement  of 
the  spermatozoa  themselves.  By  the  lashing  of  their  tails  they  wriggle  their 
way  over  the  moist  surface,  being  stimulated  to  lively  activity  probably  by  the 
opposing  ciliary  movements  in  the  epithelium  lining  the  passages.  Kraft2  has 
shown  in  the  rabbit  that,  when  spermatozoa  in  feeble  motion  are  placed  upon 
the  inner  surface  of  the  oviduct,  not  only  are  they  thrown  into  active  contrac- 
tions, but  they  move  against  the  ciliary  movement,  *.  e.  up  the  oviduct.  The 
capacity  of  the  male  cells  thus  to  respond  by  locomotion  in  the  opposite  direc- 
tion to  the  stimulating  influence  of  the  ciliary  cells  over  which  they  have  to 
pass,  is  an  interesting  adaptation.  Probably  this  is  the  directive  agency  that 
enables  the  spermatozoa  to  follow  the  right  path  to  the  ovum,  while  the  ovum, 
being  in  itself  passive,  is  by  the  same  ciliary  movement  brought  toward  the 
active  male  cell.  The  time  occupied  in  the  passage  of  the  spermatozoa  is  un- 
known in  the  human  female,  but  is  probably  short ;  in  the  rabbit  spermatozoa 
have  been  known  to  reach  the  ovary  within  two  and  three-quarter  hours  after 
copulation.  As  has  been  seen,  spermatozoa  are  probably  capable  of  living 
within  the  genital  passages  for  several  days,  when,  if  ovulation  has  not  taken 
place,  they  perish.  If,  however,  an  ovum  appears,  they  at  once  approach  and 
surround  it  in  great  numbers,  being  apparently  attracted  to  it  in  some  myste- 

^chuworski:   Abstract  in  Mnnntnnrh rift  fitr  flrtmrfdiiilf*-  u,,<l  (lyniihnlnijie,  1896,  iv.  S.  275. 
2H.  Kraft :  Pfliiger's  Archivfiir  die  gesammte  Physiologic,  1890,  xlvii. 
VOL.  II.— 30 


466  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

rious  manner.  The  work  of  Pfeffer,1  who  found  that  in  the  fertilization  of 
ferns  malic  acid  within  the  female  organs  attracts  the  spermatozoids  to  their 
vicinity,  suggests  strongly  that  also  among  animals  the  attraction  may  be  a 
chemical  one,  the  ovum  containing  or  producing  something  for  which  the  sper- 
matozoon has  an  affinity.  If  so,  the  meeting  of  the  two  germ-cells  is  an  illus- 
tration of  a  widespread  principle  of  nature  known  as  chemotaxts,  or  chemo- 
tropism.  Experimental  evidence  upon  the  subject  in  animals  is  wanting. 

Fertilization. — It  will  be  remembered  that  the  ovum  and  the  spermatozoon 
undergo  in  their  growth  the  process  of  maturation,  and  that  this  process  con- 
sists essentially  of  a  loss  of  one-half  of  the  chromosomes  of  their  nuclei.  The 
germ-cells  thus  matured  meet,  as  we  have  seen,  in  the  distal  half  of  the  Fal- 
lopian tube  and  fuse  into  one  cell,  the  process  of  fusion  being  called  fertilization 
or  impregnation.  The  details  of  fertilization  have  not  been  observed  in  the 
case  of  the  human  being,  and  the  following  account  is  generalized  from  our 
knowledge  of  the  process  in  other  mammals  and  lower  animals.  In  its  broad 
outlines  fertilization  is  probably  the  same  in  all  animals,  the  differences  being 
confined  to  details. 

The  ovum  at  the  time  of  fertilization  is  surrounded  by  the  zona  radi- 
ata  alone,  the  corona  radiata  having  been  lost.  The  spermatozoa  swarm 
about  the  zona,  lashing  their  tails  and  attempting  to  worm  their  way  through 
it.  Several  may  succeed  in  reaching  the  perivitelline  space,  but  for  some 
unknown  reason  in  most  cases  one  only  penetrates  the  substance  of  the  ovum ; 
the  others  ultimately  perish.  In  mammalian  ova  there  is  no  micropyle,  and 
apparently  the  successful  spermatozoon  may  enter  at  any  point,  the  protoplasm 
of  the  egg  rising  up  as  a  slight  protuberance  to  meet  it  (Fig.  223,  A).  In  some 
animals  the  tail  is  left  outside  to  perish ;  in  others  it  enters,  but  then  disap- 
pears ;  in  no  case  does  it  appear  to  be  of  further  use.  The  head  and  probably 
the  middle-piece  are  of  vital  importance.  The  head,  now  known  as  the  sperm- 
nucleus  or  male  pronucleus,  proceeds  by  an  unknown  method  of  locomotion 
toward  the  centre  of  the  egg,  and  becomes  enlarged  by  the  imbibition  of  liquid 
(Fig.  223,  B).  The  matured  nucleus  of  the  ovum,  or  egg-nucleus,  also  moves 
slowly  toward  the  future  meeting-place  of  the  two  nuclei,  which  is  near  the 
centre  of  the  egg.  The  two  finally  meet  (Fig.  224,  c)  and  together  form  a 
new  and  complete  nucleus,  called  the  first  segmentation-nucleus  (Fig.  224,  D). 
This  body  has  the  conventional  nuclear  structure — namely,  an  achromatic 
network  with  the  chromatic  reticulum  mingled  with  it — and  the  whole  is 
covered  by  a  nuclear  membrane.  From  the  observations  of  Van  Beneden, 
Riickert,2  Zoja,3  and  others,  it  seems  to  be  a  fact  that  the  male  and  the  female 
chromosomes  do  not  fuse  together,  but  remain  distinct  from  each  other,  per- 
haps throughout  all  the  tissue-cells.  The  chromosomes,  it  will  be  perceived, 
are  now  restored  to  the  original  number  present  in  either  germ-cell  before  its 
maturation,  hence  in  the  human  being  perhaps  sixteen,  one-half  of  them 

1 W.  Pfeffer  :    Untersuchungen  aus  dem  Botanischen  Institut  zu  Tubingen,  1884,  i. 
2  J.  Riickert:  Archiv  fur  mikroskopische  Anatomie,  1895,  xlv. 
3R.  Zoja:  Anatomischer  Anzeiger,  1896,  xi. 


REPR  OD  UCTION.  467 

having  come,  however,  from  the  male  cell  and  one-half  from  the  female  cell. 
On  the  commonly  accepted  theory  that  they  constitute  the  hereditary  sub- 
stance, the  first  segmentation-nucleus  contains  within  itself  potentially  all  the 
inherited  qualities  of  the  future  individual. 

While  the  head  of  the  spermatozoon  is  making  its  way  through  the  sub- 
stance of  the  egg  there  appears  beside  it  a  minute  cytoplasmic  body,  the 
centrosome,  and  around  the  latter  cytoplasmic  filaments  arrange  themselves 
in  the  form  of  a  star,  the  whole  body  being  known  as  the  sperm-aster  (Fig. 
223,  B).  We  have  previously  recognized  such  a  structure  in  the  ovum  at  the 
time  of  maturation,  and  have  found  it  functional  in  the  formation  of  the 
polar  bodies ;  after  maturation  it  disappears.  The  sperm-aster  accompanies 
the  sperm-nucleus,  becomes  gradually  enlarged,  and  finally  comes  to  lie,  a 
large  and  prominent  body,  beside  the  segmentation-nucleus.  The  origin  of 
its  centrosome  has  been  greatly  disputed.  Some  investigators  maintain  that 


• 


<--,. 

*.  » 


FIG.  223.— Stages  in  the  fertilization  of  the  egg  (after  Wilson).  The  drawings  were  made  from  sections 
of  the  eggs  of  the  sea-urchin,  Toxopneustes  variegatus,  Ag. 

A.  The  surface  of  the  egg  has  become  elevated  to  form  c,  the  entrance-cone  for  the  spermatozoon ;  the 
head  (/i>  and  the  middle-piece  (m)  of  the  latter  have  entered  the  egg. 

B.  Five  minutes  after  entrance  of  the  spermatozoon.   The  head,  now  the  male  pronucleus,  has  rotated 
180  degrees,  and  has  travelled  deeper  into  the  ovum.    The  cytoplasm  of  the  latter  has  become  arranged 
in  a  radiate  manner  about  the  middle-piece  of  the  spermatozoon,  now  the  centrosome,  to  form  the  sperm- 
aster  ;  the  egg-nucleus,  now  the  female  pronucleus,  is  approaching  the  sperm-nucleus ;  its  chromatin 
forms  an  irregular  reticulum. 

it  is  formed  anew  in  the  egg ;  but  the  prevalent  opinion  at  present  seems  to 
be  that  it  comes  from  the  spermatozoon  in  immediate  relation  to  the  middle- 
piece,  and  hence  is  exclusively  of  male  origin. 

There  results  from  fertilization,  it  is  perceived,  a  single  cell  complete  in 
all  its  essential  parts.  This  is  the  starting-point  of  the  new  individual.  A 
pause  or  resting  period  usually  follows  fertilization,  and  then  growth  begins. 

Segmentation. — The  process  of  growth  is  a  complex  process  of  repeated 
cell-division,  increase  in  bulk,  morphological  differentiation,  and  physiological 
division  of  labor. 

Cell-division  is  largely,  if  not  wholly,  indirect  or  mitotic.  The  term  seg- 
mentation, or  cleavage,  of  the  ovum  is  conveniently  applied  to  the  first  few 
divisions,  although  the  details  of  segmentation  are  not  different  fundamen- 
tally from  those  manifested  later  in  the  division  of  more  specialized  cells. 
Each  division  may  be  resolved  into  three  definite  acts,  which,  however, 
overlap  each  other  in  time.  The  first  act  is  characterized  by  the  appearance 
of  two  centrosomes,  each  with  its  astral  rays,  in  place  of  the  one  already 


468 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


existing  (Fig.  224,  c).  The  two  take  positions  beside  the  nucleus  (Fig.  224, 
D)  and  await  the  time  when  they  can  exert  their  specific  function.  We  have 
spoken  of  the  difference  of  opinion  regarding  the  origin  of  the  original  cen- 
trosome  of  fertilization.  The  origin  of  the  two  centrosomes  present  in  seg- 
mentation has  likewise  been  disputed.  The  question  is  of  considerable  the- 
oretical interest  in  connection  with  the  problem  of  the  physical  basis  of 


FIG.  224.— Stages  in  the  fertilization  of  the  egg  (continued  from  Fig.  223). 

c.  Ten  minutes  after  entrance  of  the  spermatozoon.  The  male  and  the  female  pronuclei  have  met 
near  the  centre  of  the  egg  and  the  fusion  has  begun ;  the  former  has  become  enlarged  and  its  chromatin 
has  become  loosely  reticulated.  The  sperm-aster  has  become  enormously  enlarged.  The  single  centro- 
some  has  been  divided  into  two,  which  lie  upon  either  side  of  the  sperm-nucleus. 

D.  Still  later  after  entrance  of  the  spermatozoon.  The  two  pronuclei  have  united  to  form  the  first 
segmentation-nucleus.  The  sperm-aster  has  become  divided  into  two  asters,  which  have  moved  to 
opposite  poles  of  the  nucleus.  The  egg  is  now  ready  to  undergo  segmentation. 

inheritance.  Certain  observers  have  claimed  that  the  centrosomes  have  a 
double  origin,  one  being  derived  from  the  male  and  one  from  the  female 
germ-cell.  Upon  this  theory  sexuality  is  shown  by  the  cytoplasmic  centro- 
somes as  well  as  by  the  nuclear  chromosomes,  and  the  inference  is  possible  that 
cytoplasm,  as  well  as  nucleus,  transmits  hereditary  qualities.  This  double 
origin  of  the  centrosomes  is  not  supported  by  trustworthy  evidence.  Other 
observers,  following  Boveri,  maintain  that  the  centrosome  of  the  sperma- 


REPRODUCTION. 


469 


\    i 


tozoon  divides  into  the  two  segmentation-centrosomes,  the  latter  hence 
exclusively  of  male  origin.  Still 
others  believe  that  the  sperm- 
centrosome  disappears,  its  place 
being  taken  by  two  new  centro- 
somes derived  from  the  cyto- 
plasm of  the  egg.  The  evi- 
dence available  at  present  does 
not  allow  a  decision  to  be  made 
between  these  two  latter  views.1 
According  to  both  of  them, 
however,  the  cleavage-centro- 
somes  are  not  male  and  female, 
and  cannot  be  regarded  as 
bearers  of  inherited  character- 
istics. These  observations  not 
only  allow,  but  tend  to 
strengthen,  the  prevailing  view 
of  the  exclusive  hereditary  role 
of  the  nucleus.  (See  below 
under  Heredity,  p.  493.) 

The  second  act  of  segmenta- 
tion is  more  complicated  than 
the  first,  and  consists  of  a  halv- 
ing of  the  nucleus.  The  nuclear 
membrane  gradually  disappears. 
The  achromatic  network  resolves 
itself  into  long  cytoplasmic  fila- 
ments arranged  in  the  form  of  a 
spindle,  and  meeting  at  the  two 
centrosomes  (Fig.  225,  A).  The 
spindle,  centrosomes,  and  asters 
form  the  body  known  as  the 
amphiaster.  The  chromatic  sub- 
stance becomes  changed  into  the 
definite  rod-like  chromosomes, 


FIG  225.— Stages  in  the  segmentation  of  the  egg  (after 
Wilson).  The  drawings  were  made  from  sections  of  eggs 
of  the  sea-urchin,  Toxopneuntes  variegatus,  Ag. 

A.  The  nuclear  membrane  has  disappeared.  Within  the 
nucleus  a  distinction  between  the  chromatic  and  the  achro- 
matic substance  has  been  made,  the  former  existing  as 
clearly  denned  chromosomes  aggregated  in  the  centre  to 
form  the  equatorial  plate,  the  achromatic  substance  exist- 
wllich  are  collected  in  the  eqiia-  ing  as  delicate  filaments  extending  in  the  form  of  a  spindle 

from  pole  to  pole. 

tonal    zone    OI    the    spindle,    and  B.  Each  chromosome  has  become  split  into  two.  and  the 

latter  are  being  pulled  toward  the  poles. 

c.  The  divergence  of  the  chromosomes  has  ceased  and 
the  latter  are  becoming  converted  into  vesicular  masses 
beside  the  centrosomes.  The  spindle  is  becoming  resolved 
into  ordinary  cytoplasm. 


constitute  the  equatorial  plate. 
Each  chromosome  proceeds  to 
split  lengthwise,  and  the  two 
halves  move  toward  the  two 
centrosomes  (Fig.  225,  B).  The  cause  of  this  movement  is  not  known. 


The 


1  For  a  critical  review  of  this  and  other  problems  in  fertilization  and  segmentation  see  E. 
B.  Wilson:  The  Cell  in  Development  and  Inheritance,  1900,  2d  ed.,  New  York. 


470 


AN  AMERICAN    TEXT-BOOK    OF  PHYSIOLOGY. 


original  idea  of  Van  Beneden,1  that  the  astral  rays  are  contractile  and 
mechanically  pull  apart  the  half-chromosomes,  is  supported  by  considerable 
but  unconvincing  evidence.  The  idea  appears  to  be  growing  that  by  reason 
of  chemical  changes  taking  place  in  the  centrosomes  the  half-chromosomes 
are  attracted  to  the  two  poles  of  the  spindle.2  Strasburger3  suggests  that 
this  attractive  influence  is  chemotaxis.  In  the  process  of  division  each 
nuclear  half  obtains  half  of  the  original  male  and  half  of  the  original 
female  chromatin,  and  hence  contains  inherited  potentialities  of  both  parents. 
After  division  each  half  gradually  assumes  the  structure  of  a  typical  resting 
nucleus  with  its  accompanying  aster  (Fig.  226). 

The  third  act  of  segmentation  consists  of  a  simple  division  of  the  cytoplasm 
into  two  equal  parts,  the  separation  taking  place  along  the  plane  of  nuclear 


FIG.  226.— Stages  in  the  segmentation  of  the  egg  (continued  from  Fig.  225). 

D.  The  vesicular  chromatic  masses  have  become  converted  into  two  typical  resting  nuclei,  each  with 
a  chromatic  network.  The  single  aster,  formerly  connected  with  each  nuclear  mass,  has  become  divided 
into  two,  which  have  taken  positions  at  opposite  poles  of  the  nuclei.  The  division  of  the  cytoplasm  is 
complete,  and  the  two  resulting  cells,  or  blastomeres,  are  resting,  preparatory  to  a  second  division  in  a 
plane  at  right  angles  to  that  of  the  first. 

division  (Fig.  226,  D).  Each  part  contains  one  of  the  new  nuclei,  and  the 
result  of  the  first  division  is  the  existence  of  two  cells,  two  blastomeres,  in 
place  of  the  one  fertilized  ovum.  The  beginning  of  differentiation  is  often 
shown  even  as  early  as  this,  for  one  blastomere  is  often  somewhat  larger  and 
less  granular  than  the  other. 

Each  blastomere  proceeds  now  to  divide  by  a  similar  mitotic  process  into 
two,  the  result  being  four  in  all,  and  by  subsequent  divisions,  eight,  sixteen, 
and  more,  the  divisions  not  proceeding,  however,  with  mathematical  regu- 
larity. By  such  repeated  mitotic  processes  the  original  fertilized  ovum 
becomes  a  mass  of  small  and  approximately  similar  cells,  the  morula,  from 
which  by  continued  increase  in  the  number  of  the  cells,  morphological  differ- 
entiation, and  physiological  division  of  labor,  the  embryo  with  all  its  functions 
is  destined  to  be  built  up. 

1  Van  Beneden  :  Archives  de  Biologie^  1883,  iv. 

*Cf.  Biitschli:   Verh.  Naturhist.  med.  Ver.  Heidelberg,  1891 ;  and'E.  B.  Wilson,  op.  cit. 

3  Strasburger :  Analomischer  Anzeiger,  1893,  viii. 


REPROD  UCT1<>\.  \  7  \ 

Polyspermy.— It  happens  occasionally  that  two  or  more  spermato/na  enter 
the  ovum  ;  such  a  phenomenon  is  known  as  tli^tn-mi/  or  /W//x/>o-///v,  according 
to  the  number  of  entering  sperms.  Each  sperm  with  its  nucleus  and  centro- 
some  becomes  a  male  pronucleus  and  proceeds  to  conjugate  with  the  female 
pnmncleiis.  In  the  case  of  dispermy  the  one  female  and  the  two  male  pro- 
nuclei  fuse  together;  each  centrosome  gives  place  as  usual  to  t\\<>.  making 
four  iu  all,  which  take  up  a  quadrilateral  position  about  the  first  segmenta- 
tion-nucleus ;  the  chromatic  figure  consists  of  two  crossed  spindles  ;  and  tin- 
egg  segments  at  once  into  four  instead  of  two  blastomeres.  Analogous  phe- 
nomena result  from  more  complex  cases  of  polyspermy.  In  such  double-  or 
multi-fertilized  eggs  development  may  proceed  to  some  distance,  but  typical 
larval  forms  are  not  produced,  and  death  occurs  early.  • 

During  cleavage  the  ovum  proceeds,  after  the  manner  of  the  non-fertilized 
ovum,  slowly  along  the  Fallopian  tube  and  enters  the  uterus.  Unlike  the  non- 
fertilized  ovum,  however,  the  morula  is  not  cast  out  of  the  body,  but  remains 
and  undergoes  further  development.  The  morphological  development  of  the 
embryo  in  utero  does  not  fall  within  the  scope  of  the  present  article.  Some 
attention  may,  however,  be  given  to  the  immediate  environment  of  the  develop- 
ing child  and  its  relations  to  the  maternal  organism. 

Decidua  Graviditatis. — While  the  segmentation  of  the  ovum  is  proceed- 
ing within  the  Fallopian  tube,  the  uterus  prepares  for  the  future  guest  by  begin- 
ning to  undergo  a  profound  change,  probably  being  stimulated  to  activity  re- 
flexly  by  centripetal  impulses  originating  in  the  walls  of  the  tube  through  con-, 
tact  with  the  ovum.  This  change  comprises  an  enlargement  of  the  whole  uterus 
and  a  great  and  rapid  growth  in  thickness  of  its  mucosa  and  its  muscular 
coat.  At  first  the  alterations  are  not  unlike  the  phenomena  of  growth  pre- 
ceding the  menstrual  flow,  but,  as  they  proceed,  they  become  much  more  pro- 
found than  those.  The  supply  of  blood  to  the  walls  is  greatly  increased,  the 
vessels  forming  large  irregular  sinuses  within  the  mucosa.  The  supply  of  lymph 
is  increased.  The  glands  become  tortuous  and  dilated  into  flattened  cavernous 
spaces,  and  their  walls  atrophy,  the  epithelium  breaking  down  except  in  their 
deepest  parts.  The  mucosa  is  thus  converted  into  a  spongy  tissue,  the  frame- 
work of  which  contains  numerous  large  irregular  cells,  derived  probably  from 
the  original  connective  tissue  and  called  decidual  ceils.  The  musculature  is 
greatly  thickened  by  an  increase,  partly  in  number  and  partly  in  size,  of  its 
constituent  fibres,  and  the  nerve-supply  is  increased.  These  general  structural 
changes  proceed  through  the  early  part  of  gestation  and  are  accompanied  by 
special  changes  to  be  discussed  later.  It  is  not  definitely  known  how  far  the 
alterations  have  gone  before  the  advent  of  the. segmented  ovum  into  the  uterus. 
With  the  latter  instead  of  the  unimpregnated  ovum  present  in  the  Fallopian 
tube,  the  hypertrophied  uterine  mucosa  does  not  break  awav  a.s  in  menstrua- 
tion, but  remains,  and  henceforth  is  called  the  decidua  graviditatis,  special 
names  being  given  to  special  parts.  Entering  the  uterus,  the  ovum  attaches 
itself  in  an  unknown  manner  to  the  wall  of  the  womb.  The  part  of  the  mucous 


472  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

membrane  that  forms  its  bed  is  henceforth  known  as  the  decidua  serotina ;  as 
the  seat  of  the  future  placenta,  it  is  physiologically  the  most  interesting  and 
important  portion  of  the  uterine  inucosa.  The  surrounding  cells  and  tissues 
are  stimulated  to  active  proliferation  and  grow  around  and  over  the  ovum, 
completely  covering  it  with  a  layer,  the  decidua  reflexa.  The  remainder  of 
the  uterine  lining  membrane  constitutes  the  decidua  vera.  Between  the  reflexa 
and  the  vera  is  the  uterine  cavity.  At  first  thickened,  the  reflexa  later  thins 
away  as  the  embryo  grows,  and  approaches  close  to  the  vera ;  finally  it  touches 
the  latter,  and  the  original  cavity  of  the  body  of  the  uterus  becomes  oblit- 
erated. By  the  sixth  month  the  reflexa  disappears,  either  coalescing  with  the 
vera  or  undergoing  total  degeneration  (Minot).  During  the  latter  half  of 
gestation  'the  vera  itself  thins  markedly.  This  atrophy  of  the  comparatively 
unimportant  reflexa  and  vera,  in  contrast  to  the  placental  hypertrophy  of  the 
serotina,  is  interesting.  The  arrangement  of  the  parts  is  well  shown  in  the 
accompanying  illustration  (Fig.  227). 

The  Petal  Membranes. — The  segmented  ovum  absorbs  nutriment  at  first 
directly  from  its  surrounding  maternal  tissues,  and  later  through  the  mediation 
of  the  placenta.  Its  growth  and  cell-division  are  active,  and  it  increases  in 
size  and  complexity.  It  early  takes  the  form  of  a  generalized  vertebrate  em- 
bryo, and  by  the  fortieth  day  begins  to  assume  distinctly  human  characteristics. 
It  becomes  surrounded  early  by  the  fetal  membranes,  which  are  two  in  num- 
ber, the  omnion  and  the  chorion  or,  as  it  is  usually  called  in  other  vertebrates, 
false  amnion.  The  amnion  is  a  thin,  transparent,  non-vascular  membrane  imme- 
diately surrounding  the  embryo  (Fig.  227).  In  origin  a  derivative  of  the  embry- 
onic somatopleure,  later  it  becomes  completely  separated  from  the  body  of  the 
embryo.  The  space  enclosed  by  the  amnion,  the  amniotic  cavity,  within  which 
the  embryo  lies,  is  traversed  by  the  umbilical  cord  and  contains  a  serous  liquid, 
the  liquor  amnii.  This  liquid,  highly  variable  in  quantity,  averages  at  full 
term  nearly  a  liter  (1J  pints).  It  has  in  general  the  composition  of  a  serous 
liquid.  It  contains  between  1  and  2  per  cent,  of  solids,  consisting  of  proteids 
(0.06-0.7  per  cent.),  mucin,  a  minute  and  variable  quantity  of  urea,  and  inor- 
ganic salts.  Its  origin,  whether  from  the  fetus,  especially  from  the  fetal 
kidneys,  or  from  the  mother,  has  been  much  discussed.  It  may  possibly 
come  in  small  part  from  the  former,  but  its  chief  origin  is  doubtless  by  trans- 
udation  from  the  maternal  blood,  as  is  indicated  by  the  ready  appearance 
within  the  amniotic  cavity  of  solutions  injected  into  the  maternal  veins,  and 
the  fact  that  the  amniotic  liquid  of  diabetic  women  contains  sugar.  It  bathes 
the  entire  surface  of  the  embryonic  body,  and  is,  moreover,  apparently  swal- 
lowed into  the  stomach,  as  the  presence  of  fetal  hairs  and  epidermal  scales 
within  the  alimentary  canal  attests.  Its  chief  functions  appear  to  be  those 
of  protecting  the  fetus  from  sudden  shocks  and  from  pressure,  maintaining  a 
constant  temperature,  and  supplying  the  fetal  body  with  water.  The  pro- 
teid  possibly  confers  upon  it  a  very  slight  nutritive  value,  and  the  minute 
quantity  of  urea  is  perhaps  indicative  of  an  unimportant  excretory  function 
of  the  fetal  kidneys.  As  growth  proceeds,  the  amnion  expands  and  becomes 
loosely  attached  to  the  outer  fetal  membrane,  the  chorion. 


REP  ROD  UCTION. 


473 


The  chorion  (Fig.  227),  or  false  amnion,  is  formed  simultaneously  with  the 
true  amnion,  and  like  it  from  somatopleure.  It  is  a  thickened  vascular  mem- 
brane, completely  surrounding  the  amnion  with  the  contained  embryo.  Be- 
tween it  and  the  amnion  there  is  at  first  a  considerable  space,  traversed  by  the 
umbilical  cord  and  filled  with  the  chorionic  fluid  (which  is  probably  of  the 
same  general  nature  as  the  amniotic  fluid).  But  later  this  space  is  obliterated 


Decidua  terotina. 
Chorion  Jrondosum. 


Muscle. 

Uterine  glands. 
Chorion  Iseve. 


Mucous  plug  within  cervical  canal. 


FIG.  227.— Diagram  of  the  human  uterus  at  the  seventh  or  eighth  week  of  pregnancy  (modified  from 
Allen  Thompson).  The  fetal  villi  are  shown  growing  into  the  sinuses  of  the  decidua  serotina  and  the 
decidua  reflexa ;  in  the  latter  they  are  becoming  atrophied.  They  are  marked  by  the  black  fetal  vessels, 
which  can  be  traced  backward  along  the  umbilical  cord  to  the  embryo.  The  placenta  comprises  the 
decidua  serotina  and  the  chorion  frondosum. 

by  the  enlargement  of  the  amnion.  Externally  the  chorion  presents,  at  first, 
a  shaggy  appearance  due  to  the  existence  of  very  numerous  columnar  pro- 
cesses, called  vitti,  extending  outward  in  all  directions  and  joining  by  their 
tips  the  decidua  serotina  and  the  decidua  reflexa.  Later  the  villi  are  aborted 
except  in  the  region  of  the  serotina,  where  they  become  more  prominent  and 
constitute  an  important  part  of  the  placenta.  The  blood-vessels  of  the  chorion 


474 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


Amnion. 


are  fetal  vessels  coming  from  the  embryonic  structure,  the  allantois.  They 
comprise  the  branches  and  uniting  capillaries  of  the  two  allantoic  or  umbilical 
arteries,  and  the  one  (at  first  two)  allantoic  or  umbilical  vein.  They  are 
especially  well  developed  within  the  villi.  As  growth  proceeds,  the  chorion 
comes  into  close  contact  with  the  deddua  reflexa,  and,  as  the  latter  disappears, 
with  the  deddua  vera;  this  portion  of  it  is  called  chorion  Iceve.  In  the  region 
of  the  deddua  serotina  it  enters  into  the  formation  of  the  placenta,  and  is 
here  called  chorion  frondosum. 

The  Placenta.  —  The  placenta  (Fig.  227),  or  organ  of  attachment  of  mother 
and  fetus,  is  a  disk-shaped  body,  approximately  20  centimeters  (7—8  inches)  in 
diameter,  attached  to  the  inner  surface  of  the  uterine  wall,  usually  either  upon 
the  dorsal  or  the  ventral  side,  and  connected  by  the  umbilical  cord  with  the 

navel  of  the  fetus.  It  consists  of 
a  maternal  part,  the  modified 
deddua  serotina,  and  a  fetal  part, 
the  modified  chorion,  intimately 
united  together.  The  modifica- 
tions of  the  serotina  consist  of  a 
degeneration  of  the  superficial 
layers  of  the  mucosa,  especially  of 
the  epithelium  and  the  glands,  and 
the  development  of  very  large 
irregular  sinuses  at  the  surface, 
into  which  the  uterine  arteries  and 
veins  freely  open.  It  is  a  disputed 
question  among  histologists  whether 
the  sinuses  are  maternal  or  fetal  in 
origin,  or  really  spaces  between 
maternal  and  fetal  tissues.  The 
modifications  of  the  chorion  con- 
sist of  a  great  increase  in  length 
and  complexity  of  branching  of 
the  villi,  a  great  development  of 
their  contained  blood-vessels,  and 
a  firm  attachment  of  their  tips  to 
the  uneven  surface  of  the  serotina, 

S°  *»*  *™»^  <=Ome  to  float 


F,G.  228,-Diagran,  of  the  placenta  (Schafer)  :  8,  pla- 

cental  sinuses,  into  which  project  the  fetal  villi,  con-  freely    Within    the     uterine     sinuses 

taining  the  red  fetal  vessels;  ds,  decidua  serotina;  sp.  i  *,      i        i      ,1      i    •                •         i  i      J 

spongy  layer,  and  m,  muscular  layer,  of  the  uterus;  a,  aild   to  be    bathed   m   «terine    blood 

uterine  artery,  and  v,  uterine  vein,  opening  into  the  (Fig.  228).       The    analogy  between 

placental  sinuses.  ,.            ,            ,    ,      .„. 

the  mammalian  placental  villi  and 

the  gills  of  a  fish,  also  highly  vascular  and  floating  in  liquid,  is  striking. 
We  shall  see  later  that  the  analogy  is  not  only  morphological  but  also 
physiological,  inasmuch  as  the  villi  have  important  respiratory  functions. 
The  bulk  of  the  placenta  is  this  intravillous  portion,  of  spongy  consistence, 


REPRODUCTION.  475 

comprising  the  maternal  sinuses  permeated  hy  the  fetal  villi  ;  this  is  in  con- 
tact upon  the  fetal  side  with  the  thin  unmodified  ehorion  covered  within  l>v 
the  amnion,  and  upon  the  maternal  side  with  the  thin  relatively  unmodified 
serotina  covered  without  l>y  the  uterine  muscle.  The  pure  maternal  blood 
brought  by  the  uterine  arteries  moves  slowly  through  the  sinuses  and  retires 
by  the  uterine  veins;  the  fetal  blood  is  propelled  by  the  fetal  heart  alono-  the 
umbilical  cord  within  the  allantoie  arteries  and  through  the  villous  capillaries, 
and  returns  by  the  allantoie  vein.  The  two  kinds  of  blood  never  mix,  but 
are  always  separated  by  the  thin  capillary  Avails  and  their  thin  villous  invest- 
ment of  connective  tissue  and  epithelium.  Thus  the  anatomical  conditions 
for  ready  diffusion  are  present,  and  this  is  the  chief  means  of  transfer 
of  nutriment  and  oxygen  from  mother  to  child,  and  of  wastes  from  child  to 
mother.  The  physiological  role  of  the  placenta  is,  therefore,  an  all-important 
and  complicated  one.  The  placenta  is,  technically,  the  nutritive  organ  of  the 
embryo. 

Nutrition  of  the  Embryo. — We  have  seen  that  a  fundamental  and  most 
striking  difference  between  the  minute  human  ovum  and  the  large  egg  of  the 
fowl  lies  in  the  relative  quantity  of  food  contained  in  the  two.  The  fowl  has 
retained  the  primitive  habit  of  discharging  the  ovum  from  the  maternal  body, 
and  discharges  within  its  shell  at  the  same  time  sufficient  food  for  the  needs  of 
the  developing  chick.  Evolution  has  endowed  the  human  mother,  in  common 
with  other  mammals,  with  the  peculiar  custom  of  retaining  the  offspring  within 
her  body  until  its  embryonic  life  is  completed,  and  of  doling  out  its  nutriment 
molecularly  throughout  the  period  of  gestation.  The  store  of  nutritive  deuto- 
plasm  with  which  the  egg  leaves  the  ovary  is,  therefore,  only  sufficient  for  the 
early  segmentative  activities.  Within  the  Fallopian  tube  absorption  from  the 
surrounding  walls  doubtless  goes  on.  Arrived  in  the  uterus  and  imbedded  in 
its  decidual  wall,  the  segmented  ovum  continues  to  take  nutriment  from  its 
immediate  environing  cells.  It  has  been  suggested,  but  without  much  basis 
of  fact,  that  the  uterine  glands,  which  at  this  time  are  greatly  dilated,  may 
furnish  a  nutritive  secretion  for  the  use  of  the  embryo;  but,  a  priori,  it  would 
seem  more  reasonable  that,  just  as  the  ovum  within  the  Graatian  follicle 
obtains  its  food  from  its  surrounding  stroma,  so  within  the  highly  vascular 
decidua  it  absorbs  directly  from  the  decidual  tissue.  But  that  this  source 
soon  proves  insufficient  for  the  rapid  growth  is  indicated  by  the  early  develop- 
ment of  the  ehorion  with  its  villi  and  the  embryonic  vascular  system.  In 
the  youngest  known  human  embryo,1  believed  to  be  scarcely  seven  days  old, 
the  villi  are  alreadv  well  marked.  From  this  time  onward  throughout  ges- 
tation the  ehorion  takes  an  important  part  in  the  embryonic  nutrition,  becom- 
ing, as  we  have  seen,  an  integral  part  of  the  placenta.  The  placenta  is  jm/- 
<'.,•<•<  11,'ncc  the  medium  of  nutritive  communication  between  mother  and  child. 

Let  us  consider  briefly  the  needs  of  the  embryo.     The  fetal  energies  must 
be  directed  almost  wholly  to  the  all-important  functions  of  growth  and  prepa- 
ration for  the  future  independent  existence.    The  organism  requires,  therefore, 
deters:  Verhnndltingen  der  deutschen  GewtMi'ift  fiir  Gynakohgie,  1897,  vii.  S.  264. 


476  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

an  abundance  of  food  containing  all  the  chief  kinds  of  food-stuffs.  With  the 
alimentary  canal  in  its  embryonic  and  functionless  state,  this  food,  when  it 
reaches  the  embryo,  must  necessarily  be  already  digested  and  ready  for  absorp- 
tion by  the  cells.  A  supply  of  oxygen,  not  necessarily  great  in  quantity,  is 
also  needed.  The  fetal  lungs  are  not  ready  for  respiration,  and  the  oxygen 
must  come  to  the  blood  by  another  channel  than  them.  Carbonic  acid  must 
be  got  rid  of,  and  through  other  than  pulmonary  paths.  Urea  and  its  fore- 
runners and  other  wastes,  probably  not  in  great  quantity,  must  be  excreted. 
The  fetal  kidneys  and  the  skin  are  probably  never  very  active,  as  is  made  rea- 
sonably certain  by  the  late  external  opening  of  the  male  urethra,  the  late 
development  of  the  cutaneous  glands,  and  the  composition  of  the  amniotic 
liquid,  into  which  they  would  naturally  pour  their  secretions.  Thus  the  paths 
of  income  and  outgo  that  are  normal  to  the  individual  after  birth  are  only 
partially  open  during  fetal  life ;  nevertheless,  the  processes  of  income  and 
outgo  must  be  performed.  The  placenta,  with*  its  close  relationship  but  non- 
communication of  maternal  and  fetal  blood-vessels,  has,  therefore,  been  evolved 
phylogenetically,  and  appears  early  in  the  course  of  ontogeny.  There  is 
brought  to  it  on  the  part  of  the  embryo  and  discharged  into  the  villous 
capillaries  a  mixed  blood,  comprising  venous  blood  from  the  various  capil- 
lary systems  of  the  body,  and  containing,  therefore,  the  carbonic  acid  and 
other  wastes  of  venous  blood,  and  a  certain  proportion  of  purified  blood 
which  has  passed  directly  by  way  of  the  ductus  venosus,  the  inferior  vena 
cava,  the  right  auricle,  the  foramen  ovale,  and  the  left  side  of  the  heart  to  the 
aorta  and  the  umbilical  arteries.  There  is  brought  to  the  placenta  on  the 
part  of  the  mother  and  discharged  into  the  sinuses  pure  arterial  blood, 
laden  with  food  and  oxygen.  Through  the  membrane  intervening  between 
maternal  and  fetal  vessels  there  pass  from  the  fetus  carbonic  acid  and  other 
wastes,  and  from  the  mother  food  (sugar,  fats,  proteids,  etc.)  and  oxy- 
gen. Back  to  the  fetal  liver  and  heart  goes  the  nutritive  and  arterialized 
blood,  and  back  to  the  maternal  excretory  organs  the  vessels  convey  the  fetal 
wastes.  The  placenta  is  thus  a  peculiar  organ  intermediate  between  the  living 
cells  of  the  embryo  on  the  one  hand  and  the  digestive  organs,  lungs,  kidneys, 
and  skin,  of  the  mother  on  the  other.  Little  is  known  of  the  actual  details 
of  the  placental  process.  The  structure  of  the  intervening  cells  indicates  that 
the  interchange  may  be  after  a  manner  analogous  to  that  taking  place  in  the 
lungs,  rather  than  to  that  of  a  typical  secreting  gland — i.  e.  that  known  physi- 
cal processes,  such  as  diffusion  and  filtration,  play  a  prominent  role.  It  has 
been  shown  by  several  investigators  that  the  fetus  may  be  poisoned  by  car- 
bonic oxide  and  strychnine,  and  may  receive  other  harmless  diffusible  sub- 
stances that  are  introduced  in  solution  into  the  maternal  circulation.  The 
mother  may  be  affected  similarly  from  the  fetal  circulation.  But,  as  in  the 
case  of  the  lungs,  so  the  placental  membrane  can  scarcely  be  regarded  as 
acting  in  the  same  passive  way  as  a  lifeless  membrane  would  act  (compare 
Respiration).  As  accessory  to  the  main  nutritive  source  it  has  been  sug- 
gested that  a  diapedesis  of  maternal  leucocytes  into  the  fetus  may  take  place. 


RE  PR  OD  UCTION.  477 

The  uterine  "-lands  arc  thought  by  some  to  afford  a  nutritive  secretion  to  the 
sinuses,  and  to  the  amniotic  liquid  lias  been  asrribed  a  nutritive  function. 
Theoretically,  these  various  means  are  not  impossible,  but  true  plaeental  dillu- 
sibn  must  be  regarded  as  the  ehief  principle  at  work.  The  result  is  that  the 
mother  relieves  the  child  of  all  the  labor  of  nutrition  except  that  connected 
directly  with  the  latter's  own  cellular  and  protoplasmic  metabolism.  The 
fetal  energies  are,  therefore,  free  to  be  expended  in  the  process  of  growl h, 
while  gestation  profoundly  affects  the  maternal  organism. 

Physiological  Effects  of  Pregnancy  upon  the  Mother. — As  might  have 
been  expected,  there  is  probably  not  one  organic  system  within  the  mother's 
body  that  is  not  more  or  less  altered  by  pregnancy,  often  morphologically,  but 
especially  in  regard  to  function.  And  such  normal  alterations  pass  so  gradu- 
ally and  so  frequently  into  genuine  pathological  conditions  that  it  is  sometimes 
difficult  to  draw  the  line  between  the  two.  The  most  marked  changes  are 
connected  with  the  body  of  the  uterus,  and  have  already  been  described.  The 
walls  of  the  cervix  uteri  become  hypertrophied,  though  to  a  less  degree  than 
the  body,  and  their  glands  secrete  a  quantity  of  mucus  that  forms  a  plug  com- 
pletely closing  the  passage-way  of  the  cervix  (Fig.  227).  The  rest  of  the 
reproductive  organs  from  the  uterus  outward  become  involved  in  the  increased 
venous  hypera3mia.  The  walls  of  the  vagina  become  infiltrated  with  serous 
liquid.  The  parts  of  the  vulva  partake  in  the  general  tumefaction.  From  the 
second  month  of  gestation  onward  the  mammary  glands  undergo  gradual  devel- 
opment as  a  preparation  for  the  post-partum  lactation.  The  increase  in  size  of 
the  laden  uterus  brings  gradually  increasing  pressure  to  bear  upon  the  abdom- 
inal viscera,  and  thus  mechanically  causes  functional  derangements  of  the 
digestive  and  the  urinary  organs.  The  stretching  of  the  abdominal  skin 
results  in  localized  ruptures  of  the  connective  tissue  of  the  cutis,  the  charac- 
teristic scars  forming  the  strice  gravidarum,  which  persist  after  pregnancy. 
Other  organic  changes  are,  however,  more  profound  than  these  mechanical  ones. 
In  accordance  with  the  increased  nutritive  labor  thrown  upon  the  mother,  the 
total  quantity  of  blood  in  her  body  is  increased,  if  we  can  reason  from  deter 
minations  made  upon  the  lower  animals.1  The  condition  of  the  blood  has 
been  disputed.  The  old  belief  was  that  the  blood  of  pregnancy  is  more  waterv 
and  contains  less  hemoglobin  than  at  other  times.  This  is  perhaps  true  for 
the  earlier  months,  but  Schroeder 2  and  others  have  shown  that  the  proportion 
of  haemoglobin  and  the  number  of  red  corpuscles  rise  above  the  normal 
during  the  later  stages.  The  work  of  the  maternal  heart  is  increased  during 
gestation.  It  is  maintained  by  some  that  the  heart  beats  more  rapidly- 
according  to  Kehrer,3  over  eighty  times  in  the  minute.  It  has  also  been 
thought,  mainly  from  the  results  of  percussion  and  from  sphvomographic 
tracings,  that  the  left  ventricle  is  hypertrophied  during  pregnancy.  IVt- 
mortem  examination  confirms  this  inference.  Pregnancy  necessarily  throus 

1O.  Spiegelberg  und  R.  Gscheidelen :   Archivfiir  Gynakologie,  1872,  iv. 

2R.  Schroeder:  Ibid.,  1890-91,  xxxix. ;  Wild:  Ibid.,  1897,  liii.  S.  363. 

8  F.  A.  Kehrer :    Ueber  die  Verdnderungen  der  Pulseurve  im  Puerperium,  1886. 


478  AN'  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

increased  labor  upon  both  the  liver  and  the  kidneys,  and  these  organs  are  prone 
to  functional  disorders.  Gastric  disturbances  are  marked  by  frequent  vomit- 
ing. A  tendency  to  increased  pigmentation  in  the  skin  is  present.  The  ner- 
vous system  is  affected,  manifesting  its  alterations  both  by  nutritional  disturb- 
ances and  by  mental  irritability,  depression  of  spirits,  disordered  senses,  easily 
passing  into  temporary  pathological  states,  and  occasionally  by  feelings  of 
heightened  well-being.  The  body-weight  usually  increases  independently  of 
the  added  weight  of  the  embryo. 

Duration  of  Gestation. — For  centuries  the  duration  of  gestation  in 
woman  has  been  commonly  regarded  as  280  days.  The  beginning  of  preg- 
nancy, the  union  of  the  ovum  and  the  spermatozoon,  however,  presents  no 
obvious  signs  by  which  it  may  be  recognized,  and  hence  the  actual  length  of 
pregnancy  in  the  human  female  is  no  more  known  than  in  other  mammals. 
The  obstetrician  is  obliged,  therefore,  to  use  artificial  schemes  in  computing  its 
probable  length.  Several  tables  have  been  published  of  the  time  elapsing 
between  a  single  coition  resulting  in  pregnancy  and  the  terminal  parturition. 
"Veit,1  in  collecting  503  such  cases  reported  by  several  obstetricians,  finds  the 
duration  to  be  from  265  to  280  days  in  396  cases,  and  longer  in  the  remaining 
107  cases,  the  variation  thus  being  marked.  It  is  obvious  that  the  date  of  the 
effective  coition  can  rarely  be  known.  One  of  the  first  and  most  evident  signs 
of  pregnancy  is  the  non-appearance  of  the  menses,  and,  probably  largely  from 
the  long-prevailing  idea  of  the  close  relation  existing  between  ovulation  and 
menstruation,  it  has  been  customary  to  regard  gestation  as  dating  from  the  last 
menstruation.  Following  Naegele,  obstetricians  estimate  the  date  of  parturi- 
tion as  280  days  from  the  first  day  of  the  last  menstruation;  and  this  simple 
but  artificial  rule  is  doubtless  approximately  correct. 

In  accordance  with  modern  biological  theories,  it  must  be  supposed  that  for 
each  species  there  has  been  developed  a  gestative  period  of  a  length  most 
favorable  to  the  continuance  of  the  species ;  this  has  been  a  matter  of  natural 
selection.  But  this  principle  does  not  account  for  the  termination  of  the  period 
in  any  individual  case.  The  proximate  cause  of  the  oncoming  of  birth  must 
be  sought  in  more  specific  anatomical  or  physiological  phenomena.  This  cause 
has  been  sought  long,  and  not  wholly  successfully.  Among  the  agents  sug- 
gested may  be  mentioned  the  pressure  which  the  uterine  tissues,  the  ganglia 
of  the  cervix,  and  the  adjacent  nerves,  receive  between  the  fetal  head  and  the 
pelvic  wall,  the  stretching  of  the  uterine  wall,  the  fatty  degeneration  of  the 
deciduse,  the  thrombosis  of  the  placental  vessels,  the  venosity  of  the  fetal 
blood  due  to  the  growing  functional  importance  of  the  fetal  right  ventricle 
acting  as  a  stimulus  to  the  placental  area,  and  a  gradual  increase  in  irritability 
of  the  uterus  as  the  nerve-supply  of  the  organ  increases.  Some  of  these,  such 
as  the  fatty  degeneration  of  the  deciduse  and  the  placental  thrombosis,  are  not 
constant  phenomena,  and  the  others  are  not  definitely  proved  to  be  efficient 
causes.  It  is  probable  that,  with  the  uterus  undoubtedly  irritable,  in  different 
1  J.  Veit :  Mutter's  Handbuch  der  Geburtshiilfe,  1888,  1. 


REPRODUCTION.  479 

cases  different  stimuli  act  to  inaugurate  the  proocss  of  birth,  and  a  priori 
several  of  the  above  causes  scciu   not  improbable  ones. 

Parturition  in  General. — Parturition,  birth,  or  labor,  is  the  process  of 
expulsion  of  the  developed  embryo,  the  membranes,  and  the  placenta  from  the 
body  of  the  mother.  It  is  executed  by  contraction  of  the  muscles  of  the  so- 
called  upper  segment  of  the  uterus  and  those  of  the  abdominal  walls.  The 
lower  segment  of  the  uterus,  comprising  approximately  that  portion  of  the 
body  lying  below  the  attachment  of  the  peritoneum,  the  cervix,  the  vagina, 
and  the  vulva,  are  largely,  if  not  wholly,  passive  in  parturition.  The  obstet- 
ricians have  found  it  convenient  to  divide  labor  into  three  stages,  although 
physiologically  these  are  not  sharply  differentiated  from  each  other.  The  first 
stage  is  characterized  by  the  dilatation  of  the  os  uteri,  the  second  by  the  expul- 
sion of  the  fetus,  the  third  by  the  expulsion  of  the  after-birth.  The  customary 
position  of  the  fetus  within  the  uterus  at  the  end  of  pregnancy  is  that  in  which 
the  head  is  downward  or  nearest  the  os,  the  back  toward  the  ventral  and  left 
side  of  the  mother,  and  the  arms  and  legs  folded  upon  the  trunk. 

First  Stage  of  Labor. — For  several  weeks  toward  the  close  of  pregnancy 
there  are  occasional  periods  when  rhythmic  muscular  contractions  pass  over  the 
uterine  walls.  These  are  mostly  painless,  and  apparently  are  not  in  themselves 
of  special  functional  importance.  The  first  stage  of  labor  is  ushered  in  by 
various  phenomena,  prominent  among  which  are  an  increase  in  the  intensity 
of  the  contractions,  their  painfulness,  and  their  frequency  and  continuance. 
In  women  they  are  confined  practically  to  the  upper  segment  of  the  uterus  and 
its  attached  ligaments,  ceasing  at  a  circular  ridge  that  projects  inward  and  is 
called  the  "contraction  ring."  For  some  reason,  at  present  disputed,  the 
lower  segment  of  the  uterus,  and  the  cervix,  are  passive.  The  contractions 
are  probably  peristaltic  in  character,  as  in  lower  animals.  Schatz1  has  graphi- 
cally recorded  the  uterine  movements  by  means  of  a  bladder  filled  with  water 
and  introduced  into  the  uterus.  During  the  earlier  part  of  parturition  the 
contractions  gradually  increase  in  intensity  up  to  a  maximum  which  they 
then  maintain.  Their  rhythm  is  somewhat  irregular ;  the  duration  of  each 
contraction  averages  about  one  minute,  and  a  pause,  which  ensues  between  suc- 
cessive contractions,  extends  from  one  and  one-half  to  several  minutes.  The 
relaxation  of  the  muscle-fibres  during  the  period  of  rest  is  incomplete,  the 
result  being  that  the  fibres  enter  gradually  into  a  tonically  contracted  state. 
Each  contraction  is  accompanied  by  a  pain,  localized  in  the  early  part  of  labor 
in  the  uterus  alone,  but  later  extending  outward,  upward  into  the  abdomen, 
and  downward  into  the  thighs.  The  pains  of  labor  vary  greatly  in  intensity 
in  individuals,  but  are  usually  more  intense  during  the  first  gestation  than 
during  later  ones.  They  are  due  chiefly  to  direct  mechanical  stimulation  of  the 
sensory  uterine  and  other  nerves  by  compression,  tension,  and  even  laceration. 

^.Schatz:  Archiv  fiir  Gynakologie,  1885-86,  xxvii.  Compare  O.  Schaeffer :  Experimentetle 
Untersuchungen  iiber  die  Wehenthatigkeit  des  menschlichen  Utems,  ausgefiihrt  mitteht  einer  neuen 
Pelotte  und  eines  neuen  Kymoyraphion,  Berlin,  1896 ;  abstract  in  Centralblatt  fiir  Gynakoloffif, 
1896,  xx.  S.  85. 


480  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

As  a  result  of  the  tonic  contraction  of  the  uterine  walls,  gradually  increas- 
ing with  each  new  peristaltic  wave,  the  uterus  becomes  gradually  narrower  in 
diameter  and  longer,  and  the  walls  press  more  and  more  firmly  upon  the  bag 
of  amniotic  liquid  containing  the  embryo.  Schatz  finds  that  the  uterine  pres- 
sure under  the  uterine  contractions  rarely  reaches  and  never  exceeds  100  milli- 
meters of  mercury.  The  direction  of  least  resistance  to  this  pressure  lies 
along  the  cervical  canal,  the  walls  of  which  do  not  take  part  in  the  uterine  labor. 
With  each  succeeding  contraction  this  canal  is  forced  wider  open  and  the  uterine 
contents  are  pressed  tightly  downward  and  into  the  cervix.  The  head  of  the 
embryo  is  preceded  by  a  bulging  portion  of  the  membrane,  filled  with  liquid 
and  forming  a  distinct  bladder-like  advance-guard.  This  bag  appears  at  the 
os  uteri,  its  contents  increase  under  the  increasing  pressure,  and  in  the  majority 
of  cases,  when  the  os  is  fully  expanded,  it  bursts  and  allows  the  amniotic  liquid 
to  escape  to  the  exterior.  In  some  cases  the  rupture  is  delayed  until  the  sec- 
ond stage  of  labor,  and  rarely  the  child  is  born  with  the  membranes  intact. 

Second  Stage  of  Labor. — The  uterine  contractions  frequently  cease  for  a 
period  following  the  rupture  of  the  membrane.  They  then  begin  anew  with 
increased  force,  and  are  accompanied  by  a  new  feature,  namely,  analogous 
vigorous  rhythmic  contractions  of  the  muscles  of  the  abdominal  walls.  These, 
following  deep  inspiration  and  accompanied  by  forced  attempts  at  expiration 
with  a  closed  glottis,  diminish  the  longitudinal  and  the  lateral  diameters  of  the 
abdominal  cavity,  compress  the  abdominal  organs,  and  help  to  augment  greatly 
the  uterine  pressure.  At  the  beginning  of  the  second  stage  the  force  of  the 
contractions  is  expended  mainly  upon  the  head  of  the  embryo,  which  lies  like 
a  plug  in  the  cervical  canal.  This  is  squeezed  gradually  through  the  os  into 
the  vagina,  followed  by  the  more  easily  passing  trunk  and  limbs.  The  con- 
tractions are  frequent,  vigorous,  and  painful,  the  pains  reaching  a  maximum 
as  the  sensitive  vulva  is  put  upon  the  stretch  and  traversed.  The  vertex  is 
usually  presented  first  to  the  exterior,  the  head  and  body  following  as  the  suc- 
cessive contractions  of  the  maternal  muscles  develop  sufficient  power  to  over- 
come the  resistance  offered  to  their  passage  by  the  surrounding  walls.  In 
the  human  female  the  vaginal  muscles  do  not  appear  to  engage  in  the  expel- 
ling act,  the  uterine  and  the  abdominal  muscles  alone  sufficing  and  finally 
forcing  the  child  wholly  outside  the  mother's  body.  In  this  gradual  manner, 
painful  and  dangerous  alike  to  mother  and  child,  the  maternal  organism  forces 
the  offspring  to  forsake  its  sheltering  and  nutritive  walls  and  begin  its  inde- 
pendent existence. 

Third  Stage  of  Labor. — During  the  later  expulsive  contractions  of  the 
second  stage  the  placenta,  being  greatly  folded  by  the  diminution  in  the  uterine 
surface  of  attachment,  is  loosened  from  the  uterine  wall  by  a  rupture  taking 
place  through  the  loose  tissue  in  the  region  of  the  blood-sinuses.  The  child, 
when  born,  is  joined  to  the  loosened  placenta  by  the  umbilical  cord,  until  the 
latter  is  tied  and  cut  by  the  obstetrician.  The  muscular  contractions,  now 
almost  painless,  continue  through  the  third  stage,  and  the  placenta  is  torn 
from  its  attachment,  everted,  and  carried  gradually  outward.  The  lining 


4X1 

membrane  of  the  uterus  from  the  placenta  outward  and  for  a  consi<l< -ruble 
depth  is  gradually  torn  free  from  the  deeper  parts  through  the  spongy  lavcr, 
and  with  the  attached  chorion  and  ainnion  follows  the  placenta.  As  a  rule, 
this  after-birth  appears  at  the  vulva  within  fifteen  minutes  alter  the  cxpuUinn 
of  the  child  ;  it  consists  of  the  placenta,  the  am n ion,  the  chorion,  the  deeidua 
/r/A'.m,  and  a  considerable  portion  of  the  deddua  vera. 

Previous  to  the  third  stage  slight  bleeding  from  laceration  of  the  passages 
occurs.  But  with  the  loosening  of  the  placenta  and  the  accompanying  rupture 
of  the  placenta!  vessels  the  maternal  blood  flows  freely  and  continues  to  flow 
from  the  uterine  wall,  chiefly  from  the  placental  area,  until  the  after-birth  is 
discharged.  The  average  loss  of  blood  amounts  to  about  400  grams.  At  the 
close  of  the  third  stage  of  labor  the  uterine  contractions  have  so  far  proceeded 
that  the  organ  is  compressed  into  a  hard  compact  mass,  the  ruptured  vessels 
are  contorted  and  compressed,  and  the  bleeding  is  thereby  largely  stopped. 
For  several  hours,  however,  slight  hemorrhage  continues  as  an  accompaniment 
of  the  post-partum  contractions,  but  finally  this  ceases  with  the  formation  of  a 
blood-clot  over  the  wounded  surface. 

The  third  stage  of  labor  may  continue  through  one  or  two  hours.  It  is 
customary,  however,  for  the  obstetrician  speedily  to  put  an  end  to  it  by  assist- 
ing the  removal  of  the  after-birth. 

Nature  of  Labor. — Our  knowledge  of  the  nature  of  the  muscular  phe- 
nomena of  labor  is  incomplete.  The  uterine  contractions  are  in  part  automatic 
and  in  part  reflex,  but  to  what  extent  the  former,  and  to  what  the  latter,  is  not 
known.  Rein1  found  that  in  the  rabbit  after  section  of  all  uterine  nerves 
normal  conception,  pregnancy,  and  birth  may  occur.  In  some  animals  uterine 
movements  may  continue  after  removal  of  the  organ  from  the  body.  Such 
and  other  observations  indicate  the  existence  of  an  automatic  contractile  power 
resident  in  the  organ  itself.  Since  nerve-cells  are  not  found  in  its  walls,  it 
seems  probable  that  the  automatism  resides  in  the  muscle  tissue.  The  uterus 
is,  moreover,  very  sensitive  to  direct  stimulation,  even  after  excision.  In  ani- 
mals higher  than  rabbits  a  connection  with  the  lumbar  spinal  cord  seems 
essential  to  normal  labor.  Goltz 2  obtained  in  dogs  conception,  pregnancy,  and 
delivery  after  section  of  the  spinal  cord  at  the  height  of  the  first  lumbar 
vertebra.  In  paraplegic  women,  with  conduction  in  the  cord  broken  in  the 
dorsal  region,  delivery  is  possible.  A  centre  for  uterine  contraction  must 
hence  be  supposed  to  exist  in  the  lumbar  cord.  Centripetal  and  centrifugal 
fibres  exist  in  both  sympathetic  and  spinal  nerves,  and  reflex  uterine  contrac- 
tions are  readily  obtained  by  stimulation  of  the  central  ends  of  the  divided 
nerve-trunks.  According  to  Langley  and  Anderson,3  in  the  cat  and  the  rab- 
bit both  the  longitudinal  and  the  circular  muscular  a>:\{~  and  the  arterie-  <>!' 
the  uterus  are  supplied  with  motor  nerve-fibres  mainly  by  the  third,  fourth, 
and  fifth  lumbar  nerves ;  the  fibres  pass  to  the  sympathetic  system,  and 

1  G.  Rein  :  Pftiiger's  Archivfiir  die  gesammte  Physiologie,  1880,  xxiii. 
2Fr.  Goltx.  :    l\n<\..  1>74.  ix. 

3  Langley  and  Anderson:  Jo  nnml  of  Physiology,  1895-96,  xix.  p.  lL"J. 
VOL.  II.— 31 


482  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

nearly  all  of  them  go  to  the  inferior  mesenteric  ganglia  and  thence  by  the 
hypogastric  nerves  to  the  uterus.  Stimulation  of  the  uterus  itself,  the  vagina, 
the  vulva,  the  sciatic  and  the  crural  nerves,  and  various  sensory  regions, 
notably  the  nipples,  causes  reflex  contractions  of  the  uterus.  The  same  result 
occurs  upon  stimulation  of  various  portions  of  the  brain,  such  as  the  medulla 
oblongata,  the  cerebellum,  the  pons,  the  corpora  qtiadrigemina,  the  optic  thala- 
mus,  the  corpus  striatum,  and  even  the  corpus  callosum.  In  woman  psychic  in- 
fluences may  call  forth  or  inhibit  uterine  contractions.  How  largely  the  well- 
known  stimulating  effects  of  the  blood  in  asphyxia  and  of  drugs,  like  ergot, 
are  due  to  central,  and  how  largely  to  direct  uterine  influence  is  undecided. 
The  regular  co-ordinated  course  of  labor  and  many  experimental  facts  make 
it  probable  that,  normally,  reflex  influences  constitute  a  large  part  of  the  proc- 
ess, the  centripetal  impulses  arising  within  the  uterus  itself,  probably  largely 
from  the  pressure  upon  the  walls  of  the  lower  segment  and  the  cervix. 
In  fact,  it  is  customary  to  speak  of  labor  as  a  complex  reflex  action.  The 
undoubted  automatism  of  the  uterine  muscle-fibres  must,  however,  be  taken 
into  account,  and  the  act  should  be  regarded  as  composed  of  both  automatic 
and  reflex  elements.  We  have  here  to  deal  with  that  variety  of  contractility 
peculiar  to  smooth  muscle,  in  which  central  and  peripheral  influences  work 
together  to  bring  about  the  result.  It  is  perhaps  not  going  too  far  to  regard  all 
such  actions,  like  that  of  the  heart,  as  primarily  automatic  and  called  out  by 
direct  stimulation,  but  as  modified  and  controlled  by  reflex  influences.  The 
parturitive  contractions  of  the  striated  muscles  of  the  abdominal  walls  are 
probably  more  generally  reflex  in  nature,  modified,  however,  by  voluntary 
efforts. 

Multiple  Conceptions. — According  to  the  records  given  by  different  stat- 
isticians, the  frequency  of  twin  births  varies  considerably  in  different  coun- 
tries. In  13,000,000  births  in  Prussia,  G.  Veit 1  found  the  number  of  twins 
to  be  1.12  per  cent.,  or  1  in  89  births.  In  the  cities  of  New  York  and 
Philadelphia  recent  reports  give  the  ratio  of  twins  to  single  births  as  1  :  120, 
or  0.83  per  cent- 
Observations  of  discharged  Graafian  follicles  in  cases  of  multiple  concep- 
tions show  that  twins  may  arise  either  from  separate  eggs  or  from  a  single  egg. 
The  presence  at  birth  of  a  double  chorion  is  commonly  regarded  as  diagnostic 
of  the  former  origin,  that  of  a  single  chorion  of  the  latter.  In  the  former 
case  the  two  ova  may  come  from  a  single  Graafian  follicle,  or  from  two  folli- 
cles situated  within  one  ovary,  or  from  both  ovaries,  direct  observation  of  the 
ovaries  themselves  being  required  to  determine  the  origin  in  any  particular 
case.  The  two  ova  are  discharged  and  fertilized  probably  at  approximately 
the  same  time.  There  are  two  distinct  amnions.  The  two  placentas  may  be 
either  fused  into  one  or  wholly  separated  from  each  other,  and  accordingly  the 
decidua  reflexa  may  be  single  or  double.  The  two  offspring  may  be  of  sep- 
arate sexes,  and  do  not  necessarily  closely  resemble  each  other.  In  cases 
where  the  two  embryos  come  from  a  single  ovum  their  origin  is  little  under- 

1  G.  Veit :  Monatsschrift  fur  Geburtskunde  und  Frauenkrankheiten,  1855,  vi. 


REPRODUCTION.  483 

stood.  It  is  conceivable  that  it  may  arise  from  the  presence  of  two  nuclei 
within  the  one  ovum.  It  is  more  probable,  however,  that  it  is  due  to  a 
mechanical  separation  of  the  blastomeres  after  the  first  cleavage  or  later  in 
segmentation.1  Driesch,2  Wilson,3  Zoja,4  and  others  have  shown  that  in  various 
invertebrates  and  the  low  vertebrate  Amphioxus,  single  blastomeres,  isolated 
from  the  rest  by  shaking  or  other  unusual  treatment,  are  capable  of  develop- 
ing into  small  but  otherwise  normal  and  complete  embryos.  No  reason  is 
obvious  why  such  an  occurrence  cannot  take  place  in  human  development,  if 
in  any  accidental  manner  within  the  Fallopian  tube  the  blastomeres  become 
separated.  Driesch  observed  in  the  sea-urchins  and  Wilson  in  Amphioxus 
that  incomplete  separation  of  blastomeres  produced  two  incomplete  organisms 
more  or  less  united  together.  It  is  not  improbable  that  even  in  man  cases 
like  the  Siamese  Twins,  and  greater  monstrosities,  may  be  similarly  accounted 
for.  In  cases  of  double  pregnancy  from  a  single  ovum  the  two  amnions  are 
usually  separate,  in  rare  cases  a  breaking  away  of  their  partition  wall  throwing 
them  into  one ;  the  two  placentas  usually  fuse  more  or  less  into  one,  the  blood- 
vessels of  the  two  halves  always  anastomosing ;  and  a  single  deddua  reflexa 
covers  both.  The  two  offspring  are  uniformly  of  the  same  sex  and  their  per- 
sonal resemblance  is  always  close. 

In  Veit's  statistics  of  13,000,000  births  in  Prussia,  triplets  occur  with  a 
frequency  of  0.012  per  cent,  or  1  in  7910,  and  quadruplets  1  in  371,126  births. 
There  are  well-authenticated  cases  of  quintuplets.  In  all  of  these  cases  a 
single  ovum  rarely,  if  ever,  contributes  more  than  two  embryos,  and  these 
are  characterized,  as  in  the  case  of  twins,  by  being  of  similar  sex,  by  pos- 
sessing a  single  chorion,  and  by  close  personal  resemblance. 

The  Determination  of  Sex. — In  most,  if  not  all,  civilized  races  more  boys 
are  born  than  girls.  This  is  shown  in  the  following  table  :6 

Boys  bom  to  1000  Girls  born  (1887-91). 
Italy 1058  England 1036 


Ireland      1055 

German  Empire 1052 

France  .  .  1046 


Connecticut 1072 

Rhode  Island .    .  1049 

Massachusetts      1046 


The  proportional  birth-rate  of  the  two  sexes  is  usually  fairly  constant  from 
year  to  year.  This  means  that  constant  regulating  factors  are  at  work. 
What  determines  sex  in  any  one  individual  is  ill  understood.  The  sexual 
organs  in  the  human  embryo  are  well  differentiated  at  the  eighth  week  of 
intra-uterine  life,  hence  the  sex  of  the  child  must  be  settled  previously  to  this 
time.  It  is  at  present  quite  impossible  to  say  whether  it  is  settled  in  the 
germ-cells  previous  to  their  union,  in  the  act  of  fertilization,  or  during  the 
early  uterine  life.  Many  facts,  both  observational  and  experimental,  and 

1  Of.  Fr.  Ahlfeld  :  Archiv  fur  Gynakologie,  ix.,  1876. 

2  H.  Driesch:  Zeiischrift  fur  wissenschaftliche  Zoologie,  liii.,  1892;  lv.,  1893;  Mittheilungen 
mis  der  Zooloyischen  Station  zu  Neapd,  xi.,  1893. 

3  E.  B.  Wilson :  Journal,  of  Morphology,  viii.,  1893. 

4  R.  Zoja :  Archiv  fiir  k'nfi'-irkelungsmechanik  der  Organismen,  ii.,  1895. 

5  Bulletin  de  Vinstitut  international  de  statistique,  vii. 


484  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

more  hypotheses,  bearing  upon  the  determination  of  sex,  have  been  brought 
forward.  The  Hofacker-Sadler  law  (Hofacker,  1828;  Sadler,  1830)  is  well 
known,  as  follows :  If  the  father  be  older  than  the  mother,  more  boys  than 
girls  will  be  born  ;  if  the  parents  be  of  equal  age,  slightly  more  girls  than 
boys ;  if  the  mother  be  older  than  the  father,  the  probability  of  girls  is  still 
greater.  Since  the  promulgation  of  this  so-called  law  facts  for  and  against  it 
have  been  brought  forward,  but  the  balance  of  evidence  seems  to  be  in  favor 
of  its  truth.  Thury  in  1863  claimed  that  the  degree  of  "ripeness"  of  the 
ovum  is  the  determining  factor — the  female  resulting  from  the  less  ripe  ovum, 
hence  the  earlier  after  its  liberation  the  egg  is  fertilized,  the  greater  is  the 
tendency  to  the  production  of  a  female  ;  the  later  the  fertilization,  the  greater 
the  probability  of  a  male.  While  it  is  not  at  all  clear  in  what  the  "  ripeness  " 
or  "  unripeness  "  of  an  ovum  consists,  breeders  have  made  use  of  this  prin- 
ciple apparently  with  success — offspring  conceived  at  the  beginning  of  "  heat " 
seem  to  be  more  usually  females.  Likewise,  it  is  frequently  believed  that  in 
human  beings  conceptions  immediately  after  menstruation  produce  a  larger 
proportion  of  females  than  later  conceptions.  Schenk 1  also  bases  his  view 
on  the  condition  of  ripeness  of  the  ovum.  He  regards  the  presence  of  sugar 
in  the  urine  of  the  pregnant  woman  as  evidence  of  incomplete  metabolism 
in  the  body,  thus  of  incomplete  nutrition  or  unripeness  of  the  ovum,  and 
hence  of  tendency  toward  femaleness  in  the  offspring.  By  means  of  a  highly 
nitrogenous  diet,  which  eliminates  the  sugar  from  the  urine  and  increases  the 
proportion  of  reducing  substances,  he  claims  to  make  the  metabolism  more 
complete,  to  insure  a  riper  ovum,  and  hence  to  make  it  probable  that  the 
offspring  will  be  a  male.  Schenk's  reasoning  is  excessively  hypothetical,  and 
his  present  facts  are  too  few  to  substantiate  his  claims.  Diising2  accepts 
Thury's  view  and  extends  it  to  the  male  element — the  younger  the  spermato- 
zoon the  greater  the  tendency  toward  the  production  of  males.  Hence  among 
animals  the  scarcity  of  one  sex  leads  to  the  more  frequent  exercise  of  its 
reproductive  function,  the  employment  of  younger  germ-cells,  and  therefore 
the  relative  increase  of  that  sex.  Further,  the  nearer  a  parent  is  to  the 
height  of  his  reproductive  capacity  the  less  will  be  the  probability  of  trans- 
mitting his  own  sex  to  the  offspring.  Nutrition  seems  to  have  some  obscure 
relation  to  the  question  of  sex.  Thus,  by  feeding  tadpoles  with  highly 
nutritious  flesh  Yung a  increased  the  percentage  of  females  from  56  to  92. 
Mrs.  Treat4  showed  that  the  butterflies  of  well-fed  caterpillars  became 
females,  those  of  starved  caterpillars  males.  Statistics  among  mammals  and 
human  beings  indicate  that  the  proportion  of  male  to  female  offspring  varies 
inversely  with  the  nutrition  of  the  parents,  especially  of  the  mother.  Thus, 

1  L.  Schenk  :  Einfluss  auf  das  Geschlechtverhdltniss,  Magdeburg,  1898.     Authorized  transla- 
tion:   The  Determination  of  Sex,  London,  1898. 

2  K.  Diising:    Jenaische  Zeitschrift  fur  Naturwissenschaft,  1883,   xvi.,  and  1884,  xvii.;    also 
published  separately,  Die  Regulierung  des  Geschlectverhdltnisses  bei  der  Vermehrung  der  Menschen, 
Tiere  und  Pflanzen,  Jena,  1884. 

3E.  Yung:   Oomptes  rendus  de  I' academic  des  sciences,  Paris,  1881,  xcii. 
4 Mrs.  Mary  Treat:   The  American  Naturalist,  1873,  vii. 


HE  PR  OD  UCTION.  485 

more  boys  are  born  in  the  country  than  in  the  city,  and  in  poor  than  in  pros- 
perous families;  the  relative  number  of  boys  is  said  to  vary  even  with  the 
price  of  food.  It  is  contended,  moreover,  and  with  some  statistical  support, 
that  in  the  human  race  an  epidemic  or  a  war,  either  of  which  affects  adversely 
the  well-being  of  the  people,  is  followed  by  a  relative  increase  of  male  births. 
Statistics  indicate  also  that  the  proportion  of  females  is  high  in  warm  climates, 
that  of  males  high  in  cold  climates.  Maupas l  found  that  sex  in  the  rotifer, 
lli/i/dtiiut  wntd,  could  be  controlled  by  altering  the  temperature  of  the  medium 
surrounding  the  egg-laying  females.  In  various  experiments  at  a  tempera- 
ture of  26°-28°  C.  81-100  per  cent,  of  the  eggs  gave  rise  to  males,  the  rest 
to  females;  at  14°-15°  C.  only  5-24  per  cent,  were  males,  the  much  lai-^-r 
majority  females.  Nussbaum 2  has  brought  Maupas's  facts  into  harmony 
with  the  facts  regarding  nutrition  by  showing  that  the  higher  temperature 
carries  with  it  a  higher  birth-rate  and  more  rapid  development,  hence  a 
greater  need  of  food  and  relative  lack  of  it  for  the  individual  ;  the  result  is 
poor  nutrition  and  the  production  of  an  excess  of  males  over  females.  It  is 
claimed,  further,  that  ethnic  intermixture  causes  a  decrease  in  the  relative 
number  of  males  born.  This  is  strongly  supported  by  a  statistical  study  by 
Ripley3  of  the  two  races  inhabiting  Belgium,  the  Walloons,  of  the  same 
origin  as  the  Kelts  in  France,  and  the  Flemish  of  German  stock.  AY  here 
these  races  are  purest,  the  number  of  boys  born  to  1000  girls  is  1064 ;  along 
the  region  where  the  two  races  come  into  contact,  however,  the  number  may 
fall  as  low  as  1043. 

The  above  considerations  are  highly  interesting  and  suggestive,  but  they 
have  not  yet  been  brought  under  general  laws  sufficiently  to  make  their  bear- 
ing upon  the  main  problem  wholly  clear.  It  is  probable  that  numerous 
factors  are  of  influence  in  the  determination  of  sex.4  The  general  deduction 
from  all  the  facts  seems  justified  that  unfavorable  nutritive  conditions  sur- 
rounding the  parents  tend  to  the  production  of  males,  favorable  conditions 
to  the  production  of  females.  The  experimental  results  indicate,  moreover, 
that  the  conditions  surrounding  the  parents  or  the  developing  embryo  are 
largely  responsible  for  the  resulting  sex.  Watase5  regards  the  embryo  as 
neutral  as  regards  sex  from  the  time  of  fertilization  up  to  a  certain  stage  in 
its  development ;  external  conditions  act  as  a  stimulus  to  the  sexless  proto- 
plasm, and  the  resulting  response  is  a  development  in  the  direction  of  either 
maleness  or  femaleness  according  to  the  nature  of  the  stimulus.  How  largely 
and  in  what  manner  this  may  be  true  of  the  human  species  is  wholly  unknown. 
Diising6  urges  that  the  various  factors  determining  sex  have  arisen  through 
natural  selection  ;  they  are  conducive  to  the  continuance  of  the  species,  and 

*E.  Maupas:  Comptes  rendus  de  I'academie  des  sciences,  Paris,  1891,  cxiii. 

2  Xussbaum  :   Archiv  fur  mikroskopische  Anatomic,  1897,  xlix.  S.  227. 

3  W.  Z.  Ripley  :   Quarterly  Publications  of  the  American  Statistical  Association,  March,  1896,  v. 

4  For  a  critical  review  of  the  various  theories  see  L.  Cohn :  Die  willkurliche  Bestimmung  des 
Geschiecht*.  2d  ed.,  Wurzburg,  1898. 

5S.  Watase:  Journal  of  Morphology,  1892,  vi. 
6  Diising  :    L«c.  fit. 


486  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

they  act  in  such  a  way  that  sex  is  in  a  certain  sense  self- regulating — the 
scarcity  of  one  sex  tends  to  the  greater  production  of  individuals  of  that  sex ; 
this  is  instanced  by  the  fact  mentioned  above  that  after  the  destruction  of 
males  by  war  relatively  more  males  are  born  than  previously. 

E.  EPOCHS  IN  THE  PHYSIOLOGICAL  LIFE  OF  THE  INDIVIDUAL. 

Fertilization  begins,  somatic  death  ends,  the  physiological  life  of  the  indi- 
vidual. Between  these  two  events  the  life-processes  go  on  gradually,  and, 
with  the  exception  of  birth,  are  marked  by  few  abrupt  changes.  It  is  some- 
times convenient  to  divide  the  individual  life  into  a  number  of  successive 
stages,  as  follows :  the  embryonic  period,  the  fetal  period,  infancy,  childhood, 
youth  or  adolescence,  maturity,  and  old  age  or  senescence.  Such  a  division, 
however,  is  not  physiologically  exact,  the  stages  are  not  sharply  limited,  and 
the  terms  are  employed  in  very  different  senses  by  different  writers.  Between 
fertilization  and  birth  the  functions  originate  and  are  developed  gradually. 
At  birth  the  environment  of  the  individual  is  abruptly  changed,  organic 
connection  with  the  mother  suddenly  ceases,  and  profound  physiological 
changes  occur.  At  this  time,  or  shortly  after  it,  the  individual  is  capable 
of  performing  all  the  functions  of  adult  life  with  the  exception  of  reproduc- 
tion, the  functions  needing,  however,  to  be  exercised  and  improved  before 
they  are  at  their  best.  From  birth  to  maturity,  therefore,  the  physiological 
history  is  mainly  a  history  of  progressive  modifications  of  function — modi- 
fications, indeed,  of  great  importance,  but  secondary  to  the  primary  fact  of 
function  itself.  The  same  may  be  said  of  the  period  of  old  age,  with  the  dif- 
ference that  here  the  modifications  of  function  are  retrogressive.  In  the  present 
book,  devoted  mainly  to  the  physiology  of  the  adult  at  the  time  of  maturity, 
little  can  be  said  of  the  origin  and  development  of  function  in  the  embryo ; 
the  modifications  of  function  at  different  periods  of  life  have  been  discussed  in 
connection  with  the  various  functions  themselves ;  certain  topics  of  special  physio- 
logical significance  have,  however,  been  left  for  brief  treatment  in  this  chapter. 

Growth  of  the  Cells,  the  Tissues,  and  the  Organs. — All  growth, 
whether  of  the  cells,  the  tissues,  or  the  organs,  is  the  result  of  no  more  than 
three  processes,  viz.  multiplication  of  cells,  enlargement  of  cells,  and  deposition 
of  intercellular  substance,  the  first  two  processes  being  the  most  potent  of  all. 
Increase  in  the  number  of  cells  is  largely,  although  not  wholly,  an  embryonic 
phenomenon ;  increase  in  the  size  of  cells  and  deposition  of  intercellular  sub- 
stance are  especially  important  from  the  later  embryonic  period  through  the 
time  of  birth  and  up  to  the  cessation  of  the  body-growth.  The  periods  of 
growth  of  the  several  tissues  differ ;  in  view  of  this  it  is  quite  impossible  to 
designate  any  period  except  that  of  death  at  which  the  growth  of  the  tissues 
wholly  terminates.  Detailed  statistics  of  the  growth  of  organs  are  wanting. 

Growth  of  the  Body  before  Birth. — The  most  obvious  result  of  growth 
of  the  cells,  the  tissues,  and  the  organs,  is  growth  or  increase  in  size  of  the 
body.  Growth  of  the  body  continues  actively  from  the  beginning  of  the  seg- 
mentation of  the  ovum  up  to  about  the  age  of  twenty-five  years,  and  results  in 


REPRODUCTION.  487 

an  increase  in  all  dimensions  and  in  weight.  In  determining  the  extent  of 
growth,  the  two  most  convenient  and  most  commonly  used  measurement-  mv 
those  of  length,  or  height,  and  weight.  For  the  embryo  the  following 
has  been  compiled  by  Hecker:1 

Table  showing  the  Average  Length  and  Weight  of  the  Human  Embryo  at 

Different  Ages. 


Month. 
Third 

Length  of  embryo  in  centimeters. 
4  to    9 

Weight  of  embryo  in  grams. 

Fourth 

10  to  17 

57 

Fifth 

18  to  27 

984 

Sixth 
Seventh   . 

28  to  34 
35  to  38 

634 

1218 

Eighth     . 

39  to  41 

1569 

Ninth 

42  to  44 

1071 

Tenth 

45  to  47 

9334 

The  length  and  the  weight  at  birth  vary  very  greatly.  The  average  measure- 
ments, as  given  for  over  450  infants  in  Great  Britain,  are,  for  height,  males 
19.5  inches,  females  19.3  inches;  for  weight,  males  7.1  pounds,  females,  6.9 
pounds.  The  weight  at  birth  is  said  to  be  greater  the  nearer  the  mother's 
age  is  to  thirty-five  years,  the  greater  the  weight  of  the  mother,  the  greater 
the  number  of  previous  pregnancies,  and  the  earlier  the  appearance  of  the  first 
menstruation.  Race  and  climate  are  also  of  influence.  Minot 2  believes  that  all 
of  these  influences  work  principally  through  prolonging  or  abbreviating  the 
period  of  gestation,  and  that  the  variations  at  birth  depend  partly  upon  the 
duration  of  gestation  and  partly  upon  individual  differences  of  the  rate  of 
growth  in  the  uterus. 

Growth  of  the  Body  after  Birth. — In  studying  the  growth  of  the  body 
after  birth  two  methods  have  been  employed,  named  the  "  generalizing  "  and 
the  "individualizing"  methods.  The  former  consists  in  deducing  the  course 
of  growth  by  averages  or  other  central  values  from  statistics  taken  from  a 
large  number  of  individuals  at  different  ages.  It  is  the  method  more  com- 
monly employed ;  it  shows  the  course  of  growth  of  the  typical  child,  but  is 
inexact  in  enabling  future  growth  to  be  predicted  in  individual  cases.  The 
individualizing  method  consists  in  measuring  the  actual  growth  of  the  same 
individual  through  successive  years ;  it  shows  well  the  relation  of  the  indi- 
vidual to  the  type  throughout  the  period  of  growth.  The  course  of  growth 
of  British  boys  and  girls  from  birth  up  to  the  age  of  twenty-four  is  graphically 
shown  in  the  accompanying  diagram  (Fig.  229).  Growth  is  here  seen  to  be 
rapid  during  the  first  five  years  of  life,  then  slower  up  to  the  tenth  or 
the  twelfth  year.  From  thence  up  to  the  fifteenth  or  the  seventeenth  year 
—that  is,  preceding  and  including  puberty — marked  acceleration  occurs, 
which  in  turn  is  followed  by  slow  increase  up  to  the  twentieth  or  the 
twenty-fifth  year.  For  from  five  to  ten  years  thereafter  slight  increase  in 

1  C.  Hecker:  Monatsschrift  fur  Gebnrtxknnde  und  Franenkrankln  /'/»//. 

2  C.  S.  Minot :  Human  Embryology,  1892. 


488 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


height  occurs,  while  from  the  accumulation  of  fat  the  weight  usually  rises 
markedly  up  to  the  fiftieth  or  the  sixtieth  year.  One  of  the  most  interesting 
results  revealed  by  statistics  is  the  relative  growth  of  the  two  sexes.  From 
birth  up  to  about  the  age  of  ten  or  twelve,  boys  show  a  slight  and  increasing 
preponderance  over  girls,  but  the  two  curves  are  nearly  parallel.  The  prepu- 
bertal  acceleration  of  growth  in  girls,  however,  precedes  that  of  boys,  and  is 
even  accompanied  by  some  check  in  the  male  growth,  with  the  result  that 
between  the  ages  of  twelve  and  fifteen  girls  are  actually  heavier  and  taller 
than  boys.  This  fact,  first  pointed  out  in  1872  by  Bowditch  l  from  observa- 


Age. 


10 


15 


20 


Years. 


25 


10 


1 

I 

140 


120 


100 


80 


60 


40 


20 


Males. 

Females. 

FIG.  229.— Diagram  showing  increase  of  stature  and  weight  of  both  sexes,  as  determined  by  the  Anthropo- 
metric  Committee  of  the  British  Association.2 

tions  on  several  thousand  Boston  school  children,  has  been  abundantly  con- 
firmed by  Pagliani  in  Italy,  Key  in  Sweden,  Schmidt  in  Germany,  Porter  in 
St.  Louis,  and  others.  At  about  fifteen  years  boys  again  take  the  lead  and 
maintain  it  throughout  life.  Boys  grow  most  rapidly  at  sixteen,  girls  at  thir- 
teen or  fourteen,  years  of  age ;  the  former  attain  their  adult  stature  approxi- 
mately at  twenty-three  to  twenty-five,  the  latter  at  twenty  to  twenty-one  years. 
The  details  of  growth  and  the  actual  measurements  vary  considerably  with 
race ;  thus  the  supremacy  of  the  American  girl  over  her  brother  appears  to  be 
less  marked  and  to  cover  a  shorter  period  than  that  of  the  English,  German, 
Swedish,  or  Italian  girl.  Children  of  well-to-do  families  are  superior  to 

1  H.  P.  Bowditch  :  Eighth  Annual  Report  of  the  State  Board  of  Health  of  Massachusetts,  1877. 

2  Roberts :  Manual  of  Anthropometry,  1878. 


REPRODUCTION.  1S<) 

others  in  both  weight  and  stature.  Beyer1  has  shown  that  systematic  exercise 
may  markedly  increase  both  height  and  weight.  Disease  may  alter  the  form 
of  the  curve  of  growth.  But  the  final  result  seems  to  depend  le>s  upon 
external  conditions  than  upon  race  and  sex.  As  an  interesting  accessory 
fact  it  was  found  by  IWter-  that  well-developed  children  take  a  higher  r.-mk 
in  school  than  less-developed  children  of  the  same  age.  If  the  percentage 
annual  increase  of  the  total  weight  be  computed,  it  is  found  to  diminish 
throughout  life,  very  rapidly  during  the  first  two  or  three  years,  later  more 
slowly  and  with  minor  variations  of  increase  and  decrease;  that  is,  as  growth 
proceeds  and  the  powers  of  the  individual  mature,  the  power  to  grow  becomes 
rapidly  less.  This  is  a  peculiar  and  most  interesting  fact,  and  has  not  been 
explained.  It  would  seem  to  signify  that  the  sum  of  the  vital  powers 
declines  from  birth  onward.  Many  facts  indicate  that  the  common  concep- 
tion, dating  from  the  time  of  Aristotle,  of  human  life  as  consisting  of  the 
three  periods  of  rise,  maturity,  and  decline,  must  give  way  to  a  more  rational 
idea  of  a  steady  decline  from  birth. 

Puberty. — By  puberty  is  meant  the  period  of  sexual  maturity,  at  which 
the  individual  becomes  able  to  reproduce.  In  the  male  the  exact  time  of  its 
onset,  characterized  primarily  by  the  appearance  of  fully  ripe  spermatozoa,  is 
not  well  known,  but  is  believed  to  be  about  one  year  later  than  in  the  female. 
In  temperate  climates,  therefore,  it  usually  appears  in  boys  not  before  the  age 
of  fifteen  ;  it  is  earlier  in  warmer  regions.  It  is  preceded  and  accompanied  by 
acceleration  in  bodily  growth,  already  spoken  of.  Other  bodily  changes,  such 
as  general  maturation  of  the  functions  of  the  reproductive  organs,  alterations  in 
the  bodily  proportions,  increase  of  strength,  and  growth  of  the  beard,  all  of  which 
are  elements  of  the  transformation  from  boyhood  to  manhood,  either  occur  at 
that  time  or  follow  soon  after.  One  of  the  most  obvious  external  changes  is 
that  of  the  voice.  Its  tone  may  fall  permanently  an  octave,  and  for  the  time  being 
become  rough,  broken,  and  uncontrollable.  This  is  due  to  a  rapid  general 
enlargement  of  the  laryngeal  cartilages  and  a  lengthening  of  the  vocal  cords. 

In  the  girl  the  oncoming  of  puberty  is  marked  more  exactly  than  in  the 
boy  by  the  appearance  of  menstruation,  in  the  majority  of  girls  in  temperate 
climates  at  the  age  of  fourteen  to  seventeen.  But  other  characteristic  anatom- 
ical and  physiological  changes  in  the  body  occur.  The  uterus,  the  external 
reproductive  organs,  and  the  breasts  become  larger,  while  the  pelvis  widens. 
The  prepubertal  acceleration  of  growth  has  been  mentioned.  Nervous  disor- 
ders are  especially  prone  to  make  their  appearance  at  this  time.  The  subcuta- 
neous layer  of  adipose  tissue  develops  and  confers  upon  the  outlines  the  grace- 
ful curves  characteristic  of  the  woman's  body.  The  mental  faculties  mature, 
and  the  girl  becomes  a  woman  earlier  and  more  rapidly  than  the  boy  a  man. 

1  H.  G.  Beyer  :  "The  Influence  of  Exercise  on  Growth,"  Journal  of  Experimental  Medicine, 
1896,  i.  p.  546.     See  also  "  The  Growth  of  U.  S.  Naval  Cadets,"  Proceedings  of  the  United  States 
Naval  Institute,  1895,  xxi.  p.  297. 

2  W.  T.  Porter:  "The  Physical  Basis  of  Precocity  and   Dulness,"  Transactions  of  the  Acad- 
emy of  Science  of  St.  Louis,  1893,  vi.,  No.  7.     See  also  "The  Growth  of  St.  Louis  Children," 
Ibid.,  1894,  vi.  No.  12. 


490  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Climacteric. — At  the  sixtieth  year  the  power  of  producing  spermato- 
zoa, and,  therefore,  the  reproductive  power  of  man,  begins  to  wane.  It  con- 
tinues, however,  in  a  diminishing  degree,  even  to  extreme  old  age,  and  there 
is  no  recognized  period  of  ending  of  the  male  sexual  life. 

In  woman,  on  the  other  hand,  the  sexual  period  continues  for  only  thirty 
to  thirty-five  years,  and  the  climacteric,  menopause,  or  change  of  life,  marks  a 
definite  ending  of  the  power  of  reproduction.  In  temperate  climates  it  occurs 
usually  between  the  ages  of  forty-four  and  forty-seven  ;  in  warmer  regions  it 
comes  early,  in  colder  late.  It  is  earlier  in  the  laboring  classes,  and  later 
where  menstruation  has  first  appeared  early.  Its  most  characteristic  feature  is 
the  cessation  of  menstruation,  which  is  a  gradual  process  extending  over  a 
period  of  two  or  three  years  and  characterized  by  irregularity  in  the  oncoming 
and  the  quantity  of  the  flow,  and  by  gradual  diminution.  But  the  cessation 
of  the  menses  is  but  one  phenomenon  in  a  long  series  of  changes  that  pro- 
foundly affect  the  whole  organism  and  endanger  life.  The  reproductive  organs 
and  the  breasts  diminish  in  size,  and  ovulation  ceases.  The  changes  in  the 
pelvic  organs  are  in  general  the  reverse  of  those  occurring  at  puberty.  The 
organic  functions  generally  are  rendered  irregular ;  dyspepsia,  cardiac  palpi- 
tation, sweating,  and  vasomotor  changes  are  frequent ;  vertigo,  neuralgia, 
rheumatism,  and  gout  are  not  rare ;  a  tendency  to  obesity  occurs,  though 
sometimes  the  reverse ;  irritability,  fear,  hysteria,  and  melancholia  may  be 
present ;  the  disposition  may  be  temporarily  altered ;  all  of  which  changes 
indicate  that  the  female  organism  at  this  time  suffers  a  profound  nervous 
shock.  The  loss  of  the  weighty  function  of  reproduction  and  the  adaptation 
to  the  new  order  of  events  are  not  accomplished  quietly. 

Senescence. — The  progressive  diminution  in  the  power  of  growth  from 
birth  onward  throughout  life  has  been  mentioned,  and  may  be  interpreted  as 
indicating  that  the  process  of  senescence  begins  with  the  beginning  of  life.1 
In  the  broadest  sense  this  is  true,  and  is  confirmed  by  a  study  of  various 
organic  functions.  In  the  more  restricted  sense  senescence  or  old  age  com- 
prises the  period  from  about  fifty  years  (in  woman  from  the  climacteric) 
onward,  during  which  there  is  a  noticeable  progressive  waning  of  the  vital 
powers.  The  leading  somatic  changes  accompanying  old  age  are  atrophic  and 
degenerative,  but  detailed  statistics  of  this  period  are  almost  wholly  wanting. 
A  marked  cellular  difference  between  the  young  and  the  old,  which  is  shown 
by  nearly  if  not  quite  all  tissues,  is  the  relatively  large  nucleus  and  small 
quantity  of  cytoplasm  in  the  young,  the  proportions  being  reversed  in  the  old. 
This  has  been  pointed  out  as  follows  by  Hodge2  in  the  nerve-cells  of  the  first 
cervical  spinal  ganglion  : 

Volume  of  Nucleoli  observ-  Pigment  Pigment 

nucleus.  able  in  nuclei.  much.  little. 

Fetus  (at  birth) 100  per  cent.  in  53  per  cent. 

Old  man  (at  ninety -two  years)     64.2     "  in    5         "  67  per  cent.  33  per  cent. 

1  Of.  C.  S.  Minot  :  Journal  of  Physiology,  1891,  xii. 

2  C.  F.  Hodge :  Anatomischer  Anzeiger,  1894,  ix. :  Journal  of  Physiology,  1894,  xvii. 


REPR  OD  UCTION.  491 

Thus  with  the  progress  of  age  the  nuclei  become  small  and  irregular  in  out- 
line, and  the  cytoplasm  pigmented,  while  the  nucleoli  arc  often  waiitiu^.  The 
nuclear  differences  are  even  more  marked  in  the  cerebral  ganglia  of  bees,  where 
moreover,  aired  individuals  possess  a  smaller  number  of  nerve-cells  than  the 
young.  The  nuclear  differences  accord  with  the  common  belief  that  the  nucleus 
is  the  formative  centre  of  the  cell.  It  has  been  shown  that  a  decrease 
in  the  weight  of  the  whole  brain  occurs  in  both  men  and  women,  beginning  in 
the  former  at  about  fifty-five  years,  in  the  latter  at  about  forty-five  years.  In 
eminent  men  the  decrease  begins  later.  The  thickness  of  the  cortex  and  the 
number  of  tangential  fibres  in  it  diminish  especially  after  fifty  years,  and  this 
probably  signifies  a  loss  of  cells.  There  is  a  decrease  in  general  brain-power, 
in  power  of  origination,  in  the  power  to  map  out  new  paths  of  conduction  and 
association  in  the  central  nervous  system  and  thus  to  form  habits.  Reaction- 
time  is  lengthened.  The  delicacy  of  the  sense-organs  is  noticeably  less,  and  in 
the  eye  the  hardening  of  the  crystalline  lens  and  the  weakening  of  the  ciliary 
muscle  diminish  the  power  of  accommodation.  The  muscles  atrophy  and  mus- 
cular strength  is  reduced.  The  pineal  gland,  ligaments,  tendons,  cartilage,  and 
the  walls  of  the  arteries,  show  a  tendency  toward  calcification,  and  the  bones 
become  more  brittle.  Subcutaneous  adipose  tissue  disappears,  but  a  fatty  de- 
generation of  cells  is  not  uncommon,  notably  in  all  varieties  of  muscle-cells, 
in  nerve-cells,  and  probably  in  gland-cells.  The  pigment  of  the  hairs  disap- 
pears. The  size  of  the  muscles,  the  liver,  the  spleen,  the  lymphatic  and  prob- 
ably the  digestive  glands,  decreases.  The  heart  and  the  kidneys  seem  to  retain 
their  adult  size.  The  vital  capacity  of  the  lungs,  the  amounts  of  carbonic  acid 
and  of  urine  excreted,  diminish.  The  rate  of  respiration  and  of  the  heart-beat 
rises  slightly.  Ovulation  is  wanting,  and  the  power  of  producing  spermatozoa 
is  lessened.  The  stature  undergoes  a  slight  and  steady  decrease.  Boas l  has 
shown  that  in  the  North  American  Indian  this  continues  from  about  thirty 
years  of  age  onward.  All  of  these  changes,  the  details  of  which  should  be  care- 
fully studied  and  reduced  to  anatomical  and  physiological  exactness,  demonstrate 
that  senescence  is  characterized  by  a  steady  diminution  of  vitality. 

Death. — Sooner  or  later  vitality  must  cease  and  the  change  that  is  called 
death  must  come.  The  term  "  death  "  is  used  in  two  senses,  according  as  it  is 
applied  to  the  whole  organism  or  to  the  individual  tissues  of  which  the  organ- 
ism is  composed.  The  former  is  distinguished  as  somatic  death,  or  death 
simply,  the  latter  as  the  death  of  the  tissues. 

Somatic  death  occurs  when  one  or  more  of  the  organic  functions  is  so  dis- 
turbed that  the  harmonious  exercise  of  all  the  functions  becomes  impossible. 
Thus,  if  the  brain  receives  a  severe  concussion,  the  co-ordination  of  the  organs 
may  be  interrupted  ;  if  the  respiration  ceases,  the  necessary  oxygen  is  withheld  ; 
if  the  heart  fails,  the  distribution  of  oxygen  and  food  and  the  collection  of 
wastes  come  to  an  end ;  if  the  kidneys  are  diseased,  the  poisonous  urea  is 
retained  within  the  tissues.  A  continuation  of  any  one  of  these  profound 
abnormal  conditions,  which  may  be  brought  about  by  accident  or  disease,  or  a 

1  F.  Boas  :  Verhandlungen  der  Berliner  Anthropologischen  Gesellschaft,  1895. 


492  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

simultaneous  occurrence  of  several  slight  disturbances  of  function,  such  as  is 
not  infrequent  in  aged  persons,  may  prevent  the  restoration  of  that  concordance 
among  the  organs  without  which  the  individual  cannot  live.  The  most  con- 
venient and  most  certain  sign  by  which  somatic  death  may  be  recognized  is  the 
absence  of  the  beat  of  the  heart,  and  in  nearly  all  cases  this  is  the  criterion 
employed.  But  it  should  be  borne  in  mind  that  the  failure  of  the  heart  to 
beat  is  but  one  of  the  causes,  and  frequently  a  very  secondary  one,  the  primary 
cause  being  then  associated  with  other  functions.  It  is  at  present  in  most  cases 
quite  impossible  to  trace  the  course  of  events  by  which  the  derangement  of  one 
function  leads  to  the  ultimate  cessation  of  individual  life. 

Death  of  the  tissues  or  of  the  living  substance  is  neither  necessarily  nor 
usually  simultaneous  with  somatic  death.  Constantly  throughout  life  the  mole- 
cules of  living  matter  are  being  disintegrated,  and  whole  cells  die  and  are  cast 
away ;  life  and  death  are  concomitants.  With  the  cessation  of  the  individual 
life  the  nervous  system  dies  almost  immediately.  "With  the  muscular  tissue  it 
is  very  different.  The  stopping  of  the  beat  of  the  heart  is  a  gradual  process, 
and,  as  Harvey  long  ago  pointed  out,  the  last  portion  to  beat,  the  ultimum 
moriens,  is  the  right  auricle.  For  many  minutes  after  death  the  heart,  if 
exposed,  will  be  found  to  be  excitable  and  to  respond  by  single  contractions  to 
single  stimuli.  Irritability  is  said  to  continue  in  the  smooth  muscle  of  the 
stomach  and  the  intestines  for  forty-five  minutes,  and  considerably  later  than 
this  the  striated  muscles  of  the  limbs  can  still  be  made  to  twitch  by  proper 
stimuli,  in  the  cat  and  rabbit  after  twelve  or  fourteen  hours.1  Gland-cells 
die  probably  within  a  few  minutes.  As  to  the  chemical  changes  undergone 
by  the  protoplasm  in  the  process  of  dying,  little  can  be  said.  The  composi- 
tion of  dead  protoplasm  is  comparatively  well  known,  that  of  living  proto- 
plasm is  at  present  largely  a  blank  ;  and,  although  investigation  has  gone  suf- 
ficiently far  to  offer  a  basis  for  several  suggestive  hypotheses,  the  latter  are  too 
abstruse  for  lucid  discussion  in  the  present  space.  Neither  in  somatic  death 
nor  in  the  death  of  the  tissues  does  the  body  lose  weight.  Within  fifteen  or 
twenty  hours  it  cools  to  the  temperature  of  the  surrounding  medium.  Rigor 
mortis,  due  to  the  coagulation  of  the  muscle-plasma  within  the  muscle-cells, 
begins  within  a  time  varying  with  the  cause  of  death  from  a  half  hour  to 
twenty  or  thirty  hours,  and  continues  upon  an  average  twenty-four  to  thirty- 
six  hours.  Then  the  tissues  soften,  and  soon  putrefactive  changes  begin. 

Theory  of  Death. — It  has  been  intimated  that  all  the  tissues  are  destined 
to  die.  An  exception  must  be  made  in  the  case  of  those  germ-cells,  both  male 
and  female,  that  are  employed  in  the  production  of  new  individuals.  They 
pass  from  one  individual,  the  parent,  to  another,  the  offspring,  and  thus  cannot 
be  said  to  undergo  death.  This  is  the  basis  of  Weismann's  theory  of  the 
origin  and  significance  of  death  in  the  organic  world.2  According  to  Weis- 
mann,  primitive  protoplasm  was  not  endowed  with  the  property  of  death. 
As  found  in  the  simplest  individuals,  like  the  Amoeba,  even  at  the  present 

1  Lee,  Adler,  and  Bulkley  :  American  Journal  of  Physiology,  1900,  iii.  p.  xxix. 

2  A.  Weismann :  Essays  upon  Heredity,  1889,  i. 


REPRODUCTION.  \\\:\ 

day,  with  a  continuance  of  the  proper  nutritive  condition  protoplasm  does  not 
grow  old  and  die;  the  single  individual  divides  into  two  and  life  continues 
unceasing,  unless  accident  or  other  untoward  event  interferes.  With  the 
progress  of  evolution,  however,  the  cells  of  the  individual  body  have  become 
differentiated  into  germ-cells  and  somatic  cells,  the  former  subserving  the 
reproduction  of  the  species,  the  latter  all  the  other  bodily  functions.  Germ- 
cells  are  passed  on  from  parent  to  offspring;  they  never  die,  they  are  immor- 
tal. Somatic  cells,  on  the  other  hand,  grow  old,  and  at  last  perish.  Death 
was,  therefore,  in  the  beginning,  not  a  necessary  adjunct  to  life ;  it  is  not  inhe- 
rent in  primitive  protoplasm,  but  has  been  acquired  along  with  the  differen- 
tiation of  protoplasm  into  germ-plasm  and  somatoplasm,  and  the  introduction 
of  a  sexual  method  of  reproduction.  It  has  been  acquired  because  it  is  to  the 
advantage  of  the  species  to  possess  it ;  in  the  simplest  cases  it  should  occur  at 
the  close  of  the  reproductive  period,  and  in  fact  it  frequently  does  occur  then. 
A  superabundance  of  aged  individuals,  after  they  have  ceased  to  be  reproduc- 
tive, would  be  detrimental  to  the  race ;  it  is  to  the  advantage  of  the  species  that 
they  be  put  out  of  the  way.  Death  of  the  individual  in  order  that  the  species 
may  survive  has,  therefore,  become  an  established  principle  of  nature.  But 
the  higher  animals  are  better  able  to  protect  themselves  from  destruction  than 
the  lower,  and,  moreover,  they  are  needed  to  rear  the  young ;  hence  in  them 
the  duration  of  life  is  frequently  prolonged  beyond  the  reproductive  period. 

Weismann's  theory  has  been  the  cause  of  much  discussion,  and  the  pros 
and  cons  have  been  set  forth  by  eminent  biological  authorities.  In  its  appli- 
cation to  the  human  race  it  would  seem  that  the  factors  of  social  evolution 
have  brought  it  about  that  the  aged  are  protected  in  the  struggle  for  existence 
for  long  after  their  reproductive  usefulness  has  ceased,  and  thus  the  working 
of  a  pitiless  biological  law  has  become  modified. 

F.  HEREDITY. 

Biologists  are  accustomed  to  recognize  two  factors  as  responsible  for  the 
character  and  actions  of  the  living  organism.  These  are  heredity  and  the 
environment.  Heredity  includes  whatever  is  transmitted,  either  as  actual  or 
as  potential  characteristics,  by  parents  to  offspring.  The  environment  com- 
prises both  material  and  immaterial  components,  such  as  food,  water,  air,  or 
other  substances  that  surround  the  organism,  and  the  forces  of  nature,  such  as 
light,  heat,  electricity,  and  gravity,  that  act  as  conditions  of  existence  or  as 
stimuli  to  action.  The  same  principles  apply  to  the  character  and  actions  of 
everv  cell  of  a  many-celled  organism,  but  here  we  must  include  in  the  envi- 
ronmental factor  the  mysterious  influences  that  are  exerted  upon  the  cell  by 
the  other  cells  of  the  body.  Of  these  two  factors  heredity  acts  from  within, 
the  environment  from  without  the  living  substance.  Among  unicellular  or- 
ganisms the  individual  begins  his  career  when  the  bit  of  protoplasm  that  con- 
stitutes his  body  is  separated  from  the  parent  bit  of  protoplasm.  Among 
higher  forms,  including  man,  the  term  individual  may  be  applied  to  the  fer- 
tilized ovum ;  the  union  of  the  ovum  and  the  spermatozoon  inaugurates  the 


494  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY, 

new  being.  From  the  inception  to  the  death  of  the  individual,  life  consists 
partly  of  manifestations  of  the  powers  conferred  by  the  germ-cells  and  partly 
of  reactions  to  environmental  influences.  In  considering  the  details  of  vital 
action  we  are  apt  to  overlook  these  fundamental  facts  and  to  evolve  narrow 
and  erroneous  views  as  to  the  causes  of  vital  phenomena.  Biologists  are 
seeking  with  increasing  vigor  to  determine  the  relative  importance  of  the  parts 
played  by  these  two  principles  in  development  and  in  daily  life.  It  is  need- 
less to  say  that  the  problem  is  a  difficult  one  and  is  still  far  from  solution. 
In  previous  chapters  of  this  book  attention  has  been  directed  more  especially 
to  the  external  than  to  the  hereditary  factor.  A  work  upon  physiology  would 
be  incomplete,  however,  if  it  did  not  include  an  examination  of  the  latter, 
especially  since  at  the  present  time  heredity  is  one  of  the  leading  subjects  of 
biological  research  and  discussion.  It  is  proposed,  therefore,  in  this  section 
to  present  a  brief  outline  of  the  facts,  the  principles,  and  the  attempted  ex- 
planations of  the  modes  of  working  of  heredity.  It  should  be  premised  that, 
because  of  the  present  incomplete  state  of  our  knowledge  of  the  facts,  the 
highly  speculative  and  involved  character  of  most  of  the  theories,  and  the  con- 
stant, active  shifting  of  ideas  and  points  of  view,  such  an  outline  must  neces- 
sarily be  incomplete  and  in  many  respects  unsatisfactory. 

Facts  of  Inheritance. — It  is  not  proposed  in  this  paragraph  to  enter  into 
a  discussion  of  the  question  as  to  whether  a  particular  vital  phenomenon  is  a 
fact  of  inheritance  or  a  reaction  to  external  influences.  For  our  present  pur- 
poses it  is  sufficient  to  record  the  common  facts  of  resemblance  to  ancestors, 
and  to  assume  that  such  resemblance,  when  present,  has  been  inherited. 
Resemblances  are  strongest  between  child  and  parents,  and  appear  in  a  dimin- 
ishing ratio  backward  along  the  ancestral  line.  Galton l  has  computed  that, 
of  the  total  heritage  of  the  child,  each  of  the  two  parents  contributes  one- 
fourth,  each  of  the  four  grandparents  one-sixteenth,  and  the  remaining  one- 
fourth  is  handed  down  by  more  remote  ancestors.  The  correctness  of  this 
estimate  has  been  disputed  by  Weismanu.  The  fact  must  not  be  overlooked 
that,  in  addition  to  and  back  of  all  the  particular  individual  features  that  are 
inherited,  a  host  of  racial  characteristics  are  transmitted — the  progeny  of  a 
given  species  belongs  to  that  species ;  the  human  being  is  the  father  of  the 
human  child,  the  child  of  Caucasian  parents  is  a  Caucasian,  of  negro  parents 
a  negro. 

Congenital  resemblances  may  be  anatomical,  physiological,  or  psychological, 
and  in  each  of  these  classes  they  may  be  normal  or  pathological.  Anatomical 
resemblances  are  the  most  commonly  recognized  of  all :  facial  features,  stature, 
color  of  eyes  and  of  hair,  supernumerary  digits,  excessive  hairiness  of  body, 
cleft  palate,  monstrosities,  and*  various  defects  of  the  eye,  such  as  those  that 
give  rise  to  hypermetropia,  myopia,  cataract,  color-blindness,  and  strabismus, 
are  all  known  examples.  Physiological  peculiarities  that  may  be  transmitted 
include  the  tendency  to  characteristic  gestures,  locomotion  and  other  muscular 
movements,  longevity  or  short  life,  tendency  to  thinness  or  obesity,  handwriting, 
1  Francis  Galton  :  Natural  Inheritance,  1889,  p.  134. 


REPRODUCTION. 

voice,  haeraatophilia  or  tendency  to  profuse  hemorrhage  from  slight  wounds, 

gout,  epilepsy,  and  asthma.  Psychological  inheritance-  ( iprise  habits  of 

mind,  talent,  artistic  and  moral  qualities,  tastes,  traits  of  character,  tempera- 
ment, ambition,  insanity  and  other  mental  diseases,  and  tendencies  to  crime 
and  to  suicide. 

Latent  Characters  ;  Revision. — Characters  that  never  appear  in  the  parent 
may  yet  he  transmitted  through  him  from  grandparent  to  child  ;  such  charac- 
ters are  called  latent.  Among  the  most  striking  latent  characters  are  those  con- 
nected with  sex.  Darwin  l  says :  "  In  every  female  all  the  secondary  male 
characters,  and  in  every  male  all  the  secondary  female  characters,  apparently 
exist  in  a  latent  state,  ready  to  be  evolved  under  certain  conditions."  Thus,  a 
girl  may  inherit  female  secondary  sexual  peculiarities  of  her  paternal  grand- 
mother that  are  latent  in  her  father,  or  a  boy  may  inherit  from  his  maternal 
grandfather  characteristics  that  never  show  in  his  mother.  An  excellent 
example  of  such  transmission,  taken  from  the  herbivora,  is  the  common  one 
of  a  bull  conveying  to  his  female  descendants  the  good  milking  qualities  of 
his  female  ancestors.  In  the  human  species  hydrocele,  necessarily  a  disease  of 
the  male,  has  been  known  to  be  inherited  from  the  maternal  grandfather,  and 
hence  must  have  been  latent  in  the  mother's  organism.  That  in  such  cases  the 
character  is  really  potential,  though  latent  in  the  intermediate  ancestor,  is 
rendered  probable  by  such  well-known  facts  as  the  appearance  of  female  cha- 
racteristics in  castrated  males,  and  of  male  characteristics  in  females  with  dis- 
eased ovaries  or  after  the  end  of  the  normal  sexual  life. 

Latency  may  be  offered  as  the  explanation  of  the  numerous  cases  of 
atavism,  or  reversion,  by  which  is  meant  the  appearance  in  an  individual  of 
peculiarities  that  were  formerly  known  only  in  the  grandparents  or  more 
remote  ancestors,  but  not  in  the  parents  of  the  individual.  This  subject  is  one 
of  the  most  important  in  the  whole  field  of  heredity.  Almost  any  character 
may  reappear  even  after  many  generations.  In  the  human  species  stronger 
likeness  to  grandparents  than  to  parents  is  a  frequent  occurrence.  The 
majority  of  the  frequent  anomalies  of  the  dissecting-room  are  regarded  as 
reversions  toward  the  simian  ancestors  of  the  human  race.  The  crossing  of 
,two  strains  develops  a  strong  tendency  to  reversion,  and  because  of  this  the  prin- 
ciple of  atavism  must  constantly  be  taken  into  account  by  breeders  of  animals 
and  growers  of  plants.  As  an  example  of  reversion  after  crossing  may  be 
mentioned  the  well-known  one,  studied  by  Darwin,  of  the  frequent  appear- 
.ance  of  marked  stripes  upon  the  legs  of  the  mule,  the  mule  being  a  hybrid 
from  the  horse  and  the  ass,  both  of  which  are  comparatively  unstriped  but 
are  undoubtedly  descended  from  a  striped  zebra-like  ancestor.  Here  the 
capacity  of  developing  stripes  is  regarded  as  latent  in  both  the  horse  and  the 
ass,  but  as  made  evident  in  the  mule  by  the  mysterious  influence  of  crossing. 
Darwin  thinks  likewise  that  the  customary  degraded  state  of  half-castes  may 
be  due  to  reversion  to  a  primitive  savage  condition  which,  usually  latent  in 

1  Charles  Darwin:  The  Variation  of  Anhnnl*  mid  Plants  under  Domestication,  1892,  vol.  ii., 
:2d  ed. 


496  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

both  civilized  and  savage  races,  is  rendered  manifest  in  the  offspring  that 
results  from  the  union  of  the  two.  Reversionary  characters  are  often  more 
prominent  during  youth  than  during  later  life — a  fact  that  has  been  quoted  in 
favor  of  their  explanation  on  the  theory  of  latency. 

Regeneration. — The  facts  of  regeneration  of  lost  parts  must  also  be  taken 
into  account  in  a  theory  of  heredity.  Such  regeneration  may  be  either  physi- 
ological or  pathological.  Physiological  or  normal  regeneration  has  reference 
to  the  reproduction  of  parts  that  takes  place  during  the  normal  life  of  the 
individual,  such  as  the  constant  growth  of  the  deeper  layers  of  the  epidermis 
to  replace  the  outer  layers  that  are  as  constantly  being  shed.  Pathological 
regeneration  refers  to  the  replacement  of  parts  lost  by  accident,  and  presents 
the  more  interesting  and  striking  examples.  The  power  of  pathological 
regeneration  in  man  and  the  higher  mammals  is  limited.  A  denuded  surface 
may  be  re-covered  with  epithelium ;  the  central  end  of  a  cut  nerve  may  grow 
anew  to  its  termination  ;  the  parts  of  a  broken  bone  may  reunite ;  muscle  may 
reappear ;  connective-tissue,  blood-corpuscles,  and  blood-vessels  may  develop 
readily;  and  in  the  healing  of  every  wound  a  regeneration  of  parts  takes 
place.  But  in  descending  the  scale  of  animal  life  the  regenerative  power 
becomes  progressively  stronger,  and  in  many  plants  and  low  animals  it  is 
marvellous.  Thus,  the  newt  may  replace  a  lost  leg,  the  crab  a  lost  claw,  the 
snail  an  eyestalk  and  eye.  If  an  earth-worm  be  cut  in  two,  one  half  may 
regenerate  a  new  half,  complete  in  all  respects.  A  hydra  may  be  chopped 
into  fragments  and  each  fragment  may  re-grow  into  a  complete  hydra.  From 
a  small  piece  of  the  leaf  of  a  begonia,  planted  in  moist  earth,  a  new  plant 
with  all  its  parts  may  arise.  It  is  evident  that  the  existing  parts  of  an  organ- 
ism, if  not  too  specialized,  possess  the  power  of  restoring  parts  that  are  lost ; 
under  ordinary  circumstances  this  power  is  latent.  The  growth  of  tumors  is 
perhaps  allied  in  nature  to  regeneration.  A  study  of  regeneration  shows  that 
in  many  cases  the  process  of  building  anew  follows  the  same  course  as  the 
original  embryonic  growth.  It  is  properly  a  phenomenon  of  heredity. 

The  Inheritance  of  Acquired  Characters. — No  topic  in  heredity  has  been 
more  debated  during  the  past  twenty  years  than  that  of  the  possibility  of  the 
transmission  to  the  offspring  of  characteristics  that  are  acquired  by  the  parents 
previous  to  the  discharge  of  the  germ-cells,  or,  in  the  case  of  the  mammalian 
female,  previous  to  parturition.  Obviously,  no  one  denies  this  possibility  in 
the  unicellular  organisms,  where  reproduction  by  fission  prevails,  for  there  the 
protoplasm  of  the  body  of  one  parent  becomes  the  substance  of  two  offspring ; 
in  the  transformation  nothing  is  lost,  and  hence  whatever  peculiarities  the  ances- 
tral protoplasm  has  acquired  are  transferred  bodily  to  the  descendants.  But 
in  multicellular  forms,  where  sexual  reproduction  exists,  the  case  is  very  dif- 
ferent, for  here  whatever  is  transmitted  is  transmitted  through  germinal  cells, 
or  germ-plasm,  as  the  hereditary  substance  contained  in  the  germ-cells  is  now 
commonly  called.  The  problem  then  resolves  itself  into  that  of  the  relation 
of  the  germ-plasm  to  the  protoplasm  of  the  rest  of  the  body,  the  so-called 
somatoplasm ;  and  the  question  to  be  answered  is  this :  Are  variations  in  the 


REPRODUCTION.  497 

• 

parental  somatoplasni  capable  of  inducing  such  changes  in  the  gerra-plasm  that 
somatic  peculiarities  appear  in  the  offspring  similar  to  those  possessed  by  the 
parent?  Weismann  classifies  all  somatic  variations  according  to  their  origin 
into  three  groups — viz.  injuries,  functional  variations,  and  variations,  mainly 
climatic,  that  depend  upon  the  environment.  The  problem  of  their  inherit- 
ance is  a  far-reaching  one,  and  upon  its  correct  solution  depend  principles  that 
are  of  much  wider  application  than  simply  to  matters  of  heredity;  for  if 
acquired  characters  can  be  inherited,  there  is  revealed  to  us  a  most  potent  fac- 
tor in  the  transformation  of  species,  and  the  whole  question  of  the  possibility 
of  use  and  disuse  as  factors  of  evolution  is  presented.  The  larger  evolutionary 
problem  need  not  here  be  considered. 

Regarding  the  problem  of  the  inheritance  of  acquired  characteristics  we  may 
say  at  once  that  it  is  not  yet  solved.  To  the  lay  mind  this  may  seem  strange, 
for  at  first  thought  it  appears  self-evident  that  parents  may  transmit  to  their 
children  peculiarities  that  they  themselves  have  acquired.  Affirmative  evidence 
seems  all  about  us,  as  witness  the  undoubted  cases  of  inheritance  of  artistic 
tastes,  of  talent,  of  traits  valuable  in  professional  life,  which  seem  to  originate 
in  the  industry  of  the  parent.  But  scientific  analysis  by  Weismann  and  others 
of  popular  impressions,  popular  anecdotes  and  hearsay  evidence,  and  accurate 
original  observation,  have  revealed  little  that  cannot  as  well  be  explained  on 
other  hypotheses.  Anatomical  and  functional  peculiarities  of  the  body  that  are 
apparently  new  often  reappear  in  successive  generations,  but  to  assume  that 
they  are  acquired  by  the  somatoplasm  and  have  become  congenital,  rather  than 
that  they  are  germinal  from  the  first,  is  unwarranted.  Direct  experiments  by 
various  investigators  are  almost  as  inconclusive.  Weismann  l  has  removed  the 
tails  of  white  mice  for  five  successive  generations,  and  yet  of  901  young  every 
individual  was  born  with  a  tail  normal  in  length  and  in  other  respects.  Bos  2 
has  experimented  similarly  upon  rats  for  ten  generations  without  observing  any 
diminution  of  the  tails.  The  practice  of  circumcision  for  centuries  has  resulted 
in  no  reduction  of  the  prepuce.  The  binding  of  the  feet  of  Chinese  girls  has 
not  resulted  in  any  congenital  malformation  of  the  Chinese  foot.  Brown- 
S6quard,3  and  later  Obersteiner,4  have  artificially  produced  epilepsy  in  guinea- 
pigs  by  various  operations  upon  the  central  nervous  system  and  the  peripheral 
nerves,  and  the  offspring  of  such  parents  have  been  epileptic.  At  first  this 
would  seem  to  amount  to  proof  of  the  actual  hereditary  transmission  of  mutila- 
tions, yet  in  these  cases  the  mutilation  itself  was  not  transmitted  ;  the  offspring 
were  weak  and  sickly  and  exhibited  a  variety  of  abnormal  nervous  and  nutri- 
tional symptoms,  among  which  was  a  tendency  toward  epileptiform  convulsions, 
the  cause  of  which  is  still  to  be  explained.  Evidence  from  palaeontology 
regarding  the  apparent  gradual  accumulation  of  the  effects  of  use  and  disuse 
throughout  a  long-continued  animal  series  seems  to  require  the  assumption  of 

1  A.  Weismann :  Essays  upon  Heredity,  vol.  i.,  1889,  p.  432. 

2  J.  B.  Bos:  Biologisches  Centralblatt,  xi.,  1891,  S.  734. 

3  E.  Brown-Sequard  :  Researches  on  Epilepsy,  etc.,  Boston,  1857;  also  various  later  papers. 

4  H.  Obersteiner :  Medizinische  Jahrbikher,  Wien   1875,8.179. 

Vor,.  IT.— 32 


498  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

such  a  principle  as  the  inheritance  of  acquired  characters,  but  even  here  the 
principle  of  natural  selection  may  perhaps  be  equally  explanatory. 

The  Inheritance  of  Diseases. — The  question  of  the  inheritance  of  diseases 
has  also  been  much  discussed.  The  same  general  principles  apply  here  as  in 
the  inheritance  of  normal  characteristics.  The  fact  has  been  mentioned  above 
that  pathological  characters,  whether  anatomical,  physiological,  or  psycholog- 
ical, are  capable  of  transmission.  If,  however,  a  pathological  character  has 
been  acquired  by  the  parent  and  is  not  inherent  in  his  own  germ-cells,  it 
is  extremely  doubtful  whether  it  can  be  passed  on  to  the  child.  A  diseased 
parent,  on  the  other  hand,  may  produce  offspring  that  are  constitutionally 
weak  or  that  are  even  predisposed  toward  the  parental  disease,  and  such  off- 
spring may  develop  the  parent's  ailment.  In  such  cases  constitutional  weakness 
or  predisposition,  and  not  actual  disease,  is  inherited ;  the  disease  itself  later 
attacks  the  weak  or  predisposed  body.  Proneness  to  mildness  or  severity  of, 
and  immunity  toward,  certain  diseases  seem  to  be  transmissible.  These  sub- 
jects, however,  are  so  little  understood,  and  the  real  meaning  of  such  terms  as 
predisposition,  inherited  constitutional  weakness,  and  inherited  immunity,  is  so 
little  known,  that  it  is  idle  to  discuss  them  here. 

Considerable  experimental  work  has  been  performed  recently  upon  the 
transmissibility  of  infectious  diseases.  Undoubtedly  infectious  diseases  cling 
to  a  particular  family  for  generations.  The  transmitted  factor  is  probably  fre- 
quently, if  not  usually,  simple  predisposition.  But  in  an  increasing  number 
of  cases  there  appears  to  be  transmission  of  a  specific  micro-organism.  Such 
transmission  is  called  germinal  when  the  micro-organism  is  conveyed  in  the 
ovum  or  the  semen,  and  placental  or  intra-uterine  when  the  micro-organism 
reaches  the  fetus  after  uterine  development  has  begun,  and  chiefly  through  the 
circulation.  Of  germinal  infections  syphilis  seems  undoubtedly  capable  of 
transmission  within  either  the  ovum  or  the  semen.  The  possibility  of  germinal 
transmission  of  tuberculosis  has  been  maintained,  but  is  not  fully  proven.  Of 
intra-uterine  infections  there  have  been  observed  in  human  beings  apparently 
undoubted  cases  of  typhoid  fever,  relapsing  fever,  scarlatina,  endocarditis, 
small-pox,  measles,  croupous  pneumonia,  anthrax,  syphilis,  and  possibly 
tuberculosis  and  Asiatic  cholera.  It  is  obvious  that  neither  germinal  nor 
placental  inheritance,  both  taking  place  through  the  medium  of  a  specific  micro- 
organism, and  not  through  the  modification  of  germ-plasm,  is  comparable  to 
inheritance  in  the  customary  sense. 

Theories  of  Inheritance. — From  early  historical  times  theories  of  inher- 
itance have  not  been  wanting.  Physical  and  metaphysical,  materialistic  and 
spiritualistic  theories  have  had  their  day.  Previous  to  the  discovery  of  the 
spermatozoon  (Hamm,  Leeuwenhoek,  1677)  all  theories  were  necessarily 
fantastic,  and  for  nearly  two  hundred  years  later  they  were  crude.  The 
theories  that  are  now  rife  may  be  said  to  date  from  1864,  when  Herbert 
Spencer  published  his  Principles  of  Biology.  Since  that  date  they  have 
become  numerous.  Even  the  modern  theories  are  highly  speculative  ;  none 
can  be  regarded  as  being  accepted  to  the  exclusion  of  all  others  by  a  large 


REPRODUCTION.  499 

majority  of  scientific  workers,  and  the  excuse  for  introducing  them  into  a 
text-book  of  physiology  is  the  hope  that  a  brief  discu.-sion  of  them  mav 
prove  suggestive,  stimulating,  and  productive  of  investigation. 

Germ-plasm. —  Germinal  substance,  germ-plasm  (Weismann),  or,  as  it  is  some- 
times called,  idioplasm  (Nageli),  must  lie  at  the  basis  of  all  scientific  theories 
of  heredity.  The  father  and  the  mother  contribute  to  the  child  the  >]><  rma- 
tozoon  and  the  ovum  respectively,  and  within  these  two  bits  of  protoplasm 
there  must  be  contained  potentially  the  qualities  of  the  two  parents.  Tlinv 
is  the  strongest  evidence  in  favor  of  the  prevailing  view  that  the  nucleus  alone 
of  each  germ-cell  is  essentially  hereditary,  or,  more  exactly,  that  the  chromatic 
substance  of  the  nucleus  is  the  sole  actual  germinal  substance.  We  have  seen 
that  the  tail  of  the  spermatozoon  is  a  locomotive  organ,  and  that  the  body  of 
the  ovum  is  nutritive  matter.  We  have  seen  also  that  the  essence  of  the 
whole  process  of  fertilization  is  a  fusion  of  the  male  and  the  female  nuclei,  or, 
more  exactly,  a  mingling  of  male  and  female  chromosomes.  Hence  most 
physiologists  agree  with  Strasburger  and  Hertwig  that  the  chromatic  substance 
of  the  nuclei  of  the  germ-cells  transmits  the  hereditary  qualities. 

As  to  the  origin  of  the  germ-plasm,  two  hypotheses  have  been  suggested. 
Spencer,  Darwin,  Galton,  and  Brooks  have  argued  in  favor  of  a  production 
of  germ-plasm  within  each  individual  by  a  collocation  within  the  reproductive 
organs  of  minute  elementary  vital  particles — "  physiological  units  "  (Spencer), 
"gemmules"  (Darwin) — which  come  from  all  parts  of  the  body;  hence  each 
part  of  the  body  has  its  representative  within  every  germ-cell.  This  hypothesis 
affords  a  ready  explanation  of  numerous  facts,  but  its  highly  speculative  cha- 
racter, the  entire  absence  of  direct  observational  or  experimental  proof  of  its 
truth,  and  the  demand  that  its  conception  makes  upon  human  credulity,  mili- 
tate against  its  general  acceptance.  Weismann,  the  promulgator  of  the  second 
hypothesis,  denies  altogether  the  formation  of  the  germ-plasm  from  the  body- 
tissues  of  the  individual,  and  maintains  its  sole  origin  from  the  germ-plasm  of 
the  parent  of  the  individual.  Through  the  parent  it  comes  from  the  grand- 
parent, thence  from  the  great-grandparent,  and  so  may  be  traced  backward 
through  families  and  tribes  and  races  to  its  origin  in  simple  unicellular 
organisms.  According  to  Weismann,  therefore,  germ-plasm  is  very  ancient 
and  is  directly  continuous  from  one  individual  to  another ;  the  parts  of  an 
individual  body  are  derivatives  of  it,  but  they  do  not  return  to  it  their  repre- 
sentatives in  the  form  of  minute  particles.  The  general  truth  of  Weismann's 
conception  can  hardly  be  denied. 

As  to  the  morphological  nature  of  germ-plasm,  two  views  likewise  are  held. 
One  school,  led  by  His  and  Weismann,  holds  that  germ-plasm  possesses  a 
complicated  architecture ;  that  the  fertilized  ovum  contains  within  its  structure 
the  rudiments  or  primary  constituents  of  the  various  cells,  tissues,  and  organs 
of  which  the  body  is  destined  to  be  composed ;  and  that  growth  is  a  develop- 
ment of  these  already  existing  germs  and  largely  independent  of  surrounding 
influences.  In  accordance  with  this  idea,  segmentation  of  the  ovum  is  specifi- 
cally a  qualitative  process,  one  blastomere  representing  one  portion  of  the 


500  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

future  adult,  another  blastomere  another  portion,  and  so  on.  This  theory 
recalls  in  a  refined  form  the  crude  theory  of  Preformation  that  was  advocated 
during  the  seventeenth  and  eighteenth  centuries  by  Haller,  Bonnet,  and  many 
others,  according  to  which  the  germ-cell  was  believed  to  contain  a  minute  but 
perfectly  formed  model  of  the  adult,  which  needed  only  to  be  enlarged  and 
unfolded  in  growth.  The  other  modern  school,  in  which  Oscar  Hertwig  is 
prominent,  maintains  that  the  fertilized  egg  is  isotropous — that  is,  that  one 
part  is  essentially  like  another  part — that  the  architecture  of  the  egg  is  rela- 
tively simple,  and  that  growth  is  largely  a  reaction  of  the  living  substance  to 
external  influences.  The  idea  of  isotropy  is  based  largely  upon  the  experi- 
mental results  of  Pfluger,  Chabry,  Driesch,  Wilson,  Boveri,  and  the  brothers 
Hertwig,  who  by  various  methods  and  in  various  animals  have  found  that 
single  blastomeres  of  a  segmenting  ovum,  when  separated  from  the  others,  will 
develop  into  normal  but  dwarfed  larvae ;  that  is,  a  portion  of  the  original  germ- 
plasm  is  capable  of  giving  rise  to  all  parts  of  the  animal.  These  results  are 
interpreted  to  signify  that  segmentation,  instead  of  being  qualitative,  is  quanti- 
tative, each  blastomere  being  like  all  the  others.  The  second  theory,  like  the 
first,  resembles  in  some  degree  a  theory  of  the  past  two  centuries,  advocated 
by  Wolff  and  Harvey,  and  known  as  the  theory  of  Epigenesis.  According  to 
this  there  was  no  preformation  in  the  germ-cells,  but  rather  a  lack  of  organi- 
zation which  during  growth,  under  guidance  of  a  mysterious  power  supposed 
to  be  resident  in  the  living  substance,  gave  place  to  differentiation  and  the 
appearance  of  definite  parts. 

Modern  microscopes  have  revealed  no  miniature  of  the  adult  in  the  egg, 
nor  has  modern  physiology  found  necessary  an  assumption  of  extra-physical 
forces  within  living  matter.  With  the  increase  of  knowledge  the  old  and 
crude  preformation  of  Haller  and  Bonnet  and  the  speculative  epigenesis  of 
Wolff  and  Harvey  have  given  place  to  the  new  preformation  and  epigenesis 
of  the  present  time,  and  all  modern  theories  of  heredity  may  be  classed  in 
the  one  or  the  other  category  or  as  intermediate  between  them.  The  mod- 
ern advocates  of  preformation  explain  hereditary  resemblance  by  the  supposed 
similarity  of  all  germ-plasm  in  any  one  line  of  descent.  The  modern  advocates 
of  epigenesis,  while  allowing  the  necessity  of  a  material  basis  of  germ-plasm, 
ascribe  hereditary  resemblance  to  similarity  of  environment  during  develop- 
ment. 

Variation. — It  is  a  commonplace  in  observation  that,  however  close  hereditary 
resemblance  may  be,  it  is  never  absolute ;  the  child  is  never  the  exact  image 
of  the  parent  either  physically  or  mentally.  Variations  from  the  parental  type 
may  be  either  acquired  by  the  offspring  subsequent  to  fertilization  or  to  birth, 
and  hence  are  to  be  traced  to  the  action  of  the  environment ;  or  they  may  be 
congenital,  that  is,  inherent  in  the  germ-plasm.  Although  it  is  not  always 
easy  in  the  case  of  any  one  variation  to  determine  to  which  class  it  belongs, 
yet  the  fact  remains  that  the  two  classes  exist;  and  a  complete  theory  of 
heredity  must  recognize  and  explain  congenital  variation  as  fully  as  congenital 
resemblance.  It  is  unnecessary  to  say  that  the  origin  of  congenital  variation 


RE  PR  OD  UCTION.  501 

is  one  of  the  much  discussed  and  still  unsettled  questions.  At  least  two  causes 
of  congenital  variations  are  commonly  recognized,  although  opinions  differ  as 
to  the  relative  importance  of  the  r6le  played  by  each.  These  causes  are  differ- 
ences in  the  nutrition  of  the  germ-plasm,  and  sexual  reproduction.  As  to  the 
former,  it  is  evident  that  the  germ-plasm  in  no  two  individuals,  even  father 
and  son,  has  exactly  identical  nutritional  opportunities.  Since  the  life  of  one 
individual  is  not  the  exact  counterpart  of  the  life  of  another,  the  germ-plasm 
of  one  individual  has  a  different  nutrition  from  that  of  another.  It  would 
hence  be  strange,  even  although  we  regard  the  germ-plasm  as  relatively  stable, 
if  with  succeeding  generations  there  did  not  appear  variations  that  are  sufficient 
to  give  rise  to  unlikeness  in  relatives.  Differences  in  the  nutrition  of  the  germ- 
plasm  in  different  individuals  are,  therefore,  a  true  cause  of  variations.  As 
regards  sexual  reproduction,  it  must  be  remembered  that  a  new  individual  is 
the  product  of  two  individuals,  that  the  two  individuals  have  descended  along 
different  genealogical  lines,  and  hence  that  the  two  conjugating  masses  of  germ- 
plasm  are  different  in  nature.  It  is  only  to  be  expected,  therefore,  that  the 
resulting  individual  shall  be  different  from  the  two  contributing  parents.  Thus 
sexual  reproduction  is  a  true  cause  of  variations. 

Having  outlined  the  main  facts  and  principles  of  heredity,  let  us  now  review 
a  few  of  the  specific  theories  that  have  been  of  value  in  clearing  the  clouded 
atmosphere. 

Darwin's  Theory  of  Pangenesis. — Darwin's  "  Provisional  Hypothesis  of 
Pangenesis"  was  published  in  1868  as  chapter  xxvii.  of  his  work  on  The  Vari- 
ations of  Animals  and  Plants  under  Domestication.  It  was  the  first  of  the 
modern  theories  to  attempt  to  cover  the  whole  ground  of  heredity ;  it  was 
accompanied  by  a  most  exhaustive  presentation  and  analysis  of  facts,  and  it 
stimulated  abundant  discussion  and  investigation.  In  Darwin's  own  words 
the  hypothesis  was  formulated  as  follows  :  "  It  is  universally  admitted  that  the 
cells  or  units  of  the  body  increase  by  cell-division  or  proliferation,  retaining 
the  same  nature,  and  that  they  ultimately  become  converted  into  the  various 
tissues  and  substances  of  the  body.  But  besides  this  means  of  increase  I  assume 
that  the  units  [cells]  throw  off  minute  granules  which  are  dispersed  throughout 
the  whole  system  ;  that  these,  when  supplied  with  proper  nutriment,  multiply 
by  self-division,  and  are  ultimately  developed  into  units  like  those  from  which 
they  were  originally  derived.  These  granules  maybe  called  gemmules.  They 
are  collected  from  all  parts  of  the  system  to  constitute  the  sexual  elements,  and 
their  development  in  the  next  generation  forms  a  new  being;  but  they  are 
likewise  capable  of  transmission  in  a  dormant  state  to  future  generations,  and 
may  then  be  developed.  Their  development  depends  on  their  union  with  other 
partially  developed  or  nascent  cells  which  precede  them  in  the  regular  course 

of  growth Gemmules  are  supposed  to  be  thrown  off  by  every  unit, 

not  only  during  the  adult  state,  but  during  each  stage  of  development  of 
every  organism ;  but  not  necessarily  during  the  continued  existence  of  the 
•same  unit.  Lastly,  I  assume  that  the  gemmules  in  their  dormant  state  have  a 
mutual  affinity  for  each  other,  leading  to  their  aggregation  into  buds  or  into 


502  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  sexual  elements.  Hence,  it  is  not  the  reproductive  organs  or  buds  which 
generate  new  organisms,  but  the  units  of  which  each  individual  is  composed. 
These  assumptions  constitute  the  provisional  hypothesis  which  I  have  called 
Pangenesis." 

Since  the  cells  of  the  body  are  represented  by  gem  mules  within  the  germ- 
cells,  Darwin's  theory  is  a  theory  of  Preformation.  It  explains  the  facts  of 
the  regeneration  of  lost  parts  by  the  assumption  that  the  gem  mules  of  the  part 
in  question  are  disseminated  throughout  the  body  and  have  only  to  unite  with 
the  nascent  cells  at  the  point  of  new  growth.  Pangenesis  explains  reversion, 
since  gemmules  may  lie  dormant  in  one  generation  and  develop  in  the  next. 
It  explains  congenital  variation,  since  the  mixture  of  maternal  and  paternal 
gemmules  is  plainly  different  from  the  two  kinds  taken  separately.  It  explains 
how  acquired  variations  may  become  congenital,  since  an  altered  part  throws 
off  altered  gemmules,  and  by  the  collocation  of  these  in  the  germ-cells  the 
alteration  may  be  transmitted.  It  thus  allows  the  transmission  of  acquired 
characters. 

Darwin's  assumptions  of  gemmules  and  their  behavior  are  pure  assump- 
tions, for  which  subsequent  investigation  has  not  provided  a  basis  of  facts. 
As  we  have  seen,  also,  the  inheritance  of  acquired  characters  is  greatly  in 
doubt,  and,  if  they  are  heritable  at  all,  they  can  be  so  only  comparatively 
feebly.  Besides  these  objections  it  was  early  found  that,  with  the  increase 
of  knowledge  of  the  facts  of  heredity,  it  was  necessary  to  modify  very  mate- 
rially the  theory  of  Pangenesis.  This  has  been  ably  done  successively  by 
Galton,1  Brooks,2  and  de  Vries.3  But  neither  the  original  theory  nor  its 
modifications  have  been  generally  accepted. 

Weismann' s  Theory. — Since  1880,  Professor  Weismann4  of  Freiburg  has 
published  numerous  essays  upon  heredity  and  allied  subjects,  in  which,  besides 
reviewing  the  views  of  others,  he  has  developed  in  detail  a  new  and  elaborate 
theory  of  his  own,  that  is  the  most  ambitious  attempt  yet  made  to  solve  the 
problem  of  inheritance.  In  the  course  of  their  development  Weismann's 
ideas  have  undergone  some  modification.  Their  leading  features  are  as 
follows : 

The  essential  hereditary  substance,  or  germ-plasm,  is  the  chromatin  of  the 
nucleus  of  the  germ-cells.  One  of  the  fundamental  tenets  of  Weismann's 
system  is  expressed  by  his  own  phrase,  "  the  continuity  of  germ-plasm."  By 
this  is  meant  that  the  germ-plasm  of  one  individual,  instead  of  arising  de  novo 
in  the  individual  by  the  collocation  of  multitudinous  "  gemmules "  derived 
from  the  body-cells,  originates  directly  from  the  germ-plasm  of  the  parent, 
thence  from  that  of  the  grandparent,  and  so  on  backward  through  all  genera- 
tions to  the  origin  of  all  germ-plasms  that  took  place  simultaneously  with  the 

1  Francis  Galton :  "A  Theory  of  Heredity,"  Journal  of  the  Anthropological  Institute,  1875. 

2  W.  K.  Brooks:  The  Laws  of 'Heredity,  1883. 

3  H.  de  Vries :  Die  Intracelluldre  Pangenesis,  1889. 

4  August  Weismann:    Essays  upon  Heredity  and   Kindred  Biological   Problems,  authorized 
translation,  vol.  i.,  1889;   vol.  ii.,  1892;    The  Germ-plasm,  authorized  translation,  1893;   The 
Effect  of  External  Influences  upon  Development,  the  Romanes  Lecture,  1894. 


REPR  OD  UCTION.  r,( );  j 

origin  of  sex — germ-plasm  is  continuous  from  individual  to  individual  along 
any  one  line  of  descent.  Wcismaim  draws  a  sharp  line  between  </rnn.-j>la*m 
and  somatoplasm,  or  body-plasm,  which  latter  comprises  all  protoplasm  that 
the  body  contains  except  the  germ-plasm.  Germ-plasm  once  originated  con- 
tinues from  generation  to  generation  ;  somatoplasm  develops  anew  in  each  gen- 
eration from  germ-plasm  by  growth  and  differentiation,  resulting  in  a  loss  of  its 
specific  germinal  character.  Germ-plasm  is  stable  in  composition  ;  somatoplasm 
is  variable.  Germ-plasm,  being  passed  on  from  parent  to  offspring,  is  immortal ; 
somatoplasm  dies  when  the  individual  dies.  Weismann  believes  that  "the 
germ-plasm  possesses  a  fixed  architecture,  which  has  been  transmitted  histori- 
cally "  and  which  represents  the  parts  of  the  future  organism.  It  consists  of 
material  particles  or  hereditary  units  called  determinants,  each  of  which  has  a 
definite  localized  position  within  the  germ-plasm.  The  determinants  are  sug- 
gestive of  Darwin's  gemmules,  yet  they  are  not  the  same,  for,  while  gem  mules 
were  supposed  to  represent  individual  cells,  determinants  are  representatives 
of  cells  or  groups  of  cells  that  are  variable  from  the  germ  onward.  Deter- 
minants consist  of  definite  combinations  of  simpler  units,  or  biophors,  which 
are  the  smallest  particles  that  can  exhibit  vital  phenomena.  Below  biophors 
there  come  in  order  of  simplicity  of  material  structure  the  molecules  and 
the  atoms  of  the  physicist.  Above  biophors  and  determinants  Weismann 
finds  it  necessary  to  assume  the  existence  of  higher  units,  named  in  order  ids 
and  idants,  the  former  being  groups  of  determinants,  and  actually  visible  as 
granules  of  chromatin,  the  latter  being  the  chromosomes  of  the  nucleus.  Each 
one  of  these  various  units  is  possessed  of  the  fundamental  vital  properties  of 
growth  and  multiplication  by  division.  Such  a  complex  system  is  Preforma- 
tion  in  an  extreme  form.  In  fertilization  idants  of  the  sperm  join  with  idants 
of  the  ovum,  and  the  resulting  segmentation  nucleus  consists  of  a  mixture  of 
paternal  and  maternal  determinants.  Within  this  mixture  there  exist  in  a 
potential  state  the  primary  constituents  of  a  considerable  number  of  forms 
which  the  future  individual  may  assume.  In  ontogeny,  or  development  of 
the  individual,  these  primary  constituents  take  two  paths :  some  of  the  ids 
remain  inactive  and  enter  the  germ-cells  of  the  embryo  for  the  production  of 
future  generations ;  other  ids  disintegrate  into  determinants,  the  determinants 
enter  the  embryonic  cells  that  result  from  segmentation,  and  there  themselves 
disintegrate  and  set  free  into  the  cytoplasm  their  constituent  biophors ;  thus 
they  determine  the  future  character  of  the  cells  of  the  organism.  The  division 
of  primary  constituents  into  those  that  shall  remain  latent  and  those  that  shall 
become  active  is  effected  largely  by  the  stimulation  of  external  influences; 
hence,  given  several  potential  formations  in  the  germ,  external  influences 
decide  which  one  shall  become  the  actual  structure  in  the  adult  or^ani-m. 
Once  set  free  and  having  become  somatoplasm,  neither  the  biophors  nor  the 
determinants  are  able  to  return  to  the  germ-cells.  In  the  adult,  germ-plasm  is 
never  capable  of  reflecting  in  any  way  the  characteristics  of  the  BOOMtOplasm 
which  surrounds  it  on  all  sides.  With  its  ancient  ancestry  it  leads  a  charmed 
existence,  largely  independent  of  environmental  changes.  It  follows  that 


504  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

characters  acquired  by  the  adult  are  incapable  of  acquisition  by  the  germ- 
plasm,  and  hence  may  not  be  transmitted.  The  non-inheritance  of  acquired 
characters  is  thus  another  of  the  fundamental  tenets  of  Weisrnanu's  theory, 
and  one  about  which  he  is  most  positive. 

If  these  two  principles  of  continuity  of  stable  germ-plasm  and  non-inheri- 
tance of  acquired  characters  be  true,  why  are  not  all  individuals  in  any  one 
line  of  descent  exactly  like  one  another?  How  is  congenital  variation  possible? 
In  the  first  place,  Weismann  allows  that  germ-plasm,  while  eminently  stable, 
is  not  absolutely  so ;  it  is  subject  to  slight  continual  changes  of  composition 
resulting  from  inequalities  in  nutrition  ;  and  "  these  very  minute  fluctuations, 
which  are  imperceptible  to  us,  are  the  primary  cause  of  the  greater  deviations 
in  the  determinants  which  we  finally  observe  in  the  form  of  individual  varia- 
tions.'' The  accumulation  of  minute  deviations  may  be  aided  greatly  by  sex- 
ual reproduction,  or,  to  use  Weismann's  more  exact  term,  which  is  equally 
applicable  to  the  combination  of  sexual  elements  in  sexual  organisms  and  to 
the  process  of  conjugation  in  the  asexual  forms,  amphimixis.  Given  the  in- 
finitesimal beginning  of  a  variation,  the  mingling  of  two  lines  of  descent,  with 
different  past  surroundings,  may  be  a  most  powerful  factor  in  strengthening 
the  deviation  and  bringing  it  into  recognition  as  a  new  character.  Moreover, 
natural  selection  becomes  here  also  potent  as  soon  as  the  variation  has  assumed 
sufficient  proportions  to  be  seized  upon  by  this  important  factor  of  evolution. 
In  cases  of  reversion  Weismann  supposes  the  determinants  to  remain  inactive 
in  the  germ-plasm  for  one  or  more  generations  and  later  to  develop.  The 
theory  accounts  for  the  regeneration  of  lost  parts  by  the  assumption  that  the 
cells  in  the  vicinity  of  the  wound,  by  the  proliferation  of  which  the  new  part 
grows,  contain,  besides  the  active  determinants  that  have  given  them  their 
specific  character,  other  determinants  that  are  latent  until  the  opportunity  for 
regeneration  arrives.  Some  cells  do  not  possess  such  latent  determinants,  and 
hence  some  parts  of  a  body  are  incapable  of  reproducing  lost  parts. 

Such  are  the  main  features  of  Weismann's  theory — a  germ-plasm  of  highly 
complex  architecture  and  independent  of  somatoplasm ;  continuity  of  germ- 
plasm  and  non-inheritance  of  acquired  somatic  characters  tending  to  preserve 
the  uniformity  of  the  species ;  slight  nutritional  variation  of  germ-plasm  and 
sexual  reproduction  tending  to  destroy  that  uniformity ;  the  result  is  inherited 
resemblance  and  congenital  variation.  The  theory  is  now  being  most  actively 
discussed. 

Theory  of  Epigenesis. — Among  epigenesists  no  one  theory  may  be  said  to 
be  pre-eminent.  The  main  features  of  the  epigenetic  conception,  already 
referred  to,  may  be  summarized  as  follows :  The  fertilized  ovum  is  isotropous, 
i.  e.  all  parts  are  essentially  alike;  germ-plasm  probably  consists  of  minute 
particles,  but  these  particles  do  not  represent  definite  cells  or  groups  of  cells 
of  the  adult;  segmentation  is  a  quantitative  process;  the  early  blastomeres 
are  essentially  alike,  and  any  one  of  them,  if  isolated  from  the  rest,  may 
give  rise  to  a  whole  organism,  although  under  ordinary  circumstances  they 
react  upon  each  other  in  bringing  about  the  resultant  individual ;  there  is 


REPRODUCTION.  505 

no  predetermination,  either  in  the  germ-cells  or  in  the  segmenting  ovum, 
of  the  ultimate  form  or  function  of  the  various  constituent  parts ;  morpho- 
logical differentiation  and  physiological  specialization  are  phenomena  of 
comparatively  late  embryonic  life,  and  the  prospective  character  of  any  one 
cell,  whether  it  is  to  be  a  muscle-cell,  gland-cell,  nerve-cell,  or  germ-cell,  is 
determined  by  the  influence  of  the  surrounding  cells  and  the  surrounding 
physical  and  chemical  conditions — "  the  prospective  character  of  each  cell  is  a 
function  of  its  location."  Extreme  epigenetic  views  are  not  so  numerous  as 
those  of  preformation.1 

The  more  moderate  thinkers  of  the  present  time  recognize  truth  in  both 
preformation  and  epigenesis,  and  are  endeavoring  by  experimental  methods  to 
determine  how  much  share  in  the  production  of  the  characteristics  of  the  off- 
spring is  to  be  ascribed  to  the  original  qualities  of  the  germ-plasm  and  how 
much  to  the  physical,  chemical,  and  physiological  phenomena  of  the  immediate 
environment  of  the  developing  embryo.  Such  experimental  work  is  per- 
formed at  present  upon  the  simpler  and  lower  animals,  mostly  marine  inverte- 
brates, and  has  reference  to  the  effect  of  changes  in  the  composition  of  the  water 
surrounding  the  embryo,  the  effects  of  various  salts,  of  changes  in  temperature, 
of  pressure,  of  electricity,  etc.,  etc.  Such  work  is  now  in  its  infancy,  but  it  is 
doubtless  destined  to  yield  results  of  the  highest  value  in  an  understanding  of 
the  true  nature  of  heredity. 

1  The  best  statement  of  a  moderate  epigenetic  theory  is  to  be  found  in  O.  Hertwig :  The 
Biological  Problem  of  To-day  ;  Preformation  or  Epigenesis  f  Authorized  translation. 


INDEX  TO  VOLUME  II. 


ABSOLUTE  muscular  force,  141 
Accelerator  u rinse  muscle,  449 
Accommodation,  306 

associated  movements  of,  311 

astigmatic,  310 

dissociation  of,  from  convergence,  312 

influence  of  drugs  on,  311 

in  old  age,  314 

mechanism  of,  309 

nervous  mechanism  of,  311 

normal  range  of,  312 

range  of,  in  hypermetropia,  314 
in  myopia,  314 

relation  of,  to  perception  of  distance,  356 

voluntary  character  of,  311 
Achromatic  lenses,  316 
Achromatism  of  the  eye,  316 
Acid  salts  of  muscle,  168 
Acidity  of  worked  muscles,  168 
Acids,  action  of,  on  nerves  and  muscles,  60 
Acquired  characters,  inheritance  of,  496 

variations,  500 

Actinic  rays  of  the  luminiferous  ether,  331 
Action  current,  diphasic,  152 
in  the  heart,  152 
in  the  muscles,  150 
in  the  nerves,  153, 183 
Adam's  apple,  425 

Adrenal  extract,  action  of,  on  muscles,  138 
Aerial  perspective,  355 

Afferent  impulses,  effect  of,  on  irritability  of  the 
central  nervous  system,  223 

neurones  of  t  he  spinal  cord,  203 

paths  in  the  cord  traced  electrically,  230 
traced  histologically,  229 
traced  physiologically,  229 
After-birth,  481 

After-effect  of  retinal  stimulation,  345 
After-images,  346 
After-loading  of  muscles,  110 
After-pressure,  sensation  of,  394 
Agamogenesis,  439 

Age,   changes   in  organization  of    the    central 
nervous  system  with,  284 

influence  of,  on  nerve-cells,  490 
on  visual  accommodation,  314 

relation  of  brain -weights  to,  276 
of  menstruation  to,  459 

specific  gravity  of  the   nervous   system  with 

changes  in,  284 

Albinos,  condition  of  the  internal  ear  in,  407 
Alcohol,  action  of,  on   conductivity  of  nerves, 
93 

effect  of,  on  nerve-currents,  156 

fumes,  action  of,  on  nerves,  60 

stimulating  action  of.  ?r> 

Alkalies,  action  of,  on  nerves  and  muscles,  60 
Allan  toic  arteries,  474 

vein,  474 
Allantois,  474 
Allochiria,  400 

Ammonia,  action  of,  on  nerves,  60 
Ammonium  salts,  action  of,  on  muscles,  138 
Amnion,  47:2 


Amniotic  cavity,  472 
Amoeba,  contractility  of,  19 
Amoeboid  movement,  19 
in  neuroblasts,  176 
in  ova,  82 
Am  phi  aster,  469 
Amphimixis  as  cause  of  congenital  variation, 

504 

Amphioxus,  reflexes  in,  212 
Amplitude  of  sound-waves,  381 
Ampulla  of  Heule,  447 
Ampullae  of  the  semicircular  canals,  372 
Ampullary  nerves,  stimulation  of,  407 
Amputation  in  man,  effects  of,  on  neurones,  196 
Anaemia  of  the  brain  during  fatigue,  288 
Anaesthesia,  contralateral,  after  hemisectiou  of 

the  cord,  233 

Anaesthetics,  action  of,  on  nerve-currents,  155 
Analgesia,  232 

following  removal  of  the  cerebellum,  272 
Analysis  of  composite  tones,  384 
Anatomy  of  the  ear,  362 
Anelectrotonus,  62 
Angular  movements  of  joints,  416 
Anisotropic  substance  of  muscle-fibres,  104 
A  nodal  contraction,  36 
Anode,  physical,  definition  of,  52 

physiological,  definition  of,  52 
Anosmia,  411 
Anterior  association  centre,  257 

roots,  recurrent  sensibility  of,  204 
Aphasia,  257 
Apraxia,  259 

Aqueous  humor,  index  of  refraction  of,  303 
Arteries,  calcification  of,  in  old  age,  491 
Arthropods,  segmental  nervous  system  of,  212 
Articular  cartilages,  415 
Articulation,  434 
Articulations,  varieties  of,  414 
Artificial  circulation  through  the  heart,  69 
through  the  muscles,  68 

stimulation  of  muscle  compared  with  normal, 

134 

Aryepiglottic  fold,  422 
Aryteno-epiglottidean  muscle,  426 
Arytenoid  cartilages,  425 

muscle,  426 
Asexual  reproduction,  439 

theory  of  the  origin  of,  441 
Aspirates,  437 

Association  centre,  anterior,  257 
middle,  257 
posterior,  •_'.">? 

cerebral,  variations  in.  -.'(in 

fibres  and  centres  of  the  cortex,  rjr.i; 

tracts,  origin  of.  from  central  cells.  •_'<>."> 
Astasia  after  removal  of  the  cerebellum.  -.'?:', 
Asthenia  from  removal  of  the  cerebellum.  :*!'.'> 
Astigmatic  accommodation,  310 
Astigmatism,  317 

detection  of,  319 

irregular,  319 

Astral  rays,  contractility  of,  470 
Atavism.  !!».") 

507 


508 


INDEX  TO    VOLUME  II. 


Ataxia  after  removal  of  the  cerebellum,  273 
Atonia  after  removal  of  the  cerebellum,  273 
Atrophy  of  the  nerve-cells  from  disuse,  195 
Atropia,  action  of,  on  accommodation,  311 
Atropin,  action  of,  on  the  eye,  325 
Attraction  sphere  of  the  ovum,  449 
Auditory  area  of  the  cortex,  253 
canal,  363 

epithelium  of  the  utricle  and  saccule,  373 
judgments,  389 
meatus,  external,  362 
nerves,  central  paths  of,  237 
cochlear  division  of,  376 
subdivisions  of,  373 
ossicles,  365 

movements  of,  367 
sensations,  limits  of,  382 
successive  contrast  in,  388 
theory  of,  380 

Auricle  of  the  external  ear,  362 
Automatism,  definition  of,  20 
Axoues,  definition  of,  21,  173 
growth  in  diameter  of,  179 
length  of,  174 

BALL-AND-SOCKET  joint,  416 

Barium  salts,  action  of,  on  muscles,  138 

Bartholini,  glands  of,  462 

Basilar  membrane,  structure  of,  377 

width  of,  380 
Bathyasthesia,  233 

Beats  in  musical  tones,  production  of,  386 
Benham's  spectrum  top,  344 
Binocular  combination  of  colors,  358 
vision,  356 

illusions  in,  359 

rivalry  of  the  fields  of  vision  in,  358 
Biophors  of  the  germ-plasm,  503 
Birds,  removal  of  cerebral  hemispheres  in,  267 
Birth,  size  of  the  child  at,  487 
Birth-rate  of  the  two  sexes,  483 
Births,  multiple,  482 

ratio  of  male  to  female,  483 
Blastomeres,  470 
Blind  spot,  328 
Blood,  amount  of,  in  the  central  nervous  system, 

288 

changes  in,  during  pregnancy,  477 
Blood-supply  of  the  central  nervous  system,  286 

relation  of,  to  irritability,  67 
Body-sense  area  of  the  cortex,  252,  254 
Body-temperature,  rise  of,  from  injury  to  the 

optic  thalami,  271 

from  lesions  in  the  corpora  striata,  271 
Bolometer,  142 

Bones,  action  of  muscles  on,  417 
Bottcher's  crystals,  445 
Brain,  curve  of  growth  of,  279 
growth  of,  278 

number  of  nerve-elements  during,  280 
relation  of,  to  growth  of  the  body,  280 
size  of  neurones  during.  281 
metabolic  activity  in,  288 
regulation  of  circulation  in,  287 
weight  of,  273 
Brain -stem,  274 

Brain- ventricles,  capacity  of,  274 
Brain- weight,  decrease  of,  in  old  age,  296 
relation  of,  to  insanity,  278 
to  sex,  276 

to  social  environment,  277 
Brain-weights  of  diiferent  races,  278 
Breaking  contraction,  point  of  origin  of,  35 
"  Breaking  "  shock,  31 
Broca's  convolution,  257 
Brown-Sequard's  paralysis,  233 
Brucin,  action  of,  on  end-plates,  27 


Bulbo-cavernosus  muscle,  449 
action  of,  in  erection,  464 
Bulimia,  404 

CAFFEIN,  action  of,  on  coagulation  of  muscle- 
plasma,  164 
Calcium  salts,  action  of,  on  the  muscles,  138 

relation  of,  to  irritability,  59 
Canalis  cochlearis,  373 

reunions,  373,  374 
Capillary  electrometer,  146 
Caput  gallinaginis,  464 
Carbon  dioxide,  action   of,  on  conductivity  in 

nerves,  93 
011  the  nerves,  60 
on  warm  spots,  398 
effect  of,  on  nerve-currents,  156 
of  the  muscle,  168 
production  of,  in  nerves,  95 
disulphide,  action  of,  on  nerves,  60 
Cardiac  palpitation  at  the  climacteric,  490 
Carnic  acid  of  muscles,  167 
Carnin  of  muscles  167 
Castration,  effects  of,  463 

on  the  voice,  431 
Cataleptic  rigor,  160 
Caudate  nucleus,  heat  centre  of,  271 
Cell,  galvanic,  29 
Cell-differentiation,  22 
Cells,  growth  of,  486 

Central  cells,  importance  of,  in  relation  to  in- 
crease of  organization,  285 
nervous  system,  amount  of  blood  in,  288 
arrangement  of  cell  groups  in,  205 
blood-supply  of,  286 
change  in  specific  gravity  of,  with  age, 

284 

condition  of,  in  sleep,  293 
conscious  phenomena  of,  172 
daily  rhythms  of,  289 
development  of,  172 
fatigue  of,  289 
general  arrangement  of,  202 

functions  of,  171 

influence  of  the  thyroid  on  growth  of,  289 
in  old  age,  295 

intensity  of  metabolism  in,  288 
medullation  of  nerves  in,  181 
operations  on,  in  frogs,  265 
organization  of.  at  different  ages,  284 
neurones  of  the  spinal  cord,  203 
stimulation  of  the  nervous  system,  208 
Centre  of  hearing,  cortical,  253 
of  rotation  of  the  eye,  298 
of  smell,  cortical,  253 
of  vision,  cortical,  253 
spinal,  of  ejaculation,  465 
of  erection,  464 
of  parturition,  481 
Centres,  association,  256 
Centrosome  of  human  spermatozoa,  444 
of  the  fertilized  egg,  468 
of  the  ovum,  449 

Cerebellum,  anatomical  connections  of.  273 
effects  of  injury  to,  272 
functions  of,  272 
senile  changes  in,  296 

Cerebral  circulation,  conditions  affecting,  288 
hemispheres,  effect  of  removal  of,  263 
relative  physiological  values  of,  259 
removal  of,  in  birds,  266 

in  dogs,  262,  267 
Cerebrin  of  nerves,  170 
Cerebrum,  heat-regulating  functions  of,  270 

removal  of,  in  dogs,  267 
Characters,  acquired,  inheritance  of,  496 
Chemical  reagents,  action  of,  on  irritability,  58 


INDEX  TO    VOLUME  II. 


509 


Chemical  stimulation  of  nerve,  25 

ton  us,  143 

< 'liniiistry  of  nerves,  !<>}» 
Chemotaxis,  466 

influence  of,  on  neuroblasts,  176 
Cherao-tropism,  -Hit 5 
Chest-voice,  432 

Chloroform,  action  of,  on  coagulation  of  muscle- 
plasma,  164 

effect  of,  on  nerve-currents,  156- 

vapor.  action  of,  on  nerves,  60 
Cholesterin  in  nerve,  169 
Chorda  tympani  nerve,  gustatory  function  of, 

410 
Chorion,  \1'.\ 

frondostun.   171 

laeve,  474 
Cluirionic  fluid,  473 

villi,  473 

Chromatic  aberration,  316 
Chromatoblasts  of  pleuronectidse,  20 
Chromosomes,  number  of,  in  the  segmentation 
nucleus,  466 

of  germ-cells,  hereditary  function  of,  499 

of  human  spermatozoa.  443 

of  the  sexual  element,  reduced  number  of, 
454 

ovarian,  changes  in,  during  maturation,  451 
number  of,  450 

reduction  of,  in  maturation  of  spermatozoa, 

445 

Chronograph,  description  of,  100 
Ciliary  ganglion,  323 

muscles,  action  of,  in  accommodation,  309 

nerves,  long,  324 
short,  311,  323 

Circulation,  artificial,  through  isolated  organs, 
68 

of  blood  in  the  retina,  322 

of  the  brain  and  cord,  286 
Circumduction,  movement  of,  416 
Climacteric,  459,  490 

ovulation  after,  456 
Climate,  influence  of,  on  age  of  puberty,  489 

on  time  of  climacteric,  490 
Clitoris,  462 

homology  of,  464 

Coagulation  of  muscle-plasma,  action  of  drugs 
on,  164 

of  myosin,  163 
Cocaine,  action  of,  on  conductivity  in  nerves,  93 

on  the  tongue,  413 
Cochlea,  anatomy  of,  374 

bony,  372 

membranous,  structure  of,  376 
Cochlear  root  of  the  auditory  nerve,  central 

paths  of,  237 

Coffee,  stimulating  action  of,  75 
Cold  and  warm  points  of  the  skin,  398 
Collaterals  of  axones,  173 

Color  of  objects,  relation  of,  to  intensity  of  illu- 
mination, 333 

sensations  in  indirect  vision,  333 
phenomena  of,  333 

theories,  335 

triangle,  334 

vision,  theories  of,  335 
Color-blindness,  338 

hereditary  transmission  of,  494 

of  the  rods,  342 

Colored  shadows  from  simultaneous  contrast,  347 
Color-mixture,  333 
Colors,  binocular  combination  of,  358 

complementary,  334 

physical  basis  of,  332 

relative  luminosity  of,  340 

saturation  of,  342 


Combinational  tones,  387 
('oinniH>iiiv.   Mcyiu-rt's,  238 

von  ( Jiddrn's,  238 

Commissures,  origin  of,  from  central  cells,  205 
Common  sensation,  definition  of,  399 

sensibility,  afferent  paths  of  tin-  nerves  of,  230 
Commutators,  method  of  using.  :;•; 
Complementary  colors,  334 

Composite  tones,  analysis  of,  384 
Conceptions,  multiple,  4H2 
Concha  of  the  external  ear,  362 
Conduction  by  contiguity,  81 

directions  of,  84 

from  neurone  to  neurone,  84 

in  branching  nerves,  80 

in  ganglion-cells,  97 

in  muscles,  80 

in  nerves,  effects  of,  95 

in  nerve-trunks,  79 

of  nerve-impulses,  direction  of,  184 
from  neurone  to  neurone,  207 

process,  nature  of,  97 

rate  of,  87 
Conductivity,  action  of  drugs  on,  93 

definition  of,  20,  77 

dependence  of,  on  protoplasmic  continuity,  77 

effect  of  constant  current  on,  94 

influences  affecting,  91 

of  muscle,  20 

of  nerves,  21 

of  the  neurone,  189 

of  ova,  22 

Condyloid  joints,  416 
Cones,  retinal,  function  of,  341 

movements  of,  331 
Confluxion  in  space-perception,  353 
Congenital  resemblances,  494 

variations,  500 

Conjugate  foci  in  a  dioptric  system,  302 
Conjugation,  440 

Conin,  action  of,  on  end-plates,  27 
Consciousness,  cerebral  origin  of,  172 
Consonants,  436 

Constant  current,  contracture  effect  of,  in  mus- 
cles, 131 

effect  of,  on  conductivity,  94 
on  muscles,  61 
on  nerves,  62 

Constrictor  nerves  of  the  iris,  323 
Continuous  contractions,  127 
Contractility,  definition  of,  17,  98 

in  vorticella,  20 

occurrence  of,  20 

of  amoebae,  19 

of  muscle,  17 

of  muscles,  adaptation  of,  to  their  normal 
functions,  108 

of  ova,  22 

of  the  astral  rays,  470 

Contraction  curve  of  muscle,  effect  of  frequent 
excitation  on,  115 

idio-muscular,  92 

of  muscles,  post-mortem,  160 
relation  of,  to  structure,  107 

remainder,  106 

wave  in  muscle,  rate  of  transmission,  87 

length  of,  88 
Contractions  from  repeated  single  stimuli,  112 

introductory,  113 

isometric,  110 

isotonic,  110 

normal,  tetanic  nature  of,  132 

of  rigor  caloris,  165 
Contracture  after  frequent  excitation,  128 

after  single  excitation,  129 

definition  of,  116 

from  fatigue,  130 


510 


INDEX  TO    VOLUME  II. 


•Contracture  in  dying  muscles,  132 
in  rigor  mortis,  128 
in  veratria  poisoning,  128 
normal,  129 
of  the  neck  muscles  after  cerebellar  injury, 

272 

pathological,  127,  132 
relation  of,  to  tetanus,  117,  122,  124 
Contractures,  127 
Contrast,  visual,  346 

in  space  perception,  352 
Convergence  of  the  eyes  in  accommodation,  311 

muscular  mechanism  of,  300 
Co-ordination   of  the  efferent  impulses  in  re- 
flexes, 214 
Copulation,  463 
Core-conductors,  158 
Cornea,  curvature  of,  303 
Corniculum  laryngis,  425 
Corn u tine,  action  of,  on  muscles,  137 
Corona  radiata,  454 

of  the  ovum,  450 
Corpora  cavernosa  of  the  penis,  448 

striata,  functions  of,  271 
Corpus  callosum,  functions  of,  270 

luteum,  455 

*•     spougiosum  of  the  penis,  448 
Corresponding  points  of  the  retinas,  359 
Cortex   cerebri,   effects   of    localized    electrical 

stimulation  of,  241 
electrical  stimulation  of,  242 
number  of  nerve-cells  in,  284 
course  of  efferent  impulses  from,  251 
latent  areas  of,  261 
Corti,  cells  of,  377 

organ  of,  structure  of,  377 
rods  of,  377 
Cortical  areas,  243 

motor,  in  man,  250 
size  of,  247 
centres,  243 
motor  control,  crossed,  251 

multiple  character  of,  250 
regions,  243 

stimulation,  inhibitory  effects  of,  224 
Cowper's  gland,  443 
histology  of,  448 
secretion  of,  446 

Crabs,  regeneration  of  lost  parts  in,  496 
Cranial  nerves,  afferent,  236 
Creatin  in  muscle,  166 
Creatinin  of  muscle,  167 
Cretinism,  sporadic,  289 
Crico-arytenoid  muscle,  lateral,  426 

posterior,  426 
Cricoid  cartilage,  425 
Crico-thyroid  muscles,  426 
Criminals,  weight  of  the  brain  in,  277 
Crista  acustica  of  the  semicircular  canals,  373 
Critical  period  of  nerves,  66 
Cross-suturing  of  nerve-trunks,  201 
Cruciate  heat  centre,  271 
Cupola  of  the  cochlea,  375 
Curare,  action  of,  26 
Currents  of  action  in  muscle,  150 

in  nerves,  153 
of  rest,  147 

theories  as  to  their  cause,  148 
Curve  of  fatigue  with  repeated  single  contrac- 
tions, 113 

of  intensity  of  sleep,  294 
of  muscle  contraction,  effect  of  frequent  exci- 
tation on,  115 

of  muscular  contractions,  100 
of  work  for  muscles,  140 
Cutaneous  sensations,  cortical  area  for,  253 
disturbance  of,  in  disease,  403 


Cutaneous  sensations,  varieties  of,  390 

temperature  points,  398 
Cytoplasmic  changes  in  nerve-cells,  182 

DANIELL  cell,  28 

Darwin's  theory  of  heredity,  501 

Death,  definition  of,  491 

of  the  tissues,  492 

somatic,  491 

theory  of,  492 
Decidua  graviditatis,  461,  471 

menstrualis,  458,  461 

reflexa,  472 

serotina,  472 

vera,  472 

Decidual  cells,  471 
Defecation,  cerebral  control  of,  270 

reflex  character  of,  213 

Degeneration  after  hemisection  of  the  spinal 
cord,  228 

following  removal  of  motor  cortical  areas,  244 

of  cut  nerves,  78 

of  muscle  after  section  of  its  nerve,  70 

of  nerve-cells,  199 

of  nerve-elements,  197 

of  nerves  after  section,  69 

reaction  of,  47,  54,  70 

Deglutition,  action  of  the  epiglottis  in,  422 
Deiters'  cells  of  the  organ  of  Corti,  377 

nucleus,  238 

Demarcation  currents  of  injured  muscle,  148 
Dendrites,  definition  of,  174 
Dermal  sensations,  cortical  area  for,  253 
path  of  conduction  for,  in  the  cord,  235 

sensibility,  area  of  distribution  of  the  nerves 

of,  231 

Descending  impulses,  course  of,  244 
Desiccation  of  nerve,  59 
Determinants  of  the  germ-plasm,  503 
Deutoplasm  of  the  ovary,  450 

ovarian,  composition  of,  451 
Development  of  nerve-cells,  176 
Dextrose,  action  of,  in  delaying  rigor  mortis, 

164 
Diapedesis    of    maternal    leucocytes    into    the 

fetus,  476 
Diarthrosis,  415 
Diaxonic  nerve-cells,  178 
Differential  tones,  387 
Diffusion  of  central  nerve-impulses,  208 

of  impulses  in  the  cord,  influences  affecting, 
217 

of  nerve-impulses,  peripheral,  218 
Digastric  muscle,  426 

Digitalin,  action  of,  on  coagulation  of  muscle- 
plasma,   164 
on  muscles,  137 

Digitalis,  action  of,  on  nerves  and  muscles,  60 
Digits,  supernumerary,  494 
Dioptric  apparatus  of  the  eye,  defects  of,  314 
Dioptrics  of  the  eye,  300 
Dioptry,  definition  of,  304 
Diphasic  current  of  action,  152 
Direction,  judgments  of,  by  means  of  auditory 
sensations.  389 

of  the  nerve-impulse,  184 
Discord,  387 

Discriminating  sensibility  of  the  skin  for  pres- 
sure, 392 

Discriminative  sensibility  for  differences  of  tem- 
perature, 397 

Discus  proligerus,  450,  454 
Diseases,  inheritance  of,  498 
Dispermy,  471 
Dispersion  of  light,  316 

Dissociation  of  the  axial  and  focal  adjustments 
of  the  eye,  312 


INDEX  TO    VOLUME  II. 


511 


Distance,  judgments  of,  by  means  of  auditory 
sensations,  389 

pen-option  of,  354 

visual  perception  of,  348 
Disuse,  effect  of,  on  muscles,  77 
Dizziness,  405 

Dogs,  removal  of  cerebrum  in,  2<>7 
Domestication,    effect   of,    on    menstruation   in 

animals,  460,  •!«;•.' 
Dorsal  nerve-roots,  efferent  fibres  in,  203 

roots,  degeneration  resulting  from  injury  to, 
287 

spinal  nerve-roots,  number  of  fibres  of,  230 
Dreams,  293 
I)u  Bote-Reymond'i  key,  30 

law  of  stimulation,  32 

theory  of  currents  of  rest,  148 
Ductus  cochlearis,  structure  of,  374 

endolymphaticus,  373 

venosus  of  the  embryo,  476 
Duration  of  electric  currents,  effect  of,  on  their 

irritating  power,  46 
Dynamic  equilibrium,  organs  of,  407 
Dyspepsia  accompanying  the  climacteric,  490 
Dyspnoea,  effect  of,  on  the  iris,  324 

EAR,  analysis  of  composite  tones  by,  384 
anatomy  of,  362 

discriminative  sensibility  of,  for  pitch,  385 
fatigue  of,  387 
imperfections  of,  388 
membranous  labyrinth  of,  372 
ossicles  of,  365 
sensibility  of,  in  perception  of  time  intervals, 

388 

Earth-worms,  regeneration  of  lost  parts  in,  496 
Efferent  fibres  of  the  optic  nerves,  240 
impulses  in  reflexes,  co-ordination  of,  214 
neurones  of  the  dorsal  spinal  nerve-roots,  203 

of  the  spinal  cord,  203 
paths  from  the  cortex,  course  of,  244 
Ejaculation,  465 
Ejaculatory  duct,  447 
Elasticity  of  muscle,  105 
Electric  currents,  correlation  of  their  duration 

with  histological  structures,  47 
effect  of,  on  muscles,  61 
on  nerves,  62 
the  duration  of,  46 
their  density,  41 
galvanic,  effect  of,  on  normal  human  nerves, 

51 
influence  of  their  direction  in  nerves,  48 

of  varying  duration  of,  47 
methods  of  detecting,  145 
spread  of,  in  moist  conductors,  41 
stimulating  effect  of,  28 
organ,  145 

Electrical  changes  in  the  retina,  331 
phenomena  of  muscle  and  nerve,  144 

of  nerves,  interpretation  of,  158 
stimulation  of  nerve,  25 

of  nerves,  law  of,  32 
Electrodes,  shielded,  41 

varieties  of,  29 

Electrostatic  changes,  stimulating  action  of,  42 
Electrotonic  changes  of  conductivity,  50 
of  irritability,  64 

in  human  nerves,  65 
twitch,  157 
Electrotonus,  62 
Embryo,  nutrition  of,  475 

rate  of  growth  of,  487 
Emmetropia.  313 
Emruetropic  eye,  312 

Encephala,     classification     of,     according     to 
weight,  275 


Encephalon,  specific  gravity  of,  275 

weight  of,  274,  27.', 
Kinl-hulhs.  M-IISOIA  .  388 
Endolymph,  372 
End-organs,  importance  of,  in  touch  sensations, 

386 

transmission  of  excitation  by  means  of,  82 
Energy  liberated  in  contracting  museles,  138 
Engelmann's  theory  of  the  nature  of  muscular 

contraction,  105 

Environment,  influence  of,  on  organisms,  493 
Epididymis,  447 
Epigenesis,  theory  of,  500,  504 
Epiglottis,  421 

Epinephrin,  action  of,  on  muscles,  138 
Equilibrium  of  the  body,  definition  of,  404 
relation  of  the  cerebellum  to,  273 
sense  of,  404 
Erection,  464 

spinal  centre  for,  464 
Erector  clitoridis  muscle,  464 
penis,  action  of,  in  erection,  464 

muscle,  449 

Eserin,  action  of,  on  nerve  and  muscle,  60 
Ether,  action    of,   on    coagulation  of    muscle- 
plasma,  164 

on  conductivity  of  nerves,  93 
effect  of,  on  nerve-currents,  155 
vapor,  action  of,  on  nerves,  60 
Eustachian  tube,  363 
function  of,  369 

Excitability,  changes  in,  during  Wallerian  de- 
generation, 69 
Exercise,  effect  of,  on  growth,  489 

on  muscular  endurance,  76 
Exhaustion  of  muscles,  72 
Explosive  consonants,  437 
Extensibility  of  muscle,  105 
External  auditory  meatus,  362 
ear,  anatomy  of,  362 
rectus  muscle,  299 

Extractives,  nitrogenous,  of  muscle,  166 
Extrapolar  region,  62 
Eye,   abnormal    positions  of,  after   cerebellar 

injury,  272 

adaptation  of,  to  light,  326 
axes  of  rotation  of,  299 
chromatic  aberration  of,  316 
constants,  changes  in,  during  accommodation, 

311 

defects  in  the  dioptric  apparatus  of,  314 
dioptric  apparatus  of,  300 
mechanical  movements  of,  298 
movements,  binocular  co-ordination  in,  300 

extent  of,  298 
muscles  of,  299 

in  nervation  of,  300 
optical  constants  of,  303 

power  of,  304 
positions  of,  299 
refractive  media  of,  302 

surfaces  of,  303 
spherical  aberration  of,  315 

FALLOPIAN  tubes,  443,  456 
False  amnion,  473 
Falsetto  register  of  the  voice,  433 
Far-point.of  vision,  312 
Fatigue,  cerebral  anaemia  from,  288 
curve  with  repeated  single  contractions,  113 
effect  of,  on  height  of  contraction,  113 
on  muscular  contraction,  130 
on  rigor  caloris,  165 

mortis,  160 

from  voluntary  muscular  contraction,  134 
in  nerve-fibres,  lit") 
of  central  nervous  system,  289 


512 


INDEX  TO    VOLUME  II. 


Fatigue  of  motor  end-organs,  83 

of  muscle,  66,  70 
recovery  from,  73 

of  nerve-cells,  136,  191 

of  nerves,  75,  96 

of  retina,  344 

relation  of,  to  sleep,  291 

theories  of,  72 

to  auditory  sensations,  387 
Fats  of  muscle,  167 

relation  of,  to  muscular  work,  74 
Female  births,  relative  number  of,  483 

pronucleus,  453 

Females,  rate  of  growth  in,  488 
Fenestra  ovalis,  363 

rotunda,  363,  375 

use  of,  376 

Ferment,  myosinogen-coagulating,  161 
Fertilization,  466 
Fetal  membranes,  472 
Field  of  vision,  binocular  rivalry  of,  358 
Fimbriae  of  the  Fallopian  tube,  456 
Fish,  bony,  removal  of  cerebral  hemisphere  in, 

263 
Fishes,  semicircular  canals  in,  407 

visual  accommodation  in,  306 
Flicker  photometry,  345 
Focal  illumination  of  the  eye,  320 
Foci,  conjugate,  302 

principal,  302 

Foramen  ovale  of  the  foetal  heart,  476 
Forced  movements  after  section  of  the  semi- 
circular canals,  405 
in  frogs,  266 
Fovea  centralis,  327 
Franklin's  theory  of  color  vision,  337 
Frictionals,  437 
Frogs,  removal  of  cerebral  hemisphere  in,  264 

striped  muscle,  time  of  single  contraction  in, 

108 

Frontal  lobes  of  the  hemispheres,  effect  of  re- 
removal  of,  262 
Fuhlsphare,  cortical,  252 
Fundamental  tone,  definition  of,  383 

GALVANI,  LUIGI,  28 

Galvanic  current,  action  of,  on  conductivity,  94 
contracture  effect  of,  on  muscles,  131 
effect  of,  on  muscles,  61 
on  nerves,  62 

of  making  and  breaking,  31 
on  normal  human  nerves,  51 
opening  and  closing  contractions  with,  38 
Galvanometers,  145 
Galvanotonus,  54,  131 
Gamogenesis,  440 
Ganglion  spirale  of  the  ear,  376 
Ganglion-cells,  conduction  in,  97 
Gaseous  exchanges  in  the  brain,  288 
Gases  of  muscle,  168 

Geminal  fibres  of  the  pyramidal  tracts,  245 
Gemmules  of  the  germ-plasma,  499 
Genio-hyoid  muscle,  426 
Germinal  spot  of  the  ovary,  450 

transmission  of  infectious  diseases,  498 

vesicle,  structure  of,  450 
Germ -plasm  as  a  basis  of  heredity,  499 

continuity  of,  502 

definition  of,  496 

morphological  nature  of,  499 

origin  of,  499 

Gestation,  duration  of,  478 
Gland-cells,  electric  currents  in,  145 
Glands  of  Bartholini,  462 

of  Littre,  448 
Glans  penis,  449 
Gliding  movements  in  joints,  416 


Glossopharyngeal  nerve,  gustatory  function  of, 

410 
Glossopharyngeus,  central  conducting  paths  of, 

236 

Glottis,  423 
oedema  of,  422 

respiratory  movements  of,A29 
Glycocoll  in  muscles,  167      ( 
Glycogen  of  muscles,  167        x^^ 
Golgi,  organ  of,  in  tendons,  402 
Graafian  follicle,  454 

Graphic  method  of  studying  muscular  contrac- 
tions, 99 

Gravity,  influence  of,  on  cerebral  circulation,  287 
Gray  matter  of  the  cerebrum,  water  contents  of, 

274 

Growth  after  birth,  487 
before  birth,  486 
increase  of  fibres  of  the  cortex  during,  282 

of  functional  neurones  during,  282 
influence  of  sex  on  the  rate  of,  488 
influences  which  modify,  489 
of  nerve-cells,  176 
Gustatory  nerves,  410 

sensations,  411 
Guttural  consonants,  437 

Gymnema  silvestre,  action  of,  on  taste-nerves. 
413 

HEMOGLOBIN  of  muscle-serum,  166 
Hair-cells  of  the  crista  acustica,  374 

of  the  organ  of  Corti,  377 
Hamulus,  376 
Harmonic  overtones,  386 
Harmony,  387 

Head  register  of  the  voice,  433 
Hearing,  362 
keenness  of,  371 
relation  of,  to  speech,  431 
Heart,  changes  in,  due  to  pregnancy,  477 
diphasic  action  currents  in,  152 
isolation  of,  69 
muscle,  rate  of  conduction  in,  89 

rigor  mortis  of.  162 
Heart-beat,  voluntary  control  of,  214 
Heat-centres,  271 

Heat-production  in  contracting  muscles,  138 
in  muscles,  142 
in  nerves,  96 
in  rigor  mortis,  160 
Heat-rays  of  ether,  331 
Height  of  contraction,  dependence  of,  on  the 

load,  111 

effect  of  temperature  on,  136 
Helicotrema,  376 
Hemianopsia,  anatomical  basis  for,  240 

from  cortical  lesions,  255 

Hemisections  of  the  cord  alternating  at  differ- 
ent levels,  230 

Brown-Sequard's  paralysis  from,  233 
degeneration  resulting  from,  228 
effect  of,  on  man,  233 

on  sensation  and  motion,  230 
in  animals,  234 
physiological  effects  of,  234 
Heredity,  definition  of,  493 

theories  of,  498 

Hering  theory  of  color  vision,  336 
Hermann's  theory  of  currents  of  rest,  148 
Heteromita,  reproduction  in,  440 
Hinge-joints,  416 
Histology  of  striped  muscle,  104 
Hofacker-Sadler  law,  484 

Holmgren  method  for  testing  color  vision,  339 
Horopter,  359 

Human  muscles,  fatigue  of,  with  artificial  stim- 
ulation, 134 


INDEX  TO    VOLUME  II. 


513 


Hunger,  404 

Hunger-centre,  clinical  evidence  for,  404 
Hydra,  regeneration  (if  lost  parts  in,  !!»»; 
Hydrocyanic  acid,  action  of,  on  coagulation  of 

muscle-plasma,  164 
Hymen.    lli'J 

Hyo-glossus  muscle,  -I'.'ti 
Hypenesthesia,  homolateral,  after  hemisection 

of  the  cord,  233 
Hypermetropia.  :u:{ 

range  of  accommodation  in,  314 
Hypoxanthin  of  muscles,  167 

IDANTS  of  the  germ-plasm,  503 
Idiomuscnlar  contraction,  27,  92,  128 
Idioplasm  as  a  basis  of  heredity,  499 
Ids  of  the  germ -pi  asm.  ."><):; 
Illusions,  visual,  in  sizes  of  objects,  354 

of  space  perception,  351 
Immunity,  inherited,  498 
Impregnation,  466 
Incus,  366 

Independent  irritability  of  muscle,  25 
Index  of  refraction  of  the  aqueous  humor,  303 
of  the  lens,  303 
of  the  vitreous  humor,  303 
Indifferent  point  of  polarized  nerves,  64 
Indirect  vision,  color  sensations  in,  333 
Induced  currents,  making  and  breaking  shocks 

with,  40 
prevention  of  spread  of,  44 

electric  currents,  stimulating  effect  of,  33 
Induction  apparatus,  schema  of,  33 
Infections,  intra-uterine,  498 
Infectious  diseases,  germinal    transmission   of, 

498 
Inferior  oblique  muscle,  299 

rectus  muscle,  299 
Inharmonic  overtones,  386 
Inheritance,  facts  of,  494 

of  acquired  characters,  496 

of  diseases,  498 

theories  of,  498 
Inhibition  from  cortical  stimulation,  224 

in  the  central  nervous  system,  224 
Inorganic  salts,  relation  of,  to  irritability,  59 
Insanity,  relation  of  brain-weight  to,  278 

variations  of  muscular  tonus  in,  220 
Insect  muscle,  time  of  contraction  in,  108 
Intensity  of  visual  sensations,  339 
Intermedius  nerve  of  Wrisberg,  central  path  of, 

236 
Internal  capsule,  grouping  of  fibres  in,  248 

ear,  anatomy  of,  371 

rectus  muscle,  299 

Intracranial    pressure,    relation    of,   to    blood- 
pressure,  287 
Intraocular  images,  320 
Intrapolar  region,  62 

'Introductory    contractions    of    a     contraction 
scries,  113 

peak  of  tetanus  curves,  124 
Inversion  of  retinal  images,  305 
Invertebrates,  conduction  in  the  nerves  of,  91 
Involuntary  muscles,  rigor  mortis  of,  162 
lon-proteid  compounds  of  muscle,  168 
Iris,  dilator  nerves  of,  324 

direct  response  to  light  by.  :;•_'! 

innervation  of.  :;•_':; 

movements  of.  in  accommodation,  309 
rate  of.  :5-.Ti 

muscles  of.  :',-}'.} 

relation  of.  to  spherical  aberration,  315 
Irradiation  in  the  retina,  .'>!!' 

of  nerve-impulses  in  the  central  nervous  sys- 
tem, 208 
Irritability,  definition  of,  20,  23 

33 


Irritability,  effect  of  Mood-supply  on,r,t; 
of  constant  currents  on.  ti-j 
of  repeated  stimuli  on,  »jr> 
of  muscle.  •_'."> 
of  nerve-fibres,  21 
of  nerves,  24 

and  muscles,  conditions  affecting,  55 
effect  of  section  on,  <i!» 
of  ova,  22 

Irritants,  classification  of,  .'.; 
conditions  determining  their  efficiency,  28 
effect  of,  on  irritability.  v> 

of  variations  in  strength  of,  39 
relation  of,  to  the  response,  24 
Ischio-cavernosi  muscles,  449 
Ischio-cavernosus,  action  of,  in  erection,  464 
Isolated  conduction  in  nerve-trunks,  79 
Isometric  contractions,  definition  of,  110 
Isotonic  contractions,  definition  of,  110 
Isotropic  substance  of  muscle-fibres,  104 

JOINTS,  classification  of,  415 
Jumping,  420 

KARYOKINETIC  figures  in  mature  nerve-cells, 

202 

Katelectrotonus,  62 
Kathodal  contraction,  35 
Kathode,  physical,  definition  of,  52 

physiological,  definition  of,  52 
Keys,  electric,  30 
Knee-kick,  reinforcement  of,  222 
Krause's  membrane,  104 

LABIA  majora,  462 

minora,  462 
Labial  consonants,  437 
Labio-dental  frictionals,  438 
Labium  tympanicum  of  the  internal  ear,  377 

vestibulare  of  the  limbus,  377 
Labor,  nature  of,  481 

stages  of,  479 
Labor-pains,  479 

Labyrinth  of  the  ear,  anatomy  of,  371 
Lactation,  ovulation  during,  456 
Lactic  acid  of  muscles,  168 
Lamina  spiralis,  372 

Laminae  of  medullary  tube  in  the  foetus,  20S 
Laryngeal  muscles,  specific  actions  of,  428 

nerve,  recurrent,  428 

superior,  428 
Laryngoscope,  429 
Larynx,  cartilages  of,  425 

closure  of,  during  muscular  effort,  423 

muscles  of,  425 

nerves  of,  428 

structure  of,  421 
Latent  areas  of  the  cortex,  261 

characters,  hereditary  transmission  of,  495 

period,  effect  of  temperature  on,  136 

of  tension  on,  110 
of  motor  end-plates,  103 
of  muscle,  in:; 
of  red  muscle.  109 
of  retinal  stimulation,  343 
of  simple  muscular  contraction,  102 
"Law  of  .contraction,"  Pfluger's,  50 
Law  of  stimulation  of  human  nerves  by  battery- 
currents,  54 
Lecithin  of  nerves,  169 
Lemniscus,  medial,  226 

sensory  paths  entering.  -'•':•"> 
Lens,  changes  in.  during  accommodation,  307 

crystalline,  changes  in,  with  old  age,  314 

curvature^  ,,f.  :\i\:\ 

opacities  in.  :;•_'! 

refractive  index  of,  M".': 


514 


INDEX  TO    VOLUME  II. 


Lens,  thickness  of,  303 

Lenticular  ganglion,  311 

Leucin,  action  of,  on  end-plates,  27 

Leucocytes,  movements  of,  19 

Leucophrys  patula,  reproduction  of,  442 

Life  of  the  individual,  stages  of,  486 

Ligaments  of  the  malleus,  366 

of  the  incus,  367 
Light,  changes  in  the  retina  produced  by,  330 

definition  of,  298 

dispersion  of,  316,  332 

monochromatic,  316 

physical  theory  of,  331 

rays  of  the  luminiferous  ether,  331 

sensations,  intensity  of,  332,  339, 

mechanism  for  the  production  of,  331 
quality  of,  332 
Light-waves,  lengths  of,  332 
Limbus  of  the  spiral  lamina,  377 
Lingual  frictiouals,  438 

nerve,  gustatory  function  of,  410 
Linguo-palatal  consonants,  437 
Liquids,  436 
Liquor  amnii,  472 

folliculi,  454 
Listing's  law,  299 
Littre,  glands  of,  448 
Load,  effect  of,  on  the  contraction  curve,  111 

of  muscle,  effect  of,  on  latent  period,  110 
Local  signs  of  sensations,  394 
Localization,  cutaneous,  variations  of,  395 

in  the  skin,  theory  of,  395 

of  cell-groups  in  the  cerebral  cortex,  241 

of  cortical  cell-groups  for  different  impulses, 
252 

of  pain  sensations,  399 

of  touch  sensations,  394 

power,  relation  of,  to  mobility,  394 
Locomotion,  420 

Locomotor  ataxy,  disturbance   of  equilibrium 
in,  405 

mechanisms,  action  of.  414 
Long  tracts  of  the  cord,  terminations  of,  235 
Long-reed  register  of  the  voice,  432 
Loudness  of  the  voice,  factors  determining,  430 

physical  cause  of,  381 

Luminiferous  ether,  rates  of  vibration  of,  331 
Luminosity,  relative,  of  spectral  colors,  340 
Luminous  sensations,  intensity  of,  339 
Lustre  in  visual  sensations,  explanation  of,  358 

MACROCEPHALIC  brains,  weight  of,  275 
Macula  acustica,  373 

lutea,  327 

Maculae  acusticae,  relation  of,  to  static  equilib- 
rium, 407 

Making  contraction,  point  of  origin,  35 
"  Making  "  shock,  31 
Male  births,  relative  number  of,  483 

pronucleus,  466 
Males,  rate  of  growth  in,  488 
Malleus,  365 

ligaments  of,  366 
Mammary  glands,  443,  462 

in  pregnancy,  477 
Manubrium  of  the  malleus,  365 
Masticatory  movements,  effect  of,  on  taste  sen- 
sations, 411 
Mastoid  antrum,  363 
Maturation  of  germ-cells,  significance  of,  454 

of  nerve-cells,  177 

of  spermatozoa,  445 

of  the  ovum,  451 
Meatus  auditorius  internus,  373 
Mechanical  stimulation  of  nerve,  25,  56 

strain,  influence  of,  on  neuroblasts,  176 

work  of  muscular  contraction,  138 


Medial  lemniscus,  226 

Medullary  sheath,  development  of,  in  the  cen- 
tral nervous  system,  181 
in  the  peripheral  nerves,  180 
significance  of,  180 
tube,  fetal,  204 

laminae  of,  in  the  foetus,  205 
Medullation,  central,  progressive  character  of, 

181 

of  nerve-fibres,  significance  of,  283 
peripheral,  180 
Medusae,  rate  of  conduction  in,  89 

staircase  contractions  in,  112 
Membrana  basilaris,  374 
flaccid  a,  365 

granulosa  of  the  Graafian  follicle,  454 
reticulata,  378 
tectoria,  377,  379 
tympani,  364 

Membrane  of  Reissner,  374,  379 
Membranous  labyrinth  of  the  ear,  372 
Menopause,  459,  490 
Menstruation,  457 
age  of  onset  of,  459 
cessation  of,  at  the  climacteric,  490 
general  disturbances  accompanying,  459 
in  animals,  460 
relation  of  ovulation  to,  456 
theory  of,  460 
Mental  activity,  relation  of  cerebral  circulation 

to,  288 

Menthol,  action  of,  on  cold  nerves,  398 
Metabolism,  intensity  of,.in  the  brain,  288 
Meynert's  commissure,  238    " 
Microcephalic  brains,  weight  of,  275 
Microcephalies,  268 
Micturition,  cerebral  control  of,  270 

reflex  character  of,  213 
Middle  ear,  362 

inflammatory  disease  of,  364 
mechanism  of,  368 
muscles  of,  369 
Migration  of  neuroblasts,  176 
Modiolus,  372 
Molecular  weight,  relation  of,  to  physiological 

action,  60 

Monochromatic  light,  316 
Mons  Veneris,  462 
Monstrosities,  congenital,  494 

origin  of,  483 

Morgagni,  ventricles  of,  422 
Morula,  470 

Motor  areas,  cortical  serial  arrangement  of,  247 
degeneration  after  removal  of,  244 
paralysis  following  removal  of,  269 
physiological  characters  of,  243 
subdivision  of,  into  centres,  247 
cen-tres,  degree  of  separateness  of,  248 

of  the  human  cortex,  250 
disturbance  from  hemisection  of  the  cord,  230 
end-plates,  latent  period  of,  103 

transmission  of  excitation  by  means  of,  82 
nerves,  fatigue  of,  96 

rate  of  conduction  in,  89 
Movements  of  joints,  varieties  of,  416 
of  spermatozoa,  444 
of  the  eyeball,  298 
Multiple  conceptions,  482 

control  from  the  cortex,  250 
Muscae  volitantes,  320 
Muscle,  accelerator  urinae,  449 
aryteno-epiglottidean,  426 
arytenoid,  426 
bulbo-cavernosus,  449 
chemistry  of,  159 
ciliary,  in  accommodation,  309 
crico-arytenoid,  lateral,  426 


INDEX  TO    VOLUME  II. 


515 


Muscle,  crico-thyroid,  426 
digastric,  426 
elasticity  of,  105 
erector  clitoridis,  464 
external  rectus,  299 
fatigue  of,  70 

frog's,  rate  of  conduction  in,  89 
gases  of,  168 

general  physiology  of,  17 
geuio-hyoid,  4'_'<i 


independent  irritability  of,  25 
inferior  oblique,  299 

rectus,  •_'!•! i 

inorganic  constituents  of,  168 
internal  rectus,  299 

limitation  of  the  rate  of  stimulation  in,  126 
mylo-hyoid,  426 
nitrogenous  extractives  of,  166 
non-nitrogenous  constituents  of,  168 
omo-hyoid,  425 
posterior  crico-arytenoid,  426 
reaction  of,  159 

red,  capacity  for  tetanic  contraction,  127 
retractor  lentis,  of  fishes,  306 
skeletal,  sensory  nerve-endings  in,  402 
specific  gravity  of,  159 
sterno-hyoid,  425 
sterno-thyroid,  425 
striated,  histology  of,  104 

optical  properties  of,  103 
stylo-hyoid,  426 
superior  oblique,  299 

rectus,  299 
thyro-arytenoid,  426 
external,  424 
internal,  424 
thyro-hyoid,  425 

Muscle-contraction,  Engelmann's  theory  of,  105 
Muscle-plasma,  161,  163 

Muscle-proteids,  precipitation  temperature  of,  166 
Muscles,  absolute  force  of,  141 
action  of,  upon  the  bones,  417 
classification  of,  18 
currents  of  action  in,  150 

of  rest  in,  147 
degeneration  of,  after  section  of  motor  nerves, 

48,  54,  70 
endurance  of,  76 
erectores  penis,  449 
human,  fatigue  of,  with  artificial  stimulation, 

134 

rate  of  conduction  in,  89 
ischio-cavernosi,  449 
of  the  eye,  299 
of  the  iris,  323 
of  the  middle  ear,  369 
rate  of  conduction  in,  89 
stapedius,  370 
tensor  tympani,  369 
Muscle-serum,  166 
Muscle-sounds,  132 
Muscle-spindles,  402 
Muscle-structure,  relation  of,  to  its  contractility, 

107 

Muscle-tonus,  143 

Muscular  contractions,  effect  of  drugs  on,  137 
of  support  on  the  height  of,  122 
of  temperature  on,  136 
graphic  record  of,  99 
influences  affecting,  107 
post-moi". 'in.  160 
source  of  energy  in,  74 
effor:  of  larynx  in,  423 

inhi  i  cortical  stimulation,  224 

itions  of  antagonistic  muscles 
in,  418 


Muscular  sensations,  cortical  area  for,  253 
definition  of,  :;!»o 

effect  of  lieniisect ion  of  the  cord  on,  '.'.;.") 
in  estimation  of  weights,  -In:; 
nature  of,  401 

path  of  conduction  for,  in  the  cord,  235 
psychological  value  of,  391 
work,  effect  of  stimulants  on.  7.~> 
Musical  sounds,  characteristics  of,  387 
tones,  beats  of,  386 
limits  in  the  pitch  of,  382 
production  of,  381 
Mydriatics,  325 
Mylo-hyoid  muscle,  426 
Myo-albumin,  166 
Myo-albumose,  166 
Myogen-fibrin,  164 
Myoglobulin,  166 
Myogram,  34 
definition  of,  100 

effect  of  temperature  on  the  form  of,  136 
Myograph,  35 
description  of,  100 
double,  of  Heriug,  36 
Myohffiinatiu,  166 
Myopia,  313 

range  of  accommodation  in,  314 
Myosin,  163 

ferment,  161,  163 
Myosin-fibrin,  164 
Myosinogen,  163 

temperature  of  heat  coagulation  of,  165 
Myotics,  325 

NAUSEA  from  disturbance  of  equilibrium,  405 
Near-point  of  vision,  312 
Negative  after-images,  346 

variation  in  muscles,  rate  of  propagation,  152 

relation  of,  to  the  contraction,  153 
of  muscle-currents,  150 
of  nerve-currents,  154 

Nerve,  chorda  tympani,  gustatory  function  of, 
410 

general  physiology  of,  17 

glossopharyngeal,   gustatory  function  of,  410 

oculomotor,  323 

recurrent  laryngeal,  428 

superior  laryngeal,  428 
Nerve-cells,  atrophy  of,  from  disuse,  195 

changes  with  age  in,  490 

chemical  changes  in,  191 

classification  of,  177 

connections  between,  206 

degeneration  of  the  cell-bodies  of,  199 

diaxouic,  178 

effect  of  exercise  on,  76 

fatigue  of,  191 

generation  of  impulses  in,  188 

growth  of,  17o' 

histological  changes  due  to  functional  activity 
in,  192 

human,  size  of,  174 

internal  structure  of,  179 

maturation  of.  177 

morphology  of,  17-'! 

number  of,  in  the  central  nervous  system, 283 

nutrition  of,  190 

nutritive  control  of,  over  nerve-fibres,  198 

of  animals,  size  of,  175 

of  spinal  ganglia,  development  of,  178 

peculiar!!  ies  of.  174 

pyramidal.  17- 

rate  of  discharge  from,  189 

regeneration  of,  -J"! 

relation  of  size  and  function  in,  17.~> 

senescence  of.  i^-_>.  nm 

significance  of  the  branches  of,  186 


516 


INDEX  TO   VOLUME  II. 


Nerve-cells,  summation  of  stimuli  in,  190 

volume  relations  of,  175 

Nerve-elements,  primitive  segmental   arrange- 
ment of,  205 

Nerve-endings  in  the  skin,  392 
Nerve-fibres,  classification  of,  21 

cortical,  increase   in   the  number  of,  during 
growth,  282 

fatigue  in,  195 

functions  of,  21 

reaction  of,  170 
Nerve-impulse,  definition  of,  25 

direction  of  the  passage  of,  184 

electrical  variation  accompanying,  183 

generation  of,  187 

in  peripheral  nerves,  183 

peripheral  diffusion  of,  218 

reversed,  in  spinal  ganglion-cells,  185 

theories  of,  97 

transmission  of,  from  neurone  to  neurone,  207 
Nerve-muscle  preparation,  34 
Nerve-trunks,  isolated  conduction  in,  79 
Nerves,  action  currents  in,  153 

auditory,  central  path  of,  237 

chemistry  of,  169 

cross-suturing  of,  200 

current  of  rest  in,  149 

degeneration  of,  after  section,  69,  78 

fatigue  of,  75 

glossopharyngeal,   central    conduction   paths 
for,  236 

in  man,  stimulation  of,  51 

law  of  stimulation  of,  with  galvanic  current,  50 

limitation  of  the  rate  of  stimulation  in,  126 

lingual,  gustatory  function  of,  410 

medullation  of,  180 

non-medullated,  rate  of  conduction  in,  90 

of  common  sensation,  central  conduction  paths 
of,  230 

of   cutaneous  sensation,    central   conduction 
paths  of,  233 

of  dermal  sensation,  area  of  distribution  of, 
231 

of  invertebrates,  rate  of  conduction  in,  91 

of  taste,  nuclei  of  origin  of,  236 

of  temperature,  397 

of  Wrisberg   (intermedius),  central  path  of, 
236 

olfactory,  central  paths  of,  241 

optic,  central  paths  of,  238 

rate  of  conduction  in,  89 

secondary  degeneration  of,  197 

specific  energy  of,  232 

trigeminal,  central  paths  of,  238 

vagus,  course  of  the  afferent  fibres  in,  236 
Nervi  erigentes,  464 
Neuroblast,  development  of,  176 
Neuro-keratin  of  nerves,  169 
Neuromuscular  spindle,  390 
Neurone,  definition  of,  173 
Neurones,  21 

afferent,  to  the  spinal  cord,  203 

changes  in  number  and  size  of,  280 

conduction  in,  97 

connection  between,  206 

double  conduction  in,  185 

increase  in  number  of,  during  growth,  282 

internal  structure  of,  179 

polarity  of,  184 

total  number  of,  283 
Nicotin,  action  of,  on  end -plates,  27 

on  sympathetic  ganglia,  219 
Nissl  method  for  study  of  nerve-cells,  195 

substance,  iron  in,  191 

of  nerve-cells,  179 
Nitrogen,  free,  of  muscles,  168 
Nitrogenous  extractives  of  muscle,  166 


Nodal  point  in  the  simplest  dioptric  system,  301 

Noaud  vital,  236 

Noises,  definition  of,  388 

Non-medullated  nerves,  rate  of  conduction  in.  90 

stimulation  fatigue  of,  180 
Non-polarizable  electrodes,  29 
Nose,  anatomy  of,  408 

respiratory  tract  of,  408 
Nutrition  of  nerve-cells,  190 

of  the  embryo,  475 
Nutritive    control    of   nerve-cell    bodies    over 

nerve-fibres,  198 
Nymphse,  462 
Nystagmus  after  cerebellar  injury,  272 

OCULOMOTOR  nerve,  ciliary  fibres  of,  311 

relation  of,  to  the  iris,  323 
Odors,  410 

(Edema  of  the  glottis,  422 
Old  age  of  the  central  nervous  system,  295 
Olfactory  area  of  the  cortex,  253 

cells,  408 

epithelium,  408 

nerves,  central  paths  of,  241 

paths  to  the  brain,  409 

stimuli,  conditions  affecting,  409 

tracts,  section  of,  in  sharks,  264 
Omo-hyoid  muscle,  425 

Ontogenetic  development  of  nerve-cells,  177 
Onychodromus,  reproduction  of,  442 
Oocyte,  451 
Ophthalmometer,  304 
Ophthalmoscope,  326 

Optic  commissure,  decussatiou  of  optic  fibres  in, 
238 

nerve,  currents  of  action  in,  154 

nerve-fibres,  number  of,  330 

nerves,  central  paths  of,  238 
cortical  centres  of,  240 
efferent  fibres  of,  240 

thalami,  functions  of,  271 
Optical  constants  of  the  eye,  303 

illusions  in  binocular  vision,  359 
of  space  perceptions,  351 

properties  of  striated  muscle,   103 
Optograms,  330 
Organ  of  Corti,  structure  of,  377 

of  Golgi  in  tendons,  402 
Organization  in  the  central  nervous  system,  285 

relation  of  educability  to  the  establishment 

of,  286 

Organs,  growth  of,  486 
Oscillatory  activity  of  the  retina,  344 
Os  orbiculare  of  the  incus,  366 
Ossicles,  auditory,  365 

of  the  ear,  movements  of,  367 
Otitis  media,  364 
Otoconia,  374 
Otoliths,  374 
Ova,  440 

number  of,  in  human  ovary,  451 
Ovaries,  443 

effect  of  removal  of,  on  menstruation,  459 

structure  of,  454 
Overtones,  definition  of,  383 

inharmonic,  386 
Oviducts,  443,  456 
Ovulation,  455 
Ovum,  chemistry  of,  450 

fertilization  of,  466 

human,  structure  of,  449 

maturation  of,  451 

physiological  properties  of,  22 

segmentation  of,  467 

stages  in  the  maturation  of,  452 
Oxygen,  storage  of,  in  muscle,  169 

supply,  relation  of,  to  irritability,  68 


INDEX  TO    VOLUME  //. 


517 


PACINIAN  body,  391 
of  the  penis,  449 
Pain  nerves,  evidence  for  the  existence  of,  232 

points  of  the  skin,  400 

sensations  of,  399 

transferred  or  sympathetic,  400 
Pale  striped  musele,  physiological  peculiarities 

of,  109 

Pangenesis,  Darwin's  theory  of,  501 
Papilla  fpliata  of  rabbits,  410 
Paradoxical  contraction,  157 
Parallax,  use,  of,  in  estimation  of  distance,  356 
Paralysis  after  removal  of  motor  areas,  269 

agitans,  296 

Hroxvu-Sequard's,  233 

homolateral,  after  hemisection  of  the  cord, 

233 

Paramo3cium,  reproduction  in,  440 
Paramyosinogen,  163 

temperature  of  heat  coagulation  of,  165 
Paresis  following  removal  of  the  cerebellum,  272 

from  inj  ury  to  motor  areas,  269 
Partial  tones,  definition  of,  383 
Parturition,  479 

spinal  centre  of,  481 

Paths  of  conduction  in  the  cord,  clinical  evi- 
dence on,  234 
Pendular  vibrations,  381 
Penis,  443 

structure  of,  448 
Perilymph,  372 
Periodic  reflexes,  216 
Peripheral  nerves,  medullation  of,  180 

reference  of  special  sensations,  400 
Pfliiger's  law  of  contraction,  50 
Phakoscope,  308 

Phalangar  process  of  the  rods  of  Corti,  378 
Phosphenes,  pressure,  305,  331 
Photometry,  345 

Phrenic  nerve,  currents  of  action  in,  154 
Phylogenetic  development  of  nerve-cells,  177 
Physiological  anode,  definition  of,  52 

kathode,  definition  of,  52 

observations  on  afferent  paths  in  the  cord,  229 

rheoscope,  148,  151 

salt  solution,  59 
Physostigmin,    action   of,  on    accommodation, 

311 

on  the  eye,  325 
Pia  mater,  weight  of,  274 
Pigment  epithelium,  retinal,  movements  of,  330 

retinal,  relation  of,  to  adaptation  of  the  eye, 

326 

Pince  myographique,  87 
Pineal  gland,  calcification  of,  in  old  age,  491 
Pinna  of  the  ear,  362 
Pitch,  limits  of  perception  of,  382 

of  musical  tones,  381 

of  the  voice,  430,  432 
Pituitary  membrane,  408 
Pivot-joint,  417 
Placenta,  474 

Placenta!  transmission   of    infectious  diseases, 
498 

villi,  474 

Plants,  regeneration  of  lost  parts  in,  496 
Pohl's  mercury  commutator,  36 
Polar  amphiaster  of  the  ripening  egg,  453 

bodies,  451 

of  the  ovum,  453 
Polarity  of  neurones,  184 
Polarization,  after-effects  of,  65 

Shysiological,  of  neuroblasts.  176 
arizing  current,  effect  of,  on  conductivity,  50 
on  muscles.  *!! 
on  nerves,  ir.' 
Pole-changers,  36 


Polyspermy,  471 
Pomum  A 'lam  i.  l •_' ". 
Positive  after-images,  :;n; 
Posterior  association-centre,  L'.")? 
Post-gangliouic  fibres  of  the  sympathetic  sys- 
tem, 219 

Posture  sense,  399 
Potassium  salts,  action  of,  on  muscles,  138 

relation  of,  to  irritability,  59 
Preformation  theory  of  heredity,  500 
Pre-ganglionic  fibres  of  the  sympathetic  system, 

Pregnancy,  effects  of,  on  the  mother,  477 

Presbyopia,  314 

Pressure,  effect  of,  on  irritability  of  nerves,  56 

influence  of,  on  conductivity,  92 
Pressure-points  of  the  skin,  396 
Pressure-sensations,  fusion  of,  394 
Pressure-sense,  delicacy  of,  392 

of  the  tympanic  membrane,  382 
Primary  position  of  the  eye,  299 

taste-sensations,  412 
Principal  foci  in  a  dioptric  system,  302 

point  of  the  simplest  dioptric  system,  301 

ray  in  the  simplest  dioptric  system,  301 
Processus  brevis  of  the  malleus,  365 

gracilis  sive  folianus  of  the  malleus,  365 
Projection  system  of  fibres,  origin  of,  from  cen- 
tral cells,  205 
Pronucleus,  female,  453 

male,  466 

Proptosis  after  cerebellar  injury,  272 
Prostate  glands,  443 
histology  of,  448 
secretion  of,  446 
Prostatic  fluid.  446,  448 
Protagon  of  medullary  substance,  170 
Proteids  of  muscle,  precipitation  temperature 
of,  166 

of  muscle-serum,  166 

relation  of,  to  muscular  work,  74 
Pseudoscopic  vision,  318,  357 
Psychical  powers  of  the  spinal  cord,  215 
Psycho-physic  law,  340 

of  Fechner,  393 
Puberty,  489 

Pupil,  changes  during  accommodation  in,  311 
in  size  of,  323 

condition  of,  in  sleep,  325 

dilator  nerves,  324 

size  of,  in  old  age,  314 
Pupillary  reflex  to  light,  323 
Purkinje-Sanson's  images,  307 
Purkinje's  figure,  321 

phenomenon,  340 

explanation  of,  342 
Purposeful  reflexes,  215 
Pyramidal  fibres,  number  of.  246 

tracts  in  the  cord,  245 
geminal  fibres  of,  245 
size  of,  252 

nerve-cells,  development  of,  178 

QUADKUPLETS,  483 

Quality  of  musical  tones,  383 

of  the  voice,  430 

Quinine,  action  of,  on  coagulation   of   muscle- 
plasma,  164 
Quintuplets,  483 

RACE,  relation  of  brain-weight  to,  278 
Range  of  accommodation,  normal,  312 
Rate  of  conduction  in  muscles,  87 

in  nerves,  89 
of   excitation,  effect   of,  on   the   contraction 

curve.  111 
of  stimulation  in  voluntary  contractions,  133 


518 


INDEX  TO    VOLUME  II. 


Rate  of  stimulation,  limitations  of,  for  muscle, 

126 

required  to  tetanize,  125 
Eeaction  of  degeneration,  48,  54,  70 

of  muscles,  159 

of  nerve-cells,  191 

of  nerve-fibres,  170 

time,  291 

in  old  age,  491 
Eecurrent  laryngeal  nerves,  428 

sensibility  of  the  anterior  roots,  204 
Eed-striped  muscles,  physiological  properties  of, 

109 

Eeduced  eye,  304 
Eeflex  actions,  simple,  208 

arc,  209 

frog,  209 

segmental  reaction,  210 

stinmlation  of  the  nervous  system,  208 

tonus  of  muscular  tissues,  220 
Eeflexes,  co-ordinated,  211 

co-ordination  of  the  efferent  impulses  in,  214 

effect  of  location  of  stimulus  on,  209 
of  strength  of  stimulus  on,  210 

from  the  isolated  cord  in  man,  213 
lumbar  cord,  213 

in  different  vertebrates,  212 

in  man,  216 

latent  period  of,  211 

of  a  purposeful  character,  215 

periodic,  216 

simple,  208 

spinal,  212 

reinforcement  of,  222 

summation  of  stimuli  in,  211 

voluntary  control  of,  214 
Eefractive  index  of  the  aqueous  humor,  303 
of  the  lens,  303 
of  the  vitreous  humor,  303 

media  of  the  eye,  302 

surfaces  of  the  eye,  303 
"Eefractory  period"  of  nerves,  57,  66 
Eegeneration  of  lost  parts,  496 

of  nerves,  78,  199 
Eegisters  of  the  voice,  432 
Eegular  astigmatism,  317 
Eeinforcement  of  reflexes,  222 

of  the  knee-kick,  222 
Eeissner,  membrane  of,  374 
Eejuvenescence  by  sexual  reproduction,  442 
Eelaxation  of  muscle,  nature  of,  99 
Eeproduction,  asexual,  439 

of  leucophrys  patula,  442 

of  onychodromus,  442 

of  stylonychia,  442 

sexual,  440 

elements  of,  440 

theory  of,  441 

Eeproductive  organs,  female,  449 
internal  secretions  of,  462 
male,  443 

process,  463 

Eesonance  of  the  ear,  388 
Eesonants,  436 

Eesonators,  analysis  of  sounds  by,  385 
Eete  vasculosum  of  the  testis,  447 
Eetina,  changes  produced  by  light  in,  330 

circulation  in,  322 

histology  of,  329 

oscillatory  activity  of,  344 

space  perceptions  by,  348 

structure  of,  327 
Eetinal  currents,  331 

images,  inversion  of,  305 
size  of,  305 

stimulation,  after-effect  of,  345 
fatigue  in,  344 


Eetinal  stimulation,  latent  period  of,  343 
laws  of,  343 

rise  to  maximum  for  different  colors,  343 
vessels,  demonstration  of,  321 
Eeversion  to  ancestral  characters,  495 
Eheocord,  41 
Eheonome,  31 

Eheoscope,  physiological,  148 
Eheoscopic  frog,  148 
Eheostat,  40 
Ehinencephalon,  241 
Eigor  caloris,  57,  164 
contracture  in,  128 
effect  of  fatigue  on,  165 
mortis,  159 

chemical  changes  accompanying,  162 
contracture  of,  128 
disappearance  of,  162 
influence  of  the  nervous  system  on,  220 
nature  of  changes  in,  161 
Eima  glottidis,  423 
respiratoria,  423 
vocalis,  423 
Eitter's  opening  tetanus,  37,  61 

tetanus,  131 

Eod-and-cone  layer,  function  of,  327 
Eod-pigment.  339.    See  also  Visual  purple. 
Eods  and  cones,  function  of,  341 

number  of,  330 
of  Corti,  377 
retinal,  function  of,  341 
Eotation,  movements  of,  416 
Eunning,  421 
Eut  of  animals,  460 

SACCULUS  of  the  internal  ear,  373 

Saccus  endolymphaticus,  373 

Saddle-joint,  416 

Salivary  secretion,  cerebral  control  of.  270 

Salts,  inorganic,  relation  of,  to  irritability,  58 

of  muscle,  168 
of   heavy  metals,   action   of,   on   nerve  and 

muscle,  60 

Santorini,  cartilage  of,  422,  425 
Sarcode  of  sponges,  contractility  of,  20 
Sarcolactic  acid,  formation   of,  in  rigor  mortis, 

161,  162 

in  clotting  of  muscle-plasma,  164 
relation  of,  to  fatigue  contracture,  131 
Sarcoplasm,  104 
Saturation  of  colors,  342 
Scala  media,  375 
tympani,  372,  375 
vestibuli,  372,  375 
Schneiderian  membrane,  408 
Scrotum,  443 

Secondary  degeneration  of  nerves,  197 
position  of  the  eye,  299 
tetanus,  150 

Secretion,  salivary,  cerebral  control  of,  270 
Secretory  nerves,  fatigue  of,  96 
Segmental  arrangement  of  nerve-elements,  206 

reactions,  reflex,  210 
Segmentation,  467 
Segmentation -centrosomes,  469 
Segmentation-nucleus,  466 
Semen,  composition  of,  445 
Semicircular  canals,  membranous,  373 
of  the  bony  labyrinth,  371 
relation  of,  to  equilibrium,  405 
section  of,  405 
Seminal  vesicles,  443 
function  of,  448 
secretion  of,  446 
Semi-vowels,  436 
Senescence  of  nerve-cells,  182 

of  the  central  nervous  system,  295 


INDEX  TO    VOLUME  II. 


519 


Senescenee,  phenomena  of,  486 
Sensation,  cutaneous,  definition  of,  390 
imiMMilar,  definition  of,  390 
of  after- pressure,  :'>!'! 
of  light,  mechanism  for   the   production   of, 

331 

of  temperature.  .">!'? 
Sense  of  equilibrium,  404 

of  touch,  392 
Sensory  areas  of  the   cortex,  determination  of, 

253 
conducting  paths  in  the  spinal  cord,  234 

continuation  of,  in  the  brain,  235 
cortical  areas  in  man,  255 

motor  responses  from.  •_'."».'! 
relative  functional  importance  of,  270 
disturbance  from  hemisection  of  the  cord,  230 
impulses,  path  of,  in  the  central  nervous  sys- 
tem, 2-,'U 
relation  of,  to  the  maintenance  of  the  erect 

posture,  419 

nerve-endings  in  skeletal  muscle,  402 
in  tendon,  402 
in  the  skin,  391 

nerves,  rate  of  conduction  in,  91 
paths,  degeneration  of,  after  section  of  the 

dorsal  roots,  227 

in  the  central  nervous  system,  226 
regions  of  the  cortex  cerebri,  252 
stimulation,  relation  of,  to  sleep,  291 
Sex,  characters  of,  442 

of  offspring,  determination  of,  483 
origin  of,  441 

relation  of  brain-weight  to,  276 
Sexual  characters,  442 
glands,  accessory,  445 
organs,  443 
reproduction,  440 

congenital  variations  resulting  from,  501 
theory  of  origin  of,  441 
Shark,  reflexes  in,  212 

removal  of  cerebral  hemispheres  in,  263 
Shrapnell's  membrane,  365 
Siamese  twins,  483 
Simple  muscular  contraction,  duration  of,  102, 

108 

explanation  of,  101 
Simultaneous  contrast,  347 
Singing  voice,  434 
Size,  increase  of  the  embryo  in,  487 

of  nerve-cells,  175 
Skiascopy,  detection  of  astigmatism  by  means 

of,  319 

Skin,  tactile  areas  of,  395 
Sleep,  291 
cause  of,  292 

condition  of  the  pupils  in,  325 
curve  of  intensity  of,  294 
effects  of  loss  of,  295 
responsiveness  to  stimuli  in,  293 
Smell,  408 

comparative  physiology  of,  409 
subjective  sensations  of,  410 
Smooth  muscle,  rate  of  conduction  in,  89 
Snails,  regeneration  of  lost  parts  in,  496 
Somatic  death,  491 
Somatoplasm,  definition  of,  496 
Somatopleure,  472 
Sound,  physical,  381 
Sounds,  quality  of.  383 
Sound-waves,  amplitude  of,  381 
composite,  384 
limits  of  perception  of,  382 
production  of,  381 
Space  illusions,  354 

Space-perception  from  visual  sensations,  347 
Specific  energies  of  nerves,  doctrine  of,  232,  399 


Specific  energy  of  the  optic  nerve.  :;:;i 
gravity  of  muscle,  !.">!) 
of  the  eneephalon,  -JT.^ 

of  the  nervous  system  at  different  ages,  284 
nerve-energy,  doctrine  of.  _>:;_',  :;«i!» 
Spectral  colors,  incomplete  saturation  of,  848 
Spectrum,  3:;-j 

luminous  intensity  of  the  colors  of,  340 
top,  Benham's,  344 
Speech,  dependence  of,  on  hearing,  431 

elements  of,  433 
Speech-centre,  257 
Sperm  atids,  445 
Spermatocytes,  445 
Spermatozoa,  440 
contractility  of,  20 
discovery  of,  443,  498 
entrance  of,  into  the  uterus,  465 
locomotion  of,  465 
maturation  of,  445 
movements  of,  444 
structure  of,  443 
Sperm-aster,  467 
Sperm-nucleus,  466 
Spermiue,  445 
Spherical  aberration,  315 
Sphincter  aui,  cerebral  control  of,  270 
iridis,  325 

urethrtE,  contraction  of,  in  erection,  464 
Spinal  cord,  afferent  paths  of,  229 
central  neurones  of,  203 
degeneration  of,  from  hemisection,  228 
efferent  neurones  of,  203 
motor  tracts  of,  245 
reflexes  in  man,  after  section  of,  213 
schematic  cross-section  of,  202 
weight  of,  274 

ganglion-cells,  development  of,  178 
nerve-roots,  section  of,  198 
Spiral  ganglion  of  the  ear,  376 
ligament  of  the  cochlea,  379 
Staircase  contractions,  66,  112 
relation  of,  to  tetanus,  124 
Standing,  418 
Stapedius  muscle,  370 
Stapes,  367 

Starvation,  effect  of,  on  the  nervous  system,  289 
Static  equilibrium,  organs  of,  407 
Stature,  relation  of  brain -weight  to,  276 
Stenson's  experiment,  67 
Stereoscope,  356 
Sterno-hyoid  muscle,  425 
Sterno-thyroid  muscle,  425 
Stimulants,  effect  of,  on  muscular  work,  75 
Stimulation  fatigue  of  non-medullated  nerves, 

180 

of  the  cortex,  241 
Stimuli,  chemical,  of  muscle,  131 
classification  of,  23 

conditions  determining  efficiency  of,  28 
effect  of  changing  intensity  of,  32 
of  their  repetition  on  irritability,  65 
of  varying  strength  of,  39 
galvanic,  con tractu re  effect  of,  in  muscles,  131 
variations  in  intensity  of,  31 
Strabismus,  300 
Striae  acusticse,  237 
gravidarum,  477 
Strychnin,  action  of,  on  diffusion  of  impulses  in 

the  cord,  217 
on  end-plates.  27 
on  sympathetic  ganglia,  219 
tetanus,  217 
Stylo-hyoid  muscle,  426 
Stylonychia,  reproduction  of,  442 
Successive  contrast,  346 
Sugar  of  muscles,  167 


520 


INDEX  TO    VOLUME  II. 


Sugar,  use  of,  in  muscular  work,  74 
Summation  of  contractions  in  muscle,  121 

of  stimuli  in  nerve-cells,  190 

in  reflex  action,  211 
Superior  laryugeal  nerve,  428 

oblique  muscle,  299 

rectus  muscle,  299 
Supernumerary  digits,  494 
Sustentacular  cells  of  the  crista  acustica,  374 
Suture  of  skull-bones,  414 
Swallowing,  action  of  the  epiglottis  in,  422 
Sylvian  heat-centre,  271 

Sympathetic  ganglia,  action  of  nicotin  on,  219 
of  strychnin  on,  219 

nerves  to  the  iris,  324 

pains,  400 

system,  connection  of,  with  the  cerebro-spinal, 

218 

post-ganglionic  fibres  of,  219 
pre-ganglionic  fibres  of,  218 

vibration,  385 
Symphysis,  414 
Syndesmosis,  414 
Synoyial  fluid,  415 
Syphilis,  hereditary  transmission  of,  498 

TACTILE  areas  of  the  skin,  395 

corpuscle,  390,  392 
Taste,  nerves  of,  410 

organs  of,  410 
Taste-buds,  410 

Taste-nerves,  nuclei  of  origin  of,  236 
Taste-perceptions,  conditions  affecting,  411 
Taste-sensations,  conditions  which  influence,  411 

primary,  412 

distribution  of,  413 
Taurin  in  muscles,  167 
Tea,  stimulating  action  of,  75 
Tectorial  membrane,  379 
Tegmen  of  the  tympanum,  364 
Temperature,  effect  of,  on  muscular  contraction, 
136 

influence  of,  on  conductivity,  92 
on  irritability,  56 
on  rigor  mortis,  161 

limits  of  muscular  contraction,  136 

nerves,  397 

of  the  blood  from  the  brain,  288 

rise  of,  from  lesions  of  corpora  striata,  271 

sense,  397 

spots  of  the  skin,  398 
Tendon  reflexes  after  cerebellar  injury,  272 

sensory  nerve-endings  in,  402 
Tension,  effect  of,  on  contraction  curve,  109 
on  irritability  of  nerves  and  muscles,  56 
on  latent  period,  110 
Tensor  tympani  muscle,  369 
Tentacles  of  Actiniae,  contractility  of,  20 
Terminal  arborizations,  definition  of,  174 
Tertiary  positions  of  the  eye,  299 
Testis,  443  / 

ducts  of,  447  -1 

histology  of,  446 
Tetanic  contractions,  height  of,  120 

relative  intensity  of,  126 
Tetanomotor,  56 
Tetanus,  66 

analysis  of,  123 

complete,  120 

curves,  introductory  peaks  of,  124 

explanation  of,  121 

strychnin  poisoning,  217 

incomplete,  117 

normal  physiological,  132 

of  the  muscles,  127 

rate  of  stimulation  required  for,  125 

Bitter's,  37,  61,  131 


Tetanus,  secondary,  150 

voluntary,  133 

Wundt's,  37 
closing,  61 
Thalamus,  cortical  connections  of,  271 

heat-centre  of,  271 
Thermal  energy  liberated  in  muscle,  141 

stimulation  of  nerve,  25 
Thermopile,  142 

Thermotaxis,  relation  of  cerebrum  to,  270 
Thirst,  404 

Thyro-arytenoid  muscles,  424,  426 
Thyro-hyoid  muscle,  425 
Thyroid  cartilage,  425 

gland,  relation  of,  to  growth  of  the  central 

nervous  system,  289 
Tigroid  of  nerve-cells,  179 
Timbre  of  musical  tones,  383,  387 
Time  intervals,  perception  of,  by  the  ear,  388 
Tissue  death,  492 
Tissues,  growth  of,  486 
Tobacco  smoke,  action  of,  on  nerves,  60 
Tones,  combinational,  387 

differential,  387 

fundamental,  383 

loudness  of,  381 

pitch  of,  381 

simple,  381 

Tongue,  distribution  of  taste-sensations  on,  413 
Tonus,  muscular,  in  the  insane,  220 
reflex  origin  of,  220 

of  muscles,  143 
Touch  illusions,  396 

sensations,  392 

localization  of,  394 
Tractus  solitarius,  236 
Tremors,  132 

Trigeminal  nerves,  central  paths  of,  238 
Triplets,  483 
Trophic  impulses  to  muscles,  70 

influence  of  neurones  on  one  another,  197 

nerves  of  the  muscles,  70 
Tubuli  recti  of  the  testis,  447 
Turtle's  striped  muscle,  time  of  contraction  in, 

108 

Twins,  482 
Tympanic  membrane,  364 

effect  of  destruction  of,  370 
pressure-sensations  of,  382 
vibrations  of,  370 
Tympanum,  363 

mechanics  of,  368 

ULTIMUM  moriens,  492 
Umbilical  arteries,  474 

vein,  474 

Umbo  of  the  tympanic  membrane,  365 
Unconsciousness,  293 
Unipolar  excitation  for  localized  excitation,  45 

nerve-cells,  development  of,  178 

stimulation,  30 

principles  of,  43 
Urea  in  muscles,  167 
Urethra,  443 

structure  of,  448 
Uric  acid  in  muscles,  167 
Uterus,  443,  456 
Utriculus  of  the  internal  ear,  373 

VAGINA,  443,  462 

Vagus,  central  path  of  the  afferent  fibres  in, 

236 
nerve,  fatigue  of,  96 

rate  of  conduction  in,  90 

Variation  of  the  offspring  in  reproduction,  500 
Variations,  somatic,  classification  of,  497 
Vas  deferens,  447 


INDEX  TO    VOLUME  II. 


521 


Vasa-deferentia,  443 

efferentia  of  the  testis,  447 
Vaso-motor  nerves  of  the  cranial  vessels,  286 
Ventral  nerve-roots,  number  of  fibres  of,  230 
Ventricles  of  Morgagni,  4VJ^ 

of  the  brain,  capacity  of,  274 
Ventricular  bauds,  422 

Veratria,  action  of,  on   coagulation  of  muscle- 
plasma,  164 

on  muscular  contraction,  129,  137 
on  nerves  and  muscles,  60 
effect  of,  on  muscular  contraction,  128 
Vertigo  in  diseases  of  the  ear  labyrinth,  406 
Vestibular  root  of  the  auditory  nerve,  central 

path  of,  237 

Vestibule  of  the  bony  labyrinth,  371 
Vibrations  of  the  tympanic  membrane,  370 

transmission  of,  through  the  labyrinth,  376 
Vision,  binocular,  356 
far-point  of,  312 
indirect,  341 
near-point  of,  312 
pseudoscopic,  357 
stereoscopic,  357 
Visual  area  of  the  cortex,  253 
impulses,  place  of  origin  of,  in  the  retina, 

327 

judgments  of  distance,  348 
of  size,  350 

and  distance,  354 
purple,  330 

adaptation  of  the  eye  by,  326 
sensation,  intensity  of,  339 
Vitelliue  membrane,  absence  of,  in  human  ova, 

450 
Vitreous  humor,  opacities  in,  321 

refractive  index  of,  303 
Vocal  cords,  false,  422 

true,  423 
Voice,  430 

changes  at  puberty  in,  489 
effect  of  age  on,  431 
pitch  of,  432 
registers  of,  432 
Voice-production,  421 
mechanism  of,  431 
Voices,  classification  of,  433 
Volta,  28 
Voltaic  pile,  28 


Voluntary  muscular  contractions,  fatigue  of.  1.11 

tetanic  character  of.  i:;:; 
rein-lions,  all'erent  paths  of. 
anatomical  mechanism  of,  226 
compared  with  reflex,  225 
von  Gudden's  commissure,  238 
Vorticella,  movements  of,  20 
Vowel-sounds,  431 

differences  in  quality  of,  385 
production  of,  434 
Vulva,  443,  462 

WALKING,  420 

Walleriau  degeneration,  changes  of  excitability 

in,  69 

of  nerve-fibres,  197 
of  nerves,  69 

Water,  percentage  of,  in  brain  and  cord,  274 
pure,  toxic  action  of,  on  nerves  and  muscles. 

58 
Weber's  law,  340 

applied  to  pressure-sensations,  393 
Weight,  increase  of,  in  the  embryo,  487 
of  the  brain  and  spinal  cord,  274 
decrease  of,  in  old  age,  296 
relation  of,  to  social  environment,  277 
of  the  child  at  birth,  487 
Weissmann's  theory  of  heredity,  502 
Whispering,  436 

Whistling  register  of  the  voice,  433 
White  matter  of  the  central  nervous  system, 

water  contents  of,  274 
Womb,  443,  456 
Work  done  by  contracting  muscles,  conditions 

affecting,  139 

by  muscular  contraction,  curve  of,  140 
Worms,  segmental  nervous  system  of,  212 
Wrisberg,  cartilages  of,  422,  425 
Wundt's  closing  tetanus,  37,  61,  131 

XANTHIN  of  muscles,  167 
YOUNG-HELMHOLTZ  theory  of  color  vision,  335 

ZOLLNER'S  lines,  351 
Zona  pellucida,  454 

of  the  ovum,  450 
radiata  of  the  ovum,  449,  450 


GENERAL  INDEX. 


ABDOMINAL  muscles,  action  of,  in  vomiting,  i. 

387 
n >piratory  action  of,  i.  407 

respiration,  definition  of,  i.  398 
Absolute  muscular  force,  ii.  141 
Absorbents,  i.  318 
Absorption,  effect  of  alcohol  on,  i.  535 

in  the  small  intestine,  i.  313 

in  the  stomach,  i.  312 

mechanism  of,  i.  312 

nature  of  process,  i.  27 

of  fats,  i.  317 

of  gases  by  liquids,  i.  414 

of  proteids,  i.  316 

of  sugars,  i.  317 

of  water  and  salts,  i.  318 

part  played  by  leucocytes  in,  i.  48 

paths  of,  i.  311 

spectrum  of  oxy haemoglobin,  i.  41 
Accelerator  centre,  cardiac,  i.  177 
respiratory,  i.  457 

nerves  of  the  heart,  i.  167,  169 

urinae  muscle,  ii.  449 
Accessory  articles  of  the  diet,  i.  357 

thyroids,  i.  268 
Accommodation,  ii.  306 

associated  movements  of,  ii.  311 

astigmatic,  ii.  310 

dissociation  of,  from  convergence,  ii.  312 

influence  of  drugs  on,  ii.  311 

in  old  age,  ii.  314 

mechanism  of,  ii.  309 

nervous  mechanism  of,  ii.  311 

normal  range  of,  ii.  312 

range  of,  in  hypermetropia,  ii.  314 
in  myopia,  ii.  314 

relation  of,  to  perception  of  distance,  ii.  356 

voluntary  character  of,  ii.  311 
Acetic  acid,  i.  536 

Acetone,  relation  of,  to  fat  metabolism,  i.  539 
Acetonitril,  i.  542 
Acetonuria,  i.  537 
Acetyl-acetic  acid,  i.  537 
Acetyl-propionic  acid,  i.  538 
Achromatic  lenses,  ii.  316 
Achromatism  of  the  eye,  ii.  316 
Achroodextrin,  i.  2a5,  566 
Acid,  acetic,  i.  536 

acetyl-acetic,  i.  537 

acetyl-propionic,  i.  538 

amido-acetic,  i.  537 

amido-ethyl-sulphonic,  i.  543 

a-aiuido-a-thiopropionic,  i.  546 

aspartic,  i.  557 

benzoic,  i.  569 

butyric,  i.  539 

capric.  i.  541 

caproic,  i.  540 

caprylir,  i.  541 

carbamic,  i.  548 

carbolic,  i.  569 

carbonic,  chemical  structure  of,  i.  545 

choleic,  i.  543 

cholic,  i.  543 


Acid,  chondroitic.  i.  578 

cynurenic,  i.  571 

diamido-acetic,  i.  551 

o-«-diamido-caproic,  i.  552 

diamido-valeric,  i.  552 

dithio-diamido-ethideue  lactic,  i.  547 

fellic,  i.  543 

formic,  i.  534 

glutamic,  i.  558 

glycerin  phosphoric,  i.  559 

glycuronic,  i.  567 

hippuric,  i.  339,  569 

homogentisic,  i.  570 

hydriodic,  i.  509 

hydrobromic,  i.  509 

hydrochloric,  i.  507 

hydrocumaric,  i.  570 

hydrofluoric,  i.  510 

iso-butyl  amido-acetic,  i.  540 

iso-valerianic,  i.  539 

lactic,  i.  545 

levulic,  i.  538 

malic,  i.  558 

mercapturic,  i.  547 

metaphosphoric,  i.  514 

methyl  amido-acetic,  i.  538 

monobasic  fatty,  i.  532 

nucleic,  i.  579 

oleic,  i.  560 

orthophosphoric,  i.  514 

oxalic,  i.  557 

oxaluric,  i.  555 

oxybutyric,  i.  548 

oxyphenyl-acetic,  i.  570 

oxyphenyl-amido-propionic,  i.  570 

palmitic,  i.  541 

parabanic,  i.  555 

phenaceturic,  i.  569 

phenyl-acetic,  i.  569 

propionic,  i.  538 

salts  of  muscle,  ii.  168 

sarco-lactic,  i.  546 

silicic,  i.  519 

stearic,  i.  541 

succinic,  i.  557 

sulphuric,  i.  506 

sulphurous,  i.  506 

thiolactic,  i.  547 

thymic,  i.  579 

uric,  i.  322,  338,  554,  557 
Acidity  of  worked  muscles,  ii.  168 
Acids,  action  of,  on  nerves  and  muscles,  ii.  60 

effect  of,  on  pancreas,  i.  236 
Acinus,  definition  of,  i.  212 
Acquired  characters,  inheritance  of,  ii.  496 

variations,  ii.  500 
Acromegaly,  i.  273 

Actinic  rays  of  the  luminiferous  ether,  ii.  331 
Action  current,  diphasic,  ii.  152 
in  the  heart,  ii.  l.Vj 
in  the  muscles,  ii.  150 
in  the  nerves,  ii.  153,  183 
Adamkiewicz  reaction  for  proteids,  i.  576 
Adam's  apple,  ii.  }-'."> 

523 


524 


GENERAL  INDEX. 


Addison's  disease,  i.  271 
Adenin,  i.  339,  554 
Adipocere,  i.  541,  560 

Adrenal  bodies,  internal  secretion  of,  i.  272 
removal  of,  i.  271 
secretory  nerves  of,  i.  272 

extract,  action  of,  on  muscles,  ii.  }38 

physiological  action  of,  i.  271 
Aerial  perspective,  ii.  355 

Afferent  impulses,  effect  of,  on  irritability  of  the 
central  nervous  system,  ii.  223 

neurones  of  the  spinal  cord,  ii.  203 

paths  in  the  cord  traced  electrically,  ii.  230 
traced  histologically,  ii.  229 
traced  physiologically,  ii.  229 

respiratory  nerves,  i.  460 
After-birth,  ii.  481 

After-effect  of  retinal  stimulation,  ii.  345 
After-images,  ii.  346 
After-loading  of  muscles,  ii.  110 
After-pressure,  sensation  of,  ii.  394 
Agamogenesis,  ii.  439 

Age,  changes  in  organization  of  the  central  ner- 
vous system  with,  ii.  284 

influence  of,  on  heat  production,  i.  482 
on  nerve-cells,  ii.  490 
on  pulse-rate,  i.  121 
on  respiration,  i.  425 
on  visual  accommodation,  ii.  314 

relation  of  body-temperature  to,  i.  469 
of  brain-weights  to,  ii.  276 
of  menstruation  to,  ii.  459 

specific  gravity  of  the  nervous  system  with 

changes  in,  ii.  284 
Air,  alveolar,  composition  of,  i.  413 

atmospheric,  composition  of,  i.  410,  413 

complemental,  i.  427 

expired,  composition  of,  i.  410 

inspired,  composition  of,  i.  410 

in  the  lungs,  renewal  of,  i.  413 

passages,  obstruction  of,  i.  452 

residual,  i.  427 

respiratory  changes  in,  i.  410 

stationary,  i.  427 

suction  of,  into  veins,  i.  97 

supplemental,  i.  427 

tidal,  volume  of,  i.  426 

variations  in  the  composition  of,  i.  435 
Albinos,  condition  of  the  internal  ear  in,  ii.  407 
Albuminates,  i.  577 
Albuminoids,  digestion  of,  in  the  stomach,  i.  297 

enumeration  of,  i.  577 

nutritive  value  of,  i.  277,  349 

properties  of,  i.  579 

protection  of  proteids  by,  i.  349 

tryptic  digestion  of,  i.  304 
Albuminous  glands,  i.  216 
Albumins,  properties  of,  i.  577 
Albumose  injections,  effect  of,  on  blood-coagu- 
lation, i.  62 
Alcaptonuria,  i.  570 
Alcohol,  absorption  of,  in  the  stomach,  i.  313 

action  of,  on  conductivity  of  nerves,  ii.  93 

amyl,  i.  539 

cerotyl,  i.  540 

cetyl,  i.  540 

effect  of,  on  nerve-currents,  ii.  156 

ethyl,  i.  535 

fumes,  action  of,  on  nerves,  ii.  60 

melicyl,  i.  540 

nutritive  value  of,  i.  358 

physiological  action  of,  i.  357,  535 

propyl,  i.  536,  538 

stimulating  action  of,  ii.  75 

toxic  effects  of,  i.  359 
Alcoholic  fermentation,  i.  535 
Alcohols,  monatomic,  i.  531 


Aldehydes,  general  properties  of,  i.  534 

Aldoses,  i.  561 

Alimentary  canal,  movements  of,  i.  369 

principles,  i.  276 

Alkalies,  action  of,  on  nerves  and  muscles,  ii.  60 
Allantoic  arteries,  ii.  474 

vein,  ii.  474 
Allantoin,  i.  555 
Allantois,  ii.  474 
Allochiria,  ii.  400 
Alloxuric  bases,  i.  338,  339,  552 
Altitude,  effect  of,  on  the  number  of  red  cor- 
puscles, i.  46 
Alveolar  air,  compostion  of,  i.  413 

capacity,  i.  427 

tension  of  carbon-dioxide,  i.  413 

of  oxygen,  i.  413 

Alveolus,  glandular,  definition  of,  i.  212 
Amido-acetic  acid,  i.  537 
Amido-acids,  properties  of,  i.  538 
Amines,  definition  of,  i.  541 
Ammonia,  action  of,  on  nerves,  ii.  60 

inhalation  of,  i.  440 

occurrence  of,  i.  511 

origin  of,  in  the  body,  i.  511 

properties  of,  i.  511 

Ainmoniacal  fermentation  of  urine,  i.  512 
Ammonium  carbamate,  i.  548 

carbonate,  i.  523 

cyanate,  i.  542 

magnesium  phosphate,  i.  527 

salts,  action  of,  on  muscles,  ii.  138 
Amnion,  ii.  472 
Amniotic  cavity,  ii.  472 

fluid,  inhibitory  effect  of,  on  respiration,  i.  464 
Amoeba,  contractility  of,  ii.  19 
Amoeboid  movement,  ii.  19 
in  neuroblasts,  ii.  176 
in  ova,  ii.  22 
of  leucocytes,  i.  48 
Amphiaster,  ii.  469 
Amphimixis  as  cause  of  congenital  variation, 

ii.  504 

Amphioxus,  reflexes  in,  ii.  212 
Ampho-peptone,  definition  of,  i.  293 
Amplitude  of  sound-waves,  ii.  381 
Ampulla  of  Henle,  ii.  447 
Ampullae  of  the  semicircular  canals,  ii.  372 
Ampullary  nerves,  stimulation  of,  ii.  407 
Amputation  in  man,  effects  of,  on  neurones,  ii. 

196 
Amygdalin,  fermentative  decomposition   of,  i. 

542 

Amyl  alcohol,  i.  539 
Amylodextrin,  i.  566 
Amyloid,  i.  578 
Amylolytic  enzyme  of  gastric  juice  in  the  dog, 

i.  296 

of  succus  entericus,  i.  308 
of  the  liver,  i.  330 

enzymes,  definition  of,  i.  280 

action  of,  in  the  body,  i.  285 
Amylopsin,  i.  232,  280 

action  of,  on  starch,  i.  566 

digestive  action  of,  i.  305 

occurrence  of,  i.  304 

properties  of,  i.  305 
Anabolism,  definition  of,  i.  19 
Anaemia  of  the  brain  during  fatigue,  ii.  288 
Anaesthesia,  contralateral,  after  hemisection  of 

the  cord,  ii.  233 
Anaesthetics,  action  of,  on  nerve-currents,  ii,  155 

effect  of,  on  body-temperature,  i.  472 
Analgesia,  ii.  232 

following  removal  of  the  cerebellum,  ii.  272 
Analysis  of  composite  tones,  ii.  384 
Anatomy  of  the  ear,  ii.  362 


GENERAL  INDEX. 


525 


Anelectrotonus,  ii.  62 

Angular  movements  of  joints,  ii.  416 

Animal  foods,  composition  of,  i.  'JT1- 

heat,  i.  4(57 

source  of,  i.  474 

Anisotropic  substance  of  muscle-fibres,  ii.  104 
Annulus  Yicusscns,  i.  l.~>!i 
Anodal  contraction,  ii.  ."><i 
Anode,  physical,  definition  of,  ii.  52 

physiological,  definition  of,  ii.  52 
Anosmia,  ii.  411 
Antallmmid,  i.  X>93 
Anterior  association  centre,  ii.  •->.')7 

roots,  recurrent  sensibility  of,  ii.  204 
Antilytic  secretion,  i.  230 
Antimony  poisoning,  i.  514 
Anti-peptone,  definition  of,  i.  293 

nature  of.  i.  302 
Antiperistalsis,  intestinal,  i.  383 

of  the  stomach,  i.  379 
Antrum  pylori,  i.  377 
Apex  beat,  i.  117 

preparation  of  the  frog's  heart,  i.  188 

ventricular,  rhythmicity  of,  i.  151 
Aphasia,  ii.  257 
Apnoea,  definition  of,  i.  440 

fatal,  i.  464 

phenomena  of,  i.  44 

relation  of  vagi  to,  i.  442 
Apoinorphia,  action  of,  i.  389 
Apraxia,  ii.  259 

Aqueous  humor,  index  of  refraction  of,  ii.  303 
Arabinose,  i.  562 
Arginin,  i.  552 
Argon  of  the  blood,  i.  417 
Aromatic  compounds  in  urine,  i.  572 

metabolism  of,  i.  568,  569 
Arsenic  poisoning,  i.  514 
Arterial  blood-pressure,  explanation  of,  i.  92 

pulse,  cause  of,  i.  93 
definition  of,  i.  139 
extinction  of,  i.  94 
Arteries,  calcification  of,  in  old  age,  ii.  491 

coronary,  i.  179 

elongation  of,  i.  140 

rate  of  flow  in,  i.  101 

Arthropods,  segmental  nervous  system  of,  ii.  212 
Articular  cartilages,  ii.  415 
Articulation,  ii.  434 
Articulations,  varieties  of,  ii.  414 
Artificial  circulation  through  the  heart,  ii.  69 
through  the  muscles,  ii.  68 

respiration,  circulatory  effects  of,  i.  453 
methods  of  maintaining,  i.  446 

stimulation  of  muscle  compared  with  normal, 

ii.  134 

Aryepiglottic  fold,  ii.  -Vl'i. 
Aryteno-epiglottidean  muscle,  ii.  426 
Arytenoid  cartilages,  ii.  425 

muscle,  ii.  426 
Asexual  reproduction,  ii.  439 

theory  of  the  origin  of,  ii.  441 
Asparagin,  i.  558 
Aspartic  acid,  i.  557 
Asphyxia,  i.  441 

circulatory  changes  in,  i.  445 

effects  of,  on  the  blood-vessels,  i.  202 
on  the  respiratory  rhythm,  i.  425 

stages  of,  i.  4-1  ."> 
Aspirates,  ii.  437 
Aspiration  of  the  thorax,  influence  of,  on  the 

circulation,  i.  77.  95 
on  the  lyinph-llmv,  i.  147 
on  venous  circulation,  i.  77.  !'•"> 
Aoimilation,  general  characteristics  of,  i.  19 
Associated  respiratory  movements,  i.  408 
Association  centre,  anterior,  ii.  257 


Association  centre,  middle,  ii.  257 
posterior,  ii.  -J.TT 

cerebral,  variations  in,  ii.  :_v,<» 

fibres  and  centres  of  the  cortex,  ii. 

tracts,  origin  of,  from  central  cells,  n 
Astasia  after  removal  of  the   cerebellum,  i, 
Asthenia  from  removal  of  t  lie  cerebellum,  n 
Astigmatic  accommodation,  ii.  310 
Astigmatism,  ii.  ::i7 

detection  of,  ii.  :;i!» 

irre-ular,  ii.  319 

Astral  rays,  contractility  of,  ii.  470 
Asymmetrical  carbon  atom,  definition  of,  i 
Atavism,  ii.  495 

Ataxia  after  removal  of  the  cerebellum,  ii.  273 
Atelectasis,  i.  396 

Atmospheric  air,  composition  of,  i.  410,  413 
Atonia  after  removal  of  the  cerebellum,  ii.  273 
Atrophy  of  the  heart  after  section  of  the  vagi,  i. 
167 

of  the  nerve-cells  from  disuse,  ii.  195 
Atropin,  action  of,  on  accommodation,  ii.  311 
on  salivary  glands,  i.  222,  229 
on  sweat  glands,  i.  260 
on  the  eye,  ii.  325 

effect  of,  on  body-temperature,  i.  472 
Attraction  sphere  of  the  ovum,  ii.  449 
Auditory  area  of  the  cortex,  ii.  253 

canal,  ii.  363 

epithelium  of  the  utricle  and  saccule,  ii.  373 

judgments,  ii.  389 

meat  us,  external,  ii.  362 

nerves,  central  paths  of,  ii.  237 
cochlear  division  of,  ii.  376 
subdivision  of,  ii.  373 

ossicles,  ii.  365 
movements  of,  ii.  367 

sensations,  limits  of,  ii.  382 
successive  contrast  in,  ii.  388 
theory  of,  ii.  380 
Augmentor  centre  of  the  heart,  i.  177 

nerves  of  the  heart,  i.  161, 167 
Auricle  of  the  external  ear,  ii.  362 

systolic  changes  in,  i.  115 
Auricles,  connection  of,  i.  135 

degree  of  emptying,  in  systole,  i.  138 

functions  of,  i.  135 

influence  of,  on  venous  blood-flow,  i.  136 

negative  pressure  in,  i.  137,  138 
Auricular  pressure,  i.  135,  137 

systole,  duration  of,  i.  124,  136 
effect  of,  on  venous  blood -flow,  i.  138 
on  ventricular  filling,  i.  137 
Auriculo-ventricular  valves,  i.  108 
Auscultation,  i.  118 
Automatism,  definition  of,  ii.  20 
Axilla,  temperature  in,  i.  468 
Axones,  definition  of;  ii.  21,  173 

growth  in  diameter  of,  ii.  179 

length  of,  ii.  174 

BACTERIAL  decomposition  in  the  intestines,  i. 

309 

Ball-and-socket  joint,  ii.  416 
Banting  diet,  i.  353 

Barium  salts,  action  of,  on  muscles,  ii.  138 
Barometric  pressures,  effect  of,  on  respiration,  i. 

434 
Bartholin,  duct  of,  i.  217 

gland-*  of.  ii.   UJ-J 
Basilar  membrane,  structure  of,  ii.  377 

width  of,  ii.  380 
P,a-i,phili'<.  i.  47 

Baths,  influence  of,  on  body-temperature,  i.  471 
Mathyasthesia.  ii.  233 

Meats  in  musical  tones,  production  of,  ii.  386 
Beckmann's  apparatus,  i.  68 


526 


GENERAL  INDEX. 


Beef-tea,  physiological  action  of,  i.  359 

Beer,  i.  535 

Beeswax,  i.  540 

Benham's  spectrum  top,  ii.  344 

Benzoic  acid,  i.  340,  569 

Benzol,  molecular  constitution  of,  i.  568 

Benzopyrol,  i.  571 

Bidder's  ganglion,  i.  148 

Bile,  amount  secreted,  i.  246,  321 

antiseptic  property  of,  i.  326 

composition  of,  i.  245,  321 

discharge  of,  from  the  gall-bladder,  i.  248,  249 

fatty  acids  of,  i.  541 

influence  of,  on  emulsification  of  fats,  i.  307 

mineral  constituents  of,  i.  530 

physiological  value  of,  i.  325 

pigments  of,  i.  245,  322 

relation  of,  to  fat  absorption,  i.  325 

secretion  of,  i.  246 

sulphur  of,  i.  507 
Bile-acids,  i.  245 

detection  of,  i.  324 

Neukomm's  test  for,  i.  545 

occurrence  of,  i.  323 

origin  of,  i.  324 

Pettenkofer's  test  for,  i.  324,  544 

relation  of,  to  fat  absorption,  i.  326 
Bile-capillaries,  i.  244 
Bile-ducts,  occlusion  of,  i.  249 
Bile-pigments,  i.  322 

chemical  properties  of,  i.  574 

Gmelin's  test  for,  i.  322,  574 

origin  of,  i.  45,  530 
Bile-salts,  i.  245 

chemistry  of,  i.  543 

circulation  of,  i.  544 
Bile-secretion,  normal  mechanism  of,  i.  248 

relation  of,  to  blood-flow  in  the  liver,  i.  247 
Bile-vessels,  motor  nerves  of,  i.  248 
Biliary  fistulae,  i.  321 
Bilicyanin,  i.  574 
Bilirubin,  i.  245,  574 
Biliverdin,  i.  245,  574 
Bilixanthin,  i.  574 
Binocular  combination  of  colors,  ii.  358 

vision,  ii.  356 

illusions  in.  ii.  359 

rivalry  of  the  fields  of  vision  in,  ii.  358 
Biophors  of  the  germ-plasm,  ii.  503 
Birds,  removal  of  cerebral  hemispheres  in,  ii.  267 
Birth,  size  of  the  child  at,  ii.  487 
Birth-rate  of  the  two  sexes,  ii.  483 
Births,  multiple,  ii.  482 

ratio  of  male  to  female,  ii.  483 
Biuret,  i.  549 
Bladder,  urinary,  movements  of,  i.  369,  390 

vaso-motor  nerves  of,  i.  209 
Blastomeres,  ii.  470 
Blind-spot,  ii.  328 
Blood,  i.  33 

amount  of,  in  the  central  nervous  system,  ii. 
288 

changes  in,  during  pregnancy,  ii.  477 

chemical  composition  of,  i.  50 

circulation  of,  i.  76 

coagulation  of,  i.  54 

defibrinated,  i.  34 

distribution  of,  in  the  body,  i.  63 

foreign,  action  of,  on  the  heart,  i.  192 

gaseous  exchanges  of,  i.  411 

general  function  of,  i.  33 

histological  structure  of,  i.  33 

identification  of,  i.  573 

oxidations  in,  i.  423 

reaction  of,  i.  34,  290 

regeneration  of,  after  hemorrhage,  i.  63 

specific  gravity  of,  i.  34 


Blood,  total  quantity  of,  in  the  body,  i.  63 

transfusion  of,  i.  64 
Blood-corpuscles,  inorganic  salts  of,  i.  50,  530 

varieties  of,  i.  33 
Blood-gases,  analyses  of,  i.  411 

extraction  of,  i.  420 

tension  of,  i.  415 
Blood-leucocytes,  i.  47 
Blood-plasma,  color  of,  i.  33 

composition  of,  i.  51 

inorganic  salts  of,  i.  50 
Blood-plates,  i.  49 
Blood-pressure,  aortic,  i.  91 

capillary,  i.  84,  93 

effect  of  the  accelerator  nerves  on,  i.  170 
of  the  depressor  nerves  on,  i.  173 
on  renal  secretion,  i.  253,  256 

mean,  definition  of,  i.  90 

methods  of  measuring,  i.  84,  85 

origin  of,  i.  91, 92 

pulmonary,  i.  91 

respiratory  changes  in,  i.  447 

venous,  i.  91,  94 
Blood-serum,  composition  of,  i.  51 

definition  of,  i.  34 

mineral  constituents  of,  i.  530 

osmotic  pressure  of,  i.  68 

Blood-supply  of  the  central  nervous  system,  ii. 
286 

relation  of,  to  irritability,  ii.  67 
Bodily  metabolism,  estimation  of,  i.  343 

movements,  effect  of,  on  lymph-flow,  i.  147 

temperature,    effect    of,    on    respiratory    ex- 
changes, i.  432 

Body-sense  area  of  the  cortex,  ii.  252,  254 
Body-temperature,  rise  of,  from   injury  to  the 

optic  thalami,  ii.  271 

from  lesions  in  the  corpora  striata,  ii.  271 
Body-weight,  influence  of,  on  heat-production, 
i.  482 

loss  of,  from  starvation,  i.  362 
Bolometer,  ii.  142 

Bones,  action  of  muscles  on,  ii.  417 
Border-cells  of  the  gastric  glands,  i.  237,  238 
Bottcher's  crystals,  ii.  445 
Brain,  circulation  in,  regulation  of,  ii.  287 

curve  of  growth  of,  ii.  279 

growth  of,  ii.  278 

number  of  nerve-elements  during,  ii.  280 
relation  of,  to  growth  of  the  body,  ii.  280 
size  of  neurones  during,  ii.  281 

metabolic  activity  in,  ii.  288 

vaso-motor  nerves  of,  i.  203 

weight  of,  ii.  273 
Brain-stem,  ii.  274 
Brain-ventricles,  capacity  of,  ii.  274 
Brain-weight,  decrease  of,  in  old  age,  ii.  296 

relation  of,  to  insanity,  ii.  278 
to  sex,  ii.  276 

to  social  environment,  ii.  277 
Brain-weights  of  different  races,  ii.  278 
Breaking  contraction,  point  of  origin  of,  ii.  35 
"  Breaking"  shock,  ii.  31 
Broca's  convolution,  ii.  257 
Bromelin,  i.  280 
Bromine,  i.  508 
Bronchial  capacity,  i.  427 

constrictor  nerves,  i.  465 
Broncho-dilator  nerves,  i.  465 
Brown-Sequard's  paralysis,  ii.  233 
Brucin,  action  of,  on  end-plates,  ii.  27 
Brunner's  glands,  i.  243 
Buffy  coat,  i.  55 

Bulbo-cavemosus  muscle,  ii.  449 
action  of,  in  erection,  ii.  464 
Bulimia,  ii.  404 
Butyric  acid,  i.  539 


GENERAL   IXI>i:X. 


527 


CADAVERIN,  i.  543 
Caflein,  i.  .">;.:; 

actiou  of,  on  body-temperature,  i.  472 
on  coagulation  of  muscle-plasma,  ii.  164 
oil  kidneys,  i.  254 
Calcium,  absortion  of,  i.  •>•-•"> 
carbonates,  i.  524 
chloride,  i.  ")2i{ 
excretion  of,  i.  526 
fluoride,  i.  .-,10,  523 
phosphates,  i.  .Vj:; 
physiological  value  of,  i.  524 
relation  of,  to  heart  muscle,  i.  151 
salts,  action  of,  on  the  heart,  i.  190 

on  the  muscles,  ii.  138 
amount  of,  in  fibrin,  i.  58 
excretion  of,  i.  .'!.~>i> 
nutritive  value  of,  i.  356 
relation  of,  to  blood-coagulation,  i.  57,  524 

to  irritability,  ii.  59 
sulphate,  i.  -Vj:; 
Calorie,  definition  of,  i.  504 
Calori metric  equivalent,  i.  478 
Caloriuietry,   direct  and   indirect,  i.   365,  475, 

478 
Canalis  cochlearis,  ii.  373 

reunions,  ii.  373,  374 
Cane  sugar,  injection  of,  i.  317 

inversion  of,  i.  565 

Capacity  of  the  heart-ventricles,  i.  105 
Capillaries,  biliary,  i.  244 
blood,  length  of,  i.  79 
permeability  of,  i.  70 
pressure  in,  i.84 
rate  of  flow  in,  i.  101 
resistance  in,  i.  81 
structure  of,  i.  80 
time  spent  by  the  blood  in,  i.  103 
secretion,  of  the  fuudic  glands,  i.  238 
Capillary  circulation,  microscopic  characters  of, 

i.  80 

electrometer,  ii.  146 
pressure,  origin  of,  i.  93 

relation  of,  to  lymph-formation,  i.  72,  75 
Capric  acid,  i.  541 
Caproic  acid,  i.  540 
Caprylic  acid,  i.  541 

Capsules,  suprarenal,  extirpation  of,  i.  271 
Caput  gallinagiuis,  ii.  464 
Carbamic  acid,  i.  548 

relation  of,  to  urea  formation,  i.  336 
Carbamide,  i.  548 

Carbo-hsemoglobin,  nature  of,  i.  39 
Carbohydrates,  absorption  of,  i.  317 
affinity  of  cell-substance  for,  i.  568 
chemistry  of,  i.  561 
combustion  equivalent  of,  i.  365 
definition  of,  i.  561 
digestion  of,  in  the  stomach,  i.  296 
dynamic  value  of,  i.  475 
fermentation  of,  in  the  intestines,  i.  310 
molecular  constitution  of,  i.  561 
nutritive  value  of,  i.  277,  353 
origin  of  fat  from,  i.  ::.">-.' 
proteid-protection  by,  i.  568 
synthesis  of,  i.  26 
Carbon  dioxide,  action  of,  on  conductivity  in 

nerves,  ii.  93 
on  the  heart,  i.  191 
on  the  nerves,  ii.  60 
on  warm  spots,  ii.  398 
dysptuea.  i.   11 1 

effect  of,  on  nerve-currents,  ii.  156 
elimination,  conditions  affecting,  i.  429 
cutaneous,  i.  1'J-J 
estimation  of.  i.  428 
inhalation,  effects  of,  i.  440 


Carbon  dioxide,  occurrence  of,  i.  r>i7 
of  imiM-le,  ii.  168 
of  the  blood,  extraction  of,  i.  517 
production  ..f.  in  nerves,  ii.  !»."> 
properties  of,  i.  518 
tension  of,  in  the  alveoli,  i.  413 

in  the  blood,  i.  416 

disulphide,  action  of,  on  nerves,  ii.  60 
equilibrium,  definition  of,  i.  345 
metabolism  of,  i.  518 
monoxide  haemoglobin,  i.  517 

absorption  spectrum  of,  i.  44 
composition  of,  i.  38 
inhalation,  i.  440 
properties  of,  i.  517 
occurrence  of,  i.  516 
properties  of,  i.  516 

Carbonic  acid,  chemical  constitution  of,  i.  545 
Carburetted  hydrogen  inhalation,  i.  440 
Cardiac  centre,  augmentor,  i.  177 

inhibitory,  i.  17<> 
cycle,  analysis  of,  i.  122 
definition  of,  i.  104 
duration  of,  i.  123 
dyspnoea,  i.  444 

excitation,  propagation  of,  during  vagus  stim- 
ulation, i.  163 
impulse,  i.  117 
nerves,  anatomy  of,  i.  159 
classification  of,  i.  171 
extrinsic,  i.  159 
of  frogs,  i.  160 
of  mammals,  i.  160 
palpitation  at  the  climacteric,  ii.  490 
Cardio-inhibitory  centre,  respiratory  variations 

in,  i.  451 

Cardio-pneumatic  movements,  i.  412 
Cardiogram,  i.  117 
Cardiometer,  i.  106 
Carnic  acid  of  muscles,  ii.  167 
Carnin,  i.  554 

of  muscles,  ii.  167 
Casein,  i.  261 
composition  of,  i.  579 
curdling  of,  by  acids,  i.  296 

by  rennin,  i.  295 
Castration,  effects  of,  ii.  463 

on  the  voice,  ii.  431 
Cataleptic  rigor,  ii.  160 
Catalysis,  i.  282,  503 
Caudate  nucleus,  heat-centre  of,  ii.  271 
Cell,  galvanic,  ii.  29 
Cell-differentiation,  i.  22 ;  ii.  22. 
Cell-division,  i.  20 
Cell-granules  of  the  glandular  epithelium,   i. 

216 

Cells,  growth  of,  ii.  486 
Central  cells,  importance  of,  in  relation  to  in- 

en-a-r  of  organization,  ii.  285 
nervous  system,  amount  of  blood  in,  ii.  288 
arrangement  of  cell  groups  in,  ii.  205 
hlood-snpply  of,  ii.  286 
change  in  specific  gravity  of,  with  age,  ii. 

284 

condition  of,  in  sleep,  ii.  293 
conscious  phenomena  of,  ii.  172 
daily  rhythms  of.  ii.  289 
development  of.  ii.  I?'.' 
fatigue  of,  ii.  289 
general  arrangement  of,  ii.  202 

functions  of,  ii.  171 
influence  of  the  thyroid  on  growth  of,  ii. 

•>!• 

in  old  age,  ii.  •_'!'.•. 
intensity  of  metabolism  in.  ii.  28* 
medullation  of  nerves  in,  ii.  181 
operation  on,  in  fro-rs.  ii 


528 


GENERAL  INDEX. 


Central  nervous  system,  organization  of,  at  dif- 
ferent ages,  ii.  284 

neurones  of  the  spinal  cord,  ii.  203 

stimulation  of  the  nervous  system,  ii.  28 
Centre,  augmentor  of  the  heart,  i.  177 

cardio-inhibitory,  i.  176 

defecation,  i.  387 

deglutition,  i.  377 

expiratory,  i.  457 

inspiratory,  i.  457 

micturition,  i.  391,  393 

of  hearing,  cortical,  ii.  253 

of  rotation  of  the  eye,  ii.  298 

of  smell,  cortical,  ii.  253 

of  vision,  cortical,  ii.  253 

peripheral  reflex,  i.  178 

respiratory,  i.  455 

salivary,  secretory,  i.  230 

spinal,  of  ejaculation,  ii.  465 
of  erection,  ii.  464 
of  parturition,  ii.  481 

sweat,  i.  260 

thermogenic,  i.  491 

vaso-motor,  i.  198 

vomiting,  i.  389 
Centres,  association,  ii.  256 
Centripetal  nerves  of  the  heart,  i.  171 
Centrosome,  i.  22 

of  human  spermatozoa,  ii.  444 

of  the  fertilized  egg,  ii.  468 

of  the  ovum,  ii.  449 
Cerebellum,  anatomical  connections  of,  ii.  273 

effects  of  injury  to,  ii.  272 

functions  of,  ii.  272 

senile  changes  in,  ii.  296 
Cerebral  circulation,  i.  203 

conditions  affecting,  ii.  288 
crossed,  i.  443 

cortex,  relation  of,  to  the  vaso-motor  centre, 
i.  202 

hemispheres,  effect  of  removal  of,  ii.  263 
relative  physiological  values  of,  ii.  259 
removal  of,  in  birds,  ii.  266 

in  dogs,  ii.  262 
Cerebrin,  i.  559 

of  nerves,  ii.  170 
Cerebrum,  heat  regulating  functions  of,  ii.  270 

removal  of,  in  dogs,  ii.  267 
Cerotyl  alcohol,  i.  540 
Cerumen,  i.  257 
Cervical  sympathetic,  vaso-motor  function  of,  i. 

193 

Cetyl  alcohol,  i.  540 

Characters,  acquired,  inheritance  of,  ii.  496 
Chemical  reagents,  action  of,  on  irritability,  ii. 
58 

stimulation  of  nerve,  ii.  25 

tonus,  ii.  143 

Chemistry  of  nerves,  ii.  169 
Chemotaxis,  ii.  466 

influence  of,  on  neuroblasts,  ii.  176 
Chemo-tropism,  ii.  466 
Chest,  effects  of  opening,  i.  115 
Chest-voice,  ii.  432 
Cheyne-Stokes  respiration,  i.  424 
Chief  cells  of  the  gastric  glands,  i.  237 
Chinese  wax,  i.  540 
Chinolin,  i.  571 

Chloral,  effect  of,  on  the  respiratory  rhythm,  i. 
425 

hydrate,  i.  536 
Chlorine,  inhalation  of,  i.  440 

occurrence  of,  i.  507 
Chlorocruorin,  i.  578 

Chloroform,  action  of,  on  coagulation  of  muscle- 
plasma,  ii.  164 

effect  of,  on  nerve-currents,  ii.  156 


Chloroform,  fate  of,  in  the  body,  i.  533 

vapor,  action  of,  on  nerves,  ii.  60 
Chocolate,  nutritive  value  of,  i.  357 
Cholagogues,  i.  246 
Cholesterin,  i.  575 
amount  of,  in  the  blood,  i.  51 
distribution  of,  i.  325 
excretion  of,  i.  325 
in  nerve,  ii.  169 
of  bile,  i.  245 
of  milk,  i.  261 
of  sebaceous  secretion,  i.  257 
Choletelin,  i.  574 
Cholin,  i.  541,  543 
Cholo-hsematin,  i.  323 
Chondroitic  acid,  i.  578 
Chondro-mucoid,  i.  578 
Chorda  tympani  nerve,  gustatory  function  of,  ii. 

410 

vaso-dilator  function  of,  i.  194 
Chordae  tendinese,  i.  109 
Chorion,  ii.  473 
frondosum,  ii.  474 
Iseve,  ii.  474 
Chorionic  fluid,  ii.  473 

villi,  ii.  473 

Chromatic  aberration,  ii.  316 
Chromatin,  i.  22,  28 

Chromatoblasts  of  pleuronectidae,  ii.  20 
Chromo-proteids,  i.  576 
Chromosomes,  i.  22,  28 

number  of,  in  the  segmentation  nucleus,  ii. 

466 

of  germ-cells,  hereditary  function  of,  ii.  499 
of  human  spermatozoa,  ii.  443 
of  the  sexual  elements,  reduced  number  of,  ii. 

454 
ovarian,  changes  in,  during   maturation,  ii. 

451 

number  of,  ii.  450 
reduction  of,  in  maturation  of  spermatozoa,  ii. 

445 

Chronograph,  description  of,  ii.  100 
Chyme,  i.  287,  381 
Ciliary  ganglion,  ii.  323 

muscles,  action  of,  in  accommodation,  ii.  309 
nerves,  long,  ii.  324 
short,  ii.  311,  323 

Circulating  proteid,  definition  of,  i.  346 
Circulation,  artificial,  through  isolated  organs, 

ii.  68 

capillary,  velocity  of,  i.  83 
cerebral,  i.  203 
of  hydriodic  acid,  i.  509 
of  hydrofluoric  acid,  i.  510 
of  the  bile,  i.  323,  324 
of  the  blood,  causes  of,  i.  77 
definition  of,  i.  76 
discovery  of,  i.  76 
in  the  retina,  ii.  322 
microscopic  appearances  of,  i.  80 
portal,  i.  77 
pulmonary,  i.  78,  103 
of  the  brain  and  cord,  ii.  286 

rate  of,  i.  79,  98 
pulmonary,  i.  103 
renal,  i.  255 
Circulation-time,  i.  79 
Circumduction,  movement  of,  ii.  416 
Climacteric,  ii.  459,  490 

ovulation  after,  ii.  456 

Climate,  influence  of,  on  age  of  puberty,  ii.  489 
on  body-temperature,  i.  469 
on  time  of  climacteric,  ii.  490 
Clitoris,  ii.  462 

hornology  of,  ii.  464 
Clothing,  influence  of,  on  heat-loss,  i.  486 


<>•/•:. YER  A  L  IM>I:X. 


529 


Clotting  of  blood,  i.  55 

of  milk.  i.  -JH5 
Clupein,  i.  .'.HI 

(.'<>_.  cliinination,  cutaneous,  i.  'J.">8,  342 
during  muscular  work,  i.  3<>1 

deep,  i.  :!«;i 

Coagulated  proteids.  properties  of.  i.  578 
Coagulating  t'li/.yincs,  definition  of.  i.  280 
Coagulation  of  tlie  blood,  accelerating  agents  of, 

i.  (il 

conditions  necessary  for,  i.  57 
description  of.  i.  •">  I 
intravascular,  i.  60 
nature  of,  i.  HO 

retarding  influences  affecting,  i.  61,  62 
theories  of.  i.  55,  56 
rime  taken  by,  i.  55 
uses  of,  i.  ">.") 
of  milk,  i.  -JIT. 

of  muscle-plasma,  action  of  drugs  on,  ii.  164 
of  myosin,  ii.  163 
Cocaine,  action  of,  on  conductivity  in  nerves,  ii. 

93 

on  the  tongue,  ii.  413 
effect  of,  on  intestinal  movements,  i.  384 
Cochlea,  anatomy  of,  ii.  374 
bony,  ii.  372 

membranous,  structure  of,  ii.  376 
Cochlear  root  of  tbe   auditory   nerve,  central 

paths  of/n.  237 
Coefficient  of  absorption  of  liquids  for  gases,  i. 

414 
Coffee,  nutritive  value  of,  i.  357 

stimulating  action  of,  ii.  75 
Cold  and  warm  points  of  the  skin,  ii.  398 

effect  of,  on  coagulation  of  the  blood,  i.  61 
Collagen,  i.  580 
Collaterals  of  axones,  ii.  173 
Colloid,  i.  578 

substance  of  the  thyroid,  secretion  of,  268 
Color  of  objects,  relation  of,  to  intensity  of  il- 
lumination, ii.  333 
sensations  in  indirect  vision,  ii.  333 

phenomena  of,  ii.  333 
theories,  ii.  335 
triangle,  ii.  334 
vision,  theories  of,  ii.  335 
Color-blindness,  ii.  338 

hereditary  transmission  of,  ii.  494 
of  the  rods,  ii.  342 
Colored  shadows  from  simultaneous  contrast,  ii. 

347 

Color-mixture,  ii.  333 
Colors,  binocular  combination  of,  ii.  358 
complementary,  ii.  334 
physical  basis 'for,  ii.  332 
relative  luminosity  of,  ii.  340 
saturation  of,  ii.  342 
Colostrum  corpuscles,  origin  of,  i.  263 

definition  of,  i.  264 
Combinational  tones,  ii.  387 
Combined  proteids,  i.  579 
Combustion,  i.  501 

equivalent  of  foods,  i.  365 
Comedones,  i.  257 
Commissure,  Meynert's,  ii.  238 

von  Gudden's,  ii.  238 

Commissures,  origin  of,  from  central  cells,  ii.  205 
Common  sensation,  definition  of.  ii.  .">!'!» 

sensibility,  afferent  paths  of  the  nerves  of, 

ii.  230 

Commutators,  method  of  using,  ii.  36 
Complemental  air.  i.  427 
Complementary  colors,  ii.  334 
Composite  tones,  analysis  of,  ii.  384 
Compressed  air,  respiration  of,  i.  452 
Conceptions,  multiple,  ii.  482 

34 


Concha  of  the  external  ear.  ii.  3fi2 
Condiments,  nutritive  value  of,  i.  .",:>!i 
Conduction  by  continuity,  ii.  M 

directions  «,f,  ii.  -[ 

from  neurone  to  neurone,  ii.  84 

in  branching  nerves,  ii.  -o 

in  ganglion-cells,  ii.  !»7 

in  muscles,  ii.  80 

in  nerve-trunks,  ii. 

in  nerves,  elleeK  of,  ii.  !»."> 

in  the  heart  of  the  contraction  wave,  i.  154 

of  nerve-impulses,  dim-Hem  of,  ii.  184 
from  neurone  to  neurone,  ii.  207 

process,  nature  of,  ii.  97 

rate  of,  ii.  87 
Conductivity,  action  of  drugs  on,  ii.  93 

definition  of,  ii.  20,  77 

dependence  of,  on  protoplasmic  continuity, 
ii.  77 

effect  of  constant  current  on,  ii.  94 

influences  affecting,  ii.  91 

of  living  matter,  i.  21 

of  muscle,  ii.  20 

of  nerves,  ii.  21 

of  neurone,  ii.  189 

of  ova,  ii.  22 
Condyloid  joints,  ii.  416 
Cones,  retinal,  function  of,  ii.  341 

movements  of,  ii.  331 
Confluxion  in  space  perception,  ii.  353 
Congenital  resemblances,  ii.  494 

variations,  ii.  500 

Congo-red  test  for  mineral  acids,  i.  289 
Con  in,  action  of,  on  end-plates,  ii.  27 
Conjugate  foci  in  a  dioptric  system,  ii.  302 
Conjugated  sulphates,  nutritive,  history  of,  i.  340 
Conjugation,  ii.  440 
Consciousness,  i.  29 

cerebral  origin  of,  ii.  172 
Consonants,  ii.  436 

Constant  current,  contracture  effect  of,  in  mus- 
cles, ii.  131 

effect  of,  on  conductivity,  ii.  94 
on  muscles,  ii.  61 
on  nerves,  ii.  62 

Constrictor  nerves  of  the  iris,  ii.  323 
Continuous  contractions,  ii.  127 
Contractility,  definition  of,  ii.  17,  98 

in  vorticella,  ii.  20 

occurrence  of,  ii.  20 

of  amcebfe,  ii.  19 

of  astral  rays,  ii.  470 

of  living  matter,  i.  21 

of  muscle,  ii.  17 

adaptation   of,   to  their  normal  functions, 
ii.  108 

of  ova,  ii.  22 

of  plain  muscle,  i.  370 

Contraction  curve  of  muscle,  effect  of  frequent 
excitations  on,  ii.  115 

id io-rnu scalar,  ii.  !>J 

of  muscles,  post-mortem,  ii.  160 
relation  of,  to  structure,  ii.  1C7 

remainder,  ii.  106 

volume  of  the  heart,  i.  105 

wave  in  muscle,  rate  of  transmission  of,  ii.  87 
length  of,  ii.  88 

of  the  heart,  rate  of  propagation  of,  i.  1":; 
Contractions  from  repeated  single  stimuli,  ii.  112 

introductory,  ii.  113 

isometric,  ii.  110 

isotonic,  ii.  110 

normal,  tetanic  nature  of,  ii.  132 

of  rigor  caloris.  ii.  !»;."• 
Contract  11  re  after  frequent  excitations,  ii.  128 

after  single  excitation,  ii.  129 

definition  of,  ii.  116 


530 


GENERAL  INDEX. 


Contracture  from  fatigue,  ii.  130 
in  dying  muscles,  ii.  132 
in  rigor  mortis,  ii.  128 
in  veratria  poisoning,  ii.  128 
normal,  ii.  129 

of  neck  muscles  after  cerebellar  injury,  ii.  272 
pathological,  ii.  127,  132 
relation  of,  to  tetanus,  ii.  117,  122,  124 
Contractures,  ii.  127 
Contrast,  ii.  346 

in  space  perception,  ii.  352 
Convergence  of  the  eyes  in  accommodation,  ii. 

311 

muscular  mechanism  of,  ii.  300 
Co-ordination  of  the  eiferent  impulses  in  re- 
flexes, ii.  214 
Copulation,  ii.  463 
Core-conductors,  ii.  158 
Cornea,  curvature  of,  ii.  303 
Corniculum  laryngis,  ii.  425 
Cornutine,  action  of,  on  muscles,  ii.  137 
Corona  radiata,  ii.  454 

of  the  ovum,  ii.  450 
Coronary  arteries,  anatomy  of,  i.  179,  180 

ligatiou  of,  i.  181,  183 
circulation,  effect  of  ventricular  systole  on,  i. 

185 

volume  of,  i.  184 
veins,  closure  of,  i.  184 
Corpora  Arantii,  i.  112 

cavernosa  of  the  penis,  ii.  448 
striata,  functions  of,  ii.  271 
Corpus  callosum,  functions  of,  ii.  270 
luteum,  ii.  455 

spongiosum  of  the  penis,  ii.  448 
Corpuscles,  colostrum,  i.  263 
of  the  blood,  i.  45 
salivary,  i.  283 

Corresponding  points  of  the  retinas,  ii.  359 
Cortex  cerebri,  connection  of,  with  the  respira- 
tory centre,  i.  463 
effects  of  localized  electrical  stimulation  of, 

ii.  241 

electrical  stimulation  of,  ii.  242 
number  of  nerve-cells  in,  ii.  284 
course  of  efferent  impulses  from,  ii.  251 
latent  areas  of,  ii.  261 
Corti,  cells  of,  ii.  377 

organ  of,  structure  of,  ii.  377 
rods  of,  ii.  377 
Cortical  areas,  ii.  243 

motor,  in  man,  ii.  250 
size  of,  ii.  247 
centres,  ii.  243 
motor  control,  crossed,  ii.  251 

multiple  character  of,  ii.  250 
regions,  ii.  243 
stimulation,  inhibitory  effects  of,  ii.  224 

vascular  effects  of,  i.  202 
Costal  respiration,  definition  of,  i.  398 
Coughing,  i.  454 
Coughs,  sympathetic,  i.  455 
Cowper's  gland,  ii.  443 
histology  of,  ii.  448 
secretion  of,  ii.  446 

Crab-extract,  lymphagogic  action  of,  i.  73 
Crabs,  regeneration  of  lost  parts  in,  ii.  496 
Cranial  nerves,  afferent,  ii.  236 
Creatin,  chemical  constitution  of,  i.  550 
in  muscle,  ii.  166 
nutritive  history  of,  i.  339,  551 
Creatinin,  i.  551 

nutritive  history  of,  i.  339 
of  muscle,  ii.  167 
Cresol,  i.  569 

elimination,  i.  340 
Cretinism,  sporadic,  ii.  289 


rico-arytenoid  muscle,  lateral,  ii.  426 

posterior,  ii.  426 
Cricoid  cartilage,  ii.  425 
Crico-thyroid  muscles,  ii.  426 
Criminals,  weight  of  the  brain  in,  ii.  277 
Crista  acustica  of  the  semicircular  canals,  ii.373 
Critical  period  of  nerves,  ii.  66 
Crossed  cerebral  circulation,  i.  443 
Cross-suturing  of  nerve-trunks,  ii.  201 
Cruciate  heat-centre,  ii.  271 
Crying,  i.  454 

Crystalloids,  diffusion  of,  i.  69 
Crystals  of  CO-hsemoglobin,  i.  40 

of  hsemin,  i.  44,  573 

of  haemoglobin,  i.  39 
Cupola  of  the  cochlea,  ii.  375 
Curare,  action  of,  ii.  26 
Currents  of  action  in  muscle,  ii.  150 
in  nerves,  ii.  153 

of  rest,  ii.  147 

theories  as  to  their  cause,  ii.  148 
Curve  of  fatigue,  with  repeated  single  contrac- 
tions, ii.  113 

of  intensity  of  sleep,  ii.  294 

of  muscle  contraction,  effect  of  frequent  ex- 
citations on,  ii.  115 

of  muscular  contractions,  ii.  100 

of  work  for  muscles,  ii.  140 
Cutaneous  nerves,  influence  of,  on  respiration, 
i.  463 

respiration,  i.  422 

secretion,  i.  257 

sensations,  cortical  area  for,  ii.  253 
disturbance  of,  in  disease,  ii.  403 
varieties  of,  ii.  390 

temperature  points,  ii,  398 
Cyauamide,  i.  542 
Cyanogen  gas,  i.  541 

inhalation,  i.  440 
Cynurenic  acid,  i.  571 
Cystein,  i.  546 
Cystin,  i.  547 

Cytology,  definition  of,  i.  31 
Cytoplasmic  changes  in  nerve-cells,  ii.  182 
Cytosin,  i.  579 

"  DANGEROUS  region,"  i.  97 
Daniell  cell,  ii.  28 
Darwin's  theory  of  heredity,  ii.  501 
Death,  definition  of,  ii.  491 

of  the  tissues,  ii.  492 

somatic,  ii.  491 

theory  of,  ii.  492 
Decidua  graviditatis,  ii.  461,  471 

menstrualis,  ii.  458,  461 

reflexa,  ii.  472 

serotina,  ii.  472 

vera,  ii.  472 
Decidual  cells,  ii.  471 

Decomposition,  bacterial,  in  the  intestines,  i.  309 
Defecation,  i.  386 

cerebral  control  of,  ii.  270 

reflex  character  of,  ii.  213 
Defibrinated  blood,  definition  of,  i.  34 

preparation  of,  i.  55 

Degeneration  after  hemisection   of  the  spinal 
cord,  ii.  228 

following  removal  of  motor  cortical  areas,  ii. 
244 

of  cut  nerves,  ii.  78 

of  muscle  after  section  of  its  nerve,  ii.  70 

of  nerve-cells,  ii.  199 

of  nerve-elements,  ii.  197 

of  nerves  after  section,  ii.  69 

reaction  of,  ii.  47 
Deglutition,  i.  372 

action  of  the  epiglottis  in,  ii.  422 


GENERAL  INDEX. 


531 


Deglutition,  aualysis  of,  i.  376 

apmra,  i.  442 
centre  for,  i.  377 
explanat  ii>ii  of,  i.  '>7f> 

nervous  regulation  of,  i.  376 
Deiters,  cells  of,  in  the  organ  of  Corti,  ii.  377 

nucleus  of,  ii.  838 

Demarcation  currents  of  injured  muscle,  ii.  148 
Demilunes,  i.  21!> 
Dendrites.  definition  of,  ii.  174 
Depressor  nerve,  i.  172,  2«:; 
Dermal  sensations,  cortical  area  for,  ii.  253 

path  of  conduction  for.  in  the  cord,  ii.  235 
sensibility,  area  of  distribution  of  the  nerves 

of;  231 

Descending  impulses,  course  of,  ii.  244 
IVsiceation  of  nerve,  ii.  59 
Determinants  of  the  germ-plasm,  ii.  503 
Deutero-proteose,  definition  of,  i.  293 
Deu  topi  asm  of  the  ovum,  ii.  450 

composition  of,  ii.  451 
Development  of  nerve-cells,  ii.  176 
Dextrose,  action  of,  in  delaying  rigor  mortis,  ii. 

164 

on  the  heart,  i.  191 
amount  of,  in  the  blood,  i.  51,  317 
origin  of,  i.  563 

oxidation  of,  in  the  tissues,  i.  317 
storage  of,  i.  563 
Diabetes  mellitus,  dextrose  excreted  in,  i.  354, 

563 

fatty  acids  in,  i.  536 
on  proteid  diet,  i.  329 
phosphorus  excretion  in,  i.  515 
relation  of  the  pancreas  to,  i.  266 
Dialysis,  definition  of,  i.  65 

of  soluble  substances,  i.  69 
Diapedesis    of    maternal   leucocytes    into    the 

foetus,  ii.  476 

Diaphoretics,  effect  of,  on  heat  dissipation,  i.  489 
Diaphragm,  movements  of,  i.  398 
Diarthrosis,  ii.  415 
Diastase,  i.  280 
Diastatic  enzymes,  i.  280,  566 
Diaxonic  nerve-cells,  ii.  178 
Dicrotic  pulse,  i.  144 

wave  of  the  pulse-curve,  i.  143 
Diet,  accessory  articles  of,  i.  357 

average,  for  man,  i.  366  t 
Dietetics,  i.  366 
Differential  manometer,  i.  131 

tones,  ii.  387 
Diffusion,  definition  of,  i.  65 

of  central  nerve-impulses,  ii.  208 

of  impulses  in  the  cord,  influences  affecting,  ii. 

217 

of  nerve-impulse,  peripheral,  ii.  218 
of  proteids,  i.  70 
through  membranes,  i.  66 
Digastric  muscle,  i.  372;  ii.  426 

ion,  action  of  alcohol  on,  i.  535 
•  ric,  i.  287 

influence  of,  on  respiratory  exchanges,  i.  431 
intestinal,  i.  299 
in  the  large  intestine,  i.  309 
of  fats,  i.  305 
of  proteids.  i.  292,  301 
of  starch,   i.  '284 
pancreatic,  i.  301,  308 
purpose  of,  i.  -2?.") 
salivary,  i.  283 
Digitalin,  action  of,  on  coagulation  of  muscle- 

plasma,  ii.  164 
on  muscles,  ii.  137 
Digitalis,  action  of,  on  nerves  and  muscles,  ii.60 

effect  of,  on  the  respiratory  rhythm,  i    12.~> 
Digits,  supernumerary,  ii.  494 


Dioptric  apparatus  of  the  eye,  defects  of,  ii.  :;i  I 
Dioptrics  of  the  eye,  ii.  ;{<H) 
Dioptry,  definition  of,  ii.  :;o| 
Dioxyacetone,  i.  558 
Dioxyplienyl-acetic  acid,  i.  570 
Diphasic  current  of  action,  i.  l.Yj 
Direction,  judgments  of,  by  means  of  auditory 
sensations,  ii. 

of  the  nerve-impulse,  ii.  184 
Disaccharides,  i.  564 

digestion  of,  i.  308 
Disassimilatiou,  definition  of,  i.  19 
Discord,  ii.  387 

Discriminating  sensibility  of  the  skin  for  pres- 
sure, ii.  :;!••_• 

Discriminative  sensibility  for  difference  of  tem- 
perature, ii.  :$«»7 
Discus  proligerus,  ii.  450,  454 
Diseases,  inheritance  of,  ii.  498 
Dispenny,  ii.  471 
Dispersion  of  light,  ii.  316 
Dissociation  of  electrolytes,  i.  67 

of  the  axial  and  focal  adjustments  of  the  eye, 

ii.  312 

Distance,  judgments  of  sensation  by  means  of 
auditory,  ii.  389 

perception  of,  ii.  354 

visual  perception  of,  ii.  348 
Disuse,  effect  of,  on  muscles,  ii.  77 
Diuretics,  action  of,  i.  254 
Dizziness,  ii.  405 

Dogs,  removal  of  cerebrum  in,  ii.  267 
Domestication,   effect  of,  on  menstruation   in 

animals,  ii.  460,  462 
Dorsal  nerve-roots,  efferent  fibres  in,  ii.  203 

roots,  degeneration  resulting  from  section  of, 
ii.  227 

spinal  nerve-roots,  number  of  fibres  of,  ii.  230 
Dreams,  ii.  293 
Drinking-water,  i.  504 
Dropsy,  i.  147 
Drowning,  phenomena  of,  i.  445 

resuscitation  from,  i.  445 
Drugs,  action  of,  on  body-temperature,  i.  472 
on  salivary  glands,  i.  222,  229 
on  sweat-glands,  i.  260 
on  therinogenesis,  i.  484 
on  thermolysis,  i.  489 
Du  Bois-Reymond's  key,  ii.  30 

law  of  stimulation,  ii.  32 

theory  of  currents  of  rest,  ii.  148 
Duct  of  Bartholin,  i.  217 

of  Rivinus,  i.  217 

of  Stenson,  i.  217 

of  Wharton,  i.  217 

of  Wirsung,  i.  231 
Ductus  cochlearis,  structure  of,  ii.  374 

endolymphaticus,  ii.  373 

venosus  of  the  embryo,  ii.  476 
Duration  of  electric  currents,  effect  of,  on  their 

irritating  power,  ii.  46 
Dynamic  equilibrium,  organs  of,  ii.  407 
Dyslysin,  i.  544 

Dyspepsia    accompanying   the   climacteric,    ii. 
490 

cause  of,  i.  309 
Dyspnosa,  definition  of,  i.  441 

effect  of,  on   the  iris.  ii.  :;•_'! 
on  intestinal  movements,  i.  386 

phenomena  of,  i.  444 

varieties  of,  i.  443,  444 

EAR,  analysis  of  composite  tones  by,  ii.  384 
anatomy  of,  ii.  362 

discriminative  sensibility  of,  for  pitch,  ii.  385 
fatigue  of,  ii.  387 
imperfections  of,  ii.  388 


532 


GENERAL  INDEX. 


Ear,  membranous  labyrinth  of,  ii.  372 
ossicles  of,  ii.  365 
sensibility  of,  in  perception  of  time  intervals, 

ii.  388 
Earth-worms,  regeneration  of  lost  parts  in,  ii. 

496 

Eck  fistula,  i.  336 
Edestine,  i.  577 

Efferent  fibres  of  the  optic  nerves,  ii.  240 
impulse  in  reflexes,  co-ordination  of,  ii.  214 
neurones  of  the  dorsal  spinal  nerve-roots,  ii. 

203 

of  the  spinal  cord,  ii.  203 
paths  from  the  cortex,  course  of,  ii.  244 
respiratory  nerves,  i.  463 
Egg  albumin,  absortion  of,  i.  315 
Ejaculation,  ii.  465 
Ejaculatory  duct,  ii.  447 
Elasticity  of  muscle,  ii.  105 
Elastin,  i.  580 
Electric  currents,  correlation  of  their  duration 

with  histological  structures,  ii.  47 
effect  of,  duration  of,  ii.  46 
on  muscles,  ii.  61 
on  nerves,  ii.  62 
their  density,  ii.  41 
galvanic,     effect    of,    on    normal     human 

nerves,  ii.  51 
influence  of  their  direction  in  nerves,  ii.  48 

varying  duration  of,  ii.  47 
methods  of  detecting,  ii.  145 
spread  of,  in  moist  conductors,  ii.  41 
stimulating  effect  of,  ii.  28 
organs,  ii.  145 

Electrical  changes  in  active  glands,  i.  231 
in  the  beating  heart,  i.  152,  153 
in  the  heart  during  vagus  stimulation,  i. 

164 

in  the  retina,  ii.  331 
phenomena  of  muscle  and  nerve,  ii.  144 

of  nerves,  interpretation  of,  ii.  158 
stimulation  of  nerve,  ii.  25 

of  nerves,  law  of,  ii.  32 
Electrodes,  shielded,  ii.  41 

varieties  of,  ii.  29 
Electrolytes,  definition  of,  i.  67 
Electrostatic  changes,  stimulating  action  of,  ii.  42 
Electrotonic  changes  of  conductivity,  ii.  50 
of  irritability,  ii.  64 

in  human  nerves,  ii.  65 
twitch,  ii.  157 
Electrotonus,  ii.  62 
Embryo,  nutrition  of,  ii.  475 

rate  of  growth  of,  ii.  487 
Emigration  of  leucocytes,  i.  83 
Emmetropia,  ii.  313 
Emmetropic  eye,  ii.  312 
Emphysema,   influence   of,  on   the  respiratory 

rhythm,  i.  424 
Emulsification  of  fats,  i.  306 

influence  of  the  bile  on,  i.  307 
Emulsions,  preparation  of,  i.  307,  559 
Encephala,  classification  of,  according  to  weight, 

ii.  275 
Encephalon,  specific  gravity  of,  ii.  275 

weight  of,  ii.  274,  275 
End-bulbs,  sensory,  ii.  392 

Endocardiac  pressure.   See  Intracardiac  pressure. 
Endolymph,  ii.  372 
End-organs,  importance  of,  in  touch  sensations, 

ii.  396 

transmission  of  excitation  by  means  of,  ii.  82 
Enemata,  nutritive,  i.  315 
Energy  liberated  in  contracting  muscles,  ii.  138 

potential,  of  foods,  i.  364 

Engelmann's  theory  of  the  nature  of  muscular 
contraction,  ii.  105 


Environment,  influence  of,  on  organisms,  ii.  493 
Enzyme  action,  theories  of,  i.  282 

glycolytic,  i.  354 
Enzymes,  classification  of,  i.  280 

composition  of,  i.  279 

definition  of,  i.  279 

effect  of,  on  blood  coagulation,  i.  63 

general  properties  of,  i.  281 

mode  of  action  of,  i.  282 

of  pancreatic  juice,  i.  232,  235,  301 

solubility  of,  i.  281 
Eosinophiles,  i.  47 
Epididymis,  ii.  447 
Epigenesis,  theory  of,  ii.  500,  504 
Epiglottis,  ii.  421 
Epiguanin,  i.  554 
Epinephriu,  i.  272,  572 

action  of,  on  muscles,  ii.  138 
Episarcin,  i.  554 
Equilibrium  of  the  body,  definition  of.  ii.  404 

relation  of  the  cerebellum  to,  ii.  273 

sense  of,  ii.  404 
Erection,  ii.  464 

of  the  heart,  i.  114 

spinal  centre  for,  ii.  464 
Erector  clitoridis  muscle,  ii.  464 

penis,  action  of,  in  erection,  ii.  464 

muscles,  ii.  449 
Erectores  spinse  muscles,  respiratory  action  of, 

i.  405 

Erythroblasts,  i.  45 
Erythrodextrin,  i.  285,  566 
Erythrose,  i.  562 

Escape  of  the  heart  from  vagus  inhibition,  i.  163 
Eserin,  action  of,  on  nerves  and  muscles,  ii.  60 
Ether,   action    of,    on    coagulation   of   muscle- 
plasma,  ii.  164 
on  conductivity  of  nerves,  ii.  93 

effect  of,  on  nerve-currents,  ii.  155 

ethyl,  i.  536 

vapor,  action  of,  on  nerves,  ii.  60 
Ethereal  sulphates,  i.  506 

of  the  urine,  i.  572 
Ethers,  properties  of,  i.  536 
Ethyl  alcohol,  i.  535 
Ethylamine,  i.  541 
Eudiometer,  i.  421 
Eupncea,  definition  of,  440 
Eustachian  tube,  ii.  363 
function  of,  ii.  369 

Excitability,  changes  in,  during  Wallerian  de- 
generation, ii.  69 

Excitation,  cardiac,  electrical  variation  in,  i.  153 
propagation  of,  i.  153,  154 

wave,  cardiac,  i.  152 
Excretin,  occurrence  of,  in  feces,  i.  320 
Excretions,  definition  of,  i.  213 
Exercise,  effect  of,  on  growth-,  ii.  489 
on  metabolism,  i.  359 
on  muscular  endurance,  ii.  76 
on  pulse-rate,  i.  121 
Exhaustion  of  muscles,  ii.  72 
Expiration,  forced,  muscles  of,  i.  407  , 

movements  of,  i.  406 
Expiratory  centre,  i.  457 
Explosive  consonants,  ii.  437 
Extensibility  of  muscle,  ii.  105 
External  auditory  meatus,  ii.  362 

ear,  anatomy  of,  ii.  362 

rectus  muscle,  ii.  299 
Extirpation  of  the  liver,  i,  336 

of  the  pancreas,  i.  266 

of  the  thyroids,  i.  268 
Extractives,  nitrogenous,  of  muscle,  ii.  166 

of  the  blood,  i.  50,  51 
Extracts,  adrenal,  i.  271 

ovarian,  i.  274 


GENERAL  INDKX. 


533 


Extracts,  testicular,  i.  273 

thyroid,  i.  '-'<>!) 
Extrapolar  region,  ii.  o'-j 
Exudations,  secretion  of,  i.  •Jl."> 
Eye,  abnormal  positions  of,  after  cerebellar  in- 
jury, ii.  272 

adaptation  of,  to  light,  ii.  •  '••:>< 

axes  of  rotation  of,  ii.  -!»!• 

chromatic  aberration  of.  ii.  ol<i 

constants,  changes  in,  during  accommodation, 

defects  in  the  dioptric  apparatus  of,  ii.  314 
dioptric  apparatus  of,  ii.  300 
mechanical  movements  of.  ii.  -.'!»- 
movements,  binocular  co-ordination  in,  ii.  300 

extent  of,  ii.  298 
muscles  of,  ii.  2SW 

innervation  of,  ii.  300 
optical  constants  of,  ii.  303 

power  of,  ii.  304 
positions  of,  ii.  299 
refractive  media  of,  ii.  302 

surfaces  of,  ii.  303 
spherical  aberration  of.  ii.  315 

FALLOPIAN  tubes,  ii.  443,  456 

False  amuion,  ii.  473 

Falsetto  register  of  the  voice,  ii.  433 

Far-point  of  vision,  ii.  312 

Fat,  affinity  of  cell-substance  for,  i.  568 

nutritive  history  of,  ii.  559 

origin  of,  from  carbohydrates,  i.  352 

from  proteid,  i.  351,  560 
Fat-absorption,  influence  of  bile  on,  i.  325 

mechanism  of,  i.  318 
Fat-combustion,  equivalent  of,  i.  365 
Fat-formation  in  the  body,  i.  351,  560 
Fatigue,  cerebral  anaemia  from,  ii.  288 

curve  with  repeated  single  contraction,  ii.  113 

effect  of,  on  height  of  contraction,  ii.  113 
on  muscular  contraction,  ii.  130 
on  rigor  caloris,  ii.  165 
mortis,  ii.  160 

from  voluntary  muscular  contraction,  ii.  134 

in  nerve-fibres,  ii.  195 
—  of  central  nervous  system,  ii.  289    *•» 

of  motor  end-organs,  ii.  83 

of  muscle,  ii.  6h'.  70 

recovery  from,  ii.  73^^ 
-    of  nerve-cells,  ii.  136T191 

of  nerves,  ii.  75,  96 

of  retina,  ii.  344 

relation  of,  to  sleep,  ii.  291 

theories  of,  ii.  72 

to  auditory  sensations,  ii.  387 
Fat-metabolism,  acetone  formation  in,  i.  537 
Fats,  absorption  of,  in  the  stomach,  i.  313 

action  of,  on  gastric  secretion,  i.  241 

digestion  of,  i.  305 

dynamic  value  of,  i.  475 

emulsification  of,  i.  306 

gastric  digestion  of,  i.  297 

nutritive  value  of,  i.  277,  350 

of  feces.  i.  319 

of  muscle,  ii.  167 

origin  of.  in  the  body.  i.  351,  560 

relation  of.  to  glycogen  formation,  i.  329 
to  muscular  work.  ii.  74 

synthesis  of,  from  fatty  acids,  i.  558 
Fatty  acids,  monobasic,  i.  532 

degeneration  in  phosphorus-poisoning,  i.  514 
Feces,  composition  of,  i.  319 
Fellic  acid,  i.  543 
Female  births,  relative  number  of,  ii.  483 

pronucleus,  ii.  •!.">:; 
Females,  rate  of  growth  in,  ii.  488 
Fenestra  oval  is,  ii.  363 


rotunda,  ii.  :;H.'{.  375 

ii>e  of,  ii.  :;TI; 

Ferment,  myo*iaofen-<x>«cul»ting,  ii.  161 
Fermentation,  alcoholic,  i.  535 

lactic,  i.  .")!."> 

Ferments,  unorua nixed,  i.  279 
Ferralin,  i 
Ferric  ph., 
Ferrosulphide,  i.  528 
Fertilization,  ii.  466 
Fetal  membranes,  ii.  17_' 
Fever,  body-temperature  in,  i.  l?j 
cause  of,  i.  473 
effect  of,  on  blood  coagulation,  i.  55 

on  the  respiratory  centre,  i.  458 
heat  dissipation  in,  i.  4*!) 
Fibrillar  contraction  of  the  heart,  i.  181,  183 
Fibrin  ferment,  i.  56 

absence  of,  in  circulating  blood,  i.  61 
nature  of,  i.  57 
origin  of,  i.  59 
preparation  of,  i.  59 
mode  of  deposition  of,  i.  54,  55 
Fibrin-globulin,  i.  56 
Fibrinogen,  i.  53,  54 
Fibrinoplastin,  i.  56 
Fictitious  meal,  effect  of,  on  gastric  secretion,  i. 

239 

Field  of  vision,  binocular  rivalry  of,  ii.  358 
Filtration  processes  in  secretion,  i.  213,  215 
Fimbrise  of  the  Fallopian  tube,  ii.  456 
Fish,  bony,  removal  of  cerebral  hemispheres  in, 

ii.  263 

semicircular  canals  in,  ii.  407 
visual  accommodation  in,  ii.  306 
Flavors,  nutritive  value  of,  i.  359 
Flicker  photometry,  ii.  345 
Fluorine,  occurrence  of,  i.  510 
Focal  illumination  of  the  eye,  ii.  320 
Foci,  conjugate,  ii.  302 

principal,  ii.  302 

Food,  combustion  equivalent  of,  i.  365 
definition  of,  i.  275 
dynamic  value  of,  i.  364 
effect  of,  on  respiratory  activity,  i.  431 
energy  liberated  by,  i.  474 
influence  of,  on  thermogenesis,  i.  484 
rate  of  movement  of,  in  the  intestines,  i.  314 
Food-stuffs,  classification  of,  i.  276 
composition  of,  i.  278 
Liebig's  classification  of,  i.  346 
Foramen  ovale  of  the  foetal  heart,  ii.  476 
Force  of  ventricular  systole  during  vagus  stim- 
ulation, i.  i«;:; 

Forced  movements  after  section  of  the  semicir- 
cular canals,  ii.  405 
in  frogs,  ii.  266 
Formic  acid,  i.  534 
aldehyde,  i.  533 
Formose,  synthesis  of,  i.  533 
Fovea  centralis,  ii.  :;-J7 
Franklin's  theory  of  color  vision,  ii.  337 
Frequency  of  respiration,  conditions  affect in<r. 

i.'  I'.T. 

relation  of,  to  the  pulse-rate,  i.  426 
Frictionals,  ii.  l.'!7 
Frogs,  removal  of  cerebral  hemispheres  in,  ii. 

-.'.;} 
striped  muscle  of,  time  of  single  contraction 

in.  ii.  108 

Frontal  lobes  of  the  hemispheres,  effect  of  re- 
moval of.  ii.  262 
Fuhlspiire,  cortical,  ii.  •-'."•_> 
Fundamental  tone,  definition  of,  ii.  383 

GALACTOSK,  i.  r,<;-2,  564 
Gall-bladder,  motor  nerves  of,  i.  248 


534 


GENERAL  INDEX. 


Galvani,  Luigi,  ii.  28 

Galvanic  current,  action  of,  on  conductivity,  ii. 

94 

contracture  effect  of,  on  muscles,  ii.  131 
effect  of,  on  heart  apex,  i.  150 
on  muscles,  ii.  61 
on  nerves,  ii.  62 

of  making  and  breaking,  ii.  31 
on  normal  human  nerves,  ii.  51 
opening  and  closing  contractions  with,  ii.  38 
Galvanometers,  ii.  145 
Galvanotoiius,  ii.  54,  131 
Gamogenesis,  ii.  440 
Ganglion  spirale  of  the  ear,  ii.  376 

submaxillary,  i.  219 
Ganglion-cells,  conduction  in,  ii.  97 

of  the  heart,  i.  148 
Gas  analysis,  i.  421 

Gaseous  exchanges  in  the  brain,  ii.  288 
interchanges  in  the  lungs,  i.  410,  417 

in  the  tissues,  i.  419 
Gas-pump,  description  of,  i.  420 
Gases,  absorption  of,  i.  414 

in  the  blood,  respiratory  changes  in,  i.  411 
in  the  large  intestine,  i.  320 
law  of  partial  pressure  of,  i.  413 
of  muscle,  ii.  168 
of  the  saliva,  i.  221 
poisonous,  inhalation  of,  i.  440 
solutions  of,  i.  415 
Gastric  digestion  of  proteids,  i.  292 
fistulse,  i.  288 

glands,  histology  of,  i.  237 
secretory  changes  in,  i.  242 
value  of,  i.  299 
juice,  acidity  of,  i.  289 

action  of,  on  carbohydrates,  i.  296 

on  milk,  i.  296 
antiseptic  property  of,  i.  288 
artificial,  preparation  of,  i.  291 
composition  of,  i.  238,  288 
methods  of  obtaining,  i.  287 
mineral  constituents  of,  i.  530 
secretion,  inhibition  of,  i.  241 
nervous  regulation  of,  i.  239 
normal  mechanism  of,  i.  240 
relation  of,  to  the  character  of  the  diet,  i. 

241 

stimulants  for,  i.  241 

Gelatin,  digestion  of,  in  the  stomach,  i.  297 
nutritive  value  of,  i.  349 
proteid,  protecting  power  of,  i.  567 
Gelatoses,  i.  297 

Geminal  fibres  of  the  pyramidal  tracts,  ii.  245 
Gemmules  of  the  germ-plasma,  ii.  499 
Genio-hyoid  muscle,  ii.  426 

function  of,  in  mastication,  i.  372 
Gerhardt's  reaction,  i.  537 
Germinal  spot  of  the  ovary,  ii.  450 
transmission  of  infectious  diseases,  ii.  498 
vesicle,  structure  of,  ii.  450 
Germ-plasm  as  a  basis  of  heredity,  ii.  499 
continuity  of,  ii.  502 
definition  of,  ii.  496 
morphological  nature  of,  ii.  499 
origin  of,  ii.  499 
Gestation,  duration  of,  ii.  478 
Gland,  adrenal,  i.  271 
mammary,  i.  262 
pancreatic,  i.  231,  266 
parathyroid,  i.  268 
parotid,  i.  217 
sublingual,  i.  217 
submaxillary,  i.  217 
thyroid,  i.  267 

Gland-cells,  electric  currents  in,  ii.  145 
selective  activity  of,  i.  27 


|  Glands,  albuminous,  histology  of,  i.  216 

Brunner's,  i.  243 

cutaneous,  i.  257 

gastric,  i.  237 

intestinal,  i.  243 

Lieberkiihn's,  i.  243 

mucous,  histology  of,  i.  216 

of  Bartholin,  ii.  462 

of  Littre,  ii.  448 

salivary,  i.  215 

sebaceous,  i.  257 

serous,  definition  of,  i.  216 

structure  of,  i.  211 

sweat,  i.  259 
Glans  penis,  ii.  449 
Glauber's  salt,  i.  522 
Gliding  movements  in  joints,  ii.  416 
Globin,  i.  37 

Globulicidal  action  of  serum,  i.  36 
Globulins,  i.  577 

Glomeruli,  renal,  secretory  function  of,  i.  253 
Glossopharyngeal  nerve,  gustatory  function  of, 
ii.  410 

nerves,  influence  of,  on  respiration,  i.  462 
Glossopharyngeus,  central  conduction  paths  for, 

ii.  236 
Glottis,  ii.  423  » 

oedema  of,  ii.  422 

respiratory  movements  of,  i.  408 ;  ii.  429 
Glucosamin,  i.  564 
Glucoses,  i.  562 

synthesis  of,  i.  563 
Glutamic  acid,  i.  558 
Glutamin,  i.  558 
Glutolin,  i.  53 
Glutoses,  i.  297 
Glycerin,  i.  558 

aldehyde,  i.  558 

phosphoric  acid,  i.  559. 
Glycerose,  i.  558 
Glycocoll,  i.  537,  543 

in  muscles,  ii.  167 

nutritive  history  of,  i.  538 
Glycogen,  i.  566 

amount  of,  in  the  liver,  i.  327 

demonstration  of,  in  the  liver,  i.  327 

distribution  of,  i.  330 

effect  of  exercise  on,  i.  361 
of  starvation  on,  i.  362 
of  sugars  on,  i.  328 

function  of,  i.  329 

of  muscles,  i.  330 ;  ii.  167 

origin  of,  i.  326,  327 

properties  of,  i.  327,  566 
Glycogen-eliminatiou  of  the  liver,  i.  265 
Glycogen-formation,  effect  of  proteid  diet  on,  i. 

328 

Glycogen-formers,  i.  328 
Glycogenic  theory,  i.  329 
Glycolysis,  i.  354 
Glycolytic  enzyme,  i.  280,  354 

origin  of,  i.  267 
Glyco-proteids,  i.  576,  578 
Glycosazones,  i.  562 
Glyco-secretory  nerves,  i.  248 
Glycoses,  i.  562 
Glycosuria  after  pancreas  extirpation,   i.  266, 

563 

Glycuronic  acid,  i.  567 
Gmelin's  test  for  bile-pigments,  i.  322,  574 
Goblet  cells,  i.  216 
Goitre,  i.  269 

Golgi,  organ  of,  in  tendons,  ii.  402 
Gout,  i.  557 

Graafian  follicles,  ii.  454 
Grammeter,  i.  477 
Gram-molecular  solution,  i.  67 


GENERAL  INDEX. 


536 


Graphic    method    of    studying    muscular  con 

tractions,  ii.  99 
Gravity,  influence  of,  on  cerebral  circulation,  ii. 

287 
Gray  matter  of  the  cerebrum,  water  contents  of, 

ii.  274 

Growth  after  birth,  ii.  487 
before  birth,  ii.  l-<: 
increase  of  fibres  of  the  cortex  during,  ii.  282 

of  functional  neurones  during,  ii.  •'-•' 
influence  of  sex  on  the  rate  of,  ii.  488 
influences  which  modify,  ii.  489 
of  nerve-cells,  ii.  176 
Guanidin.  i.  550 
Guanin,  i.  :«9f  554 
Gunzburg's  reagent,  i.  508 
Gustatory  nerves,  ii.  410 

sensations,  ii.  411 
Guttural  consonants,  ii.  437 
Gymnemra  silvestre,  action  of,  on  taste-nerves, 
ii.  413 

H.EMATIN,  i.  37,  44,  573 
Haematogen,  i.  356 

composition  of,  i.  579 

nutritive  value  of,  i.  528 
Haeinatoidiu,  i.  44,  323,  574 
Haematopoiesis,  definition  of,  i.  45 
Haematopoietic  tissues,  embryonic,  i.  46 
Hsematoporphyrin,  i.  44,  574 
Haemerythriu,  i.  578 
Haemin,  i.  44,  573 
Haeniochromogen,  i.  37,  44,  573 
Haemocyanin,  i.  578 
Haemoglobin,  i.  573 

absorption  spectra  of,  i.  43 

action  of.  on  carbonates,  i.  517 

affinity  of,  for  COa,  i.  417 

amount  of,  i.  38 

compounds  of,  with  gases,  i,  38 

condition  of.  in  the  corpuscles,  i.  35 

crystals  of,  i.  39 

decomposition  products  of,  i.  37 

derivatives  of,  i.  44 

distribution  of,  in  animals,  i.  37 

elementary  composition  of,  i.  37 

molecular  formula  of,  i.  37,  38 

nature  of,  i.  37 

of  muscle-serum,  ii.  166 

oxygen  capacity  of,  i.  416 
Hair-cells  of  the  crista  acustica,  ii.  374 

of  the  organ  of  Corti,  ii.  377 
Hamulus,  ii.  376 
Harmonic  overtones,  ii.  386 
Harmony,  ii.  387 
Hawking,  i.  454 
Head  register  of  the  voice,  ii.  433 

vaso-motor  nerves  of,  i.  204 
Hearing,  ii.  362 

keenness  of,  ii.  371 

relation  of,  to  speech,  ii.  431 
Heart,  anaemia  of,  i.  183 

artificial  stimulation  of,  i.  156 

augmentor  nerves  of,  i.  167 

cause  of  rhythmic  beat  of,  i.  148 

centripetal  nerves  of,  i.  171 

changes  in,  due  to  pregnancy,  ii.  477 
in  form  of,  i.  113 
in  position  of.  i.  114 
in  size  of,  i.  112 

compensatory  pause  of,  i.  156 

diphasic  action  currents  in,  ii.  152 

electrical  currents  of,  i.  152 

erection  of,  i.  114 

fibrillar  contraction  of,  i.  181 

heat  produced  by.  i.  108 

human,  output  of,  i.  106 


Heart,  intrinsic  nerves  of,  i.  148 
illation  of,  i.  148,  1>7  ;   ii.iiii 
lymphatics  of,  i.  186 
muscle,  atrophy  of,  after  section  of  the  vagi, 

i.  167 

conduction  of  the  contraction  wave  by ,  i   1  ."4 
normal  stimulus  of,  i.  ir.i 
rate  of  conduction  in,  ii.  89 
rhythinicity  of,  i.  151 
rigor  mortis  of,  ii.  162 
nutrition  of,  i.  179 
position  of,  i.  117 
pumping  action  of,  i.  78 
refractory  period  of,  i.  156 
suction -pump  action  of,  i.  134 
tetanus  of,  i.  165 
vaso-motor  nerves  of,  i.  206 
work  done  by,  i.  107 
Heart-beat,  abnormal  sequence  of,  i.  152 
conduction  of,  from  auricles  to  ventricles,  i. 

155 

effect  of  blood-supply  on,  i.  186 
genesis  of,  i.  149,  150 
heat  produced  by,  i.  108 
rate  of,  i.  121 

voluntary  control  of,  ii.  214 
Heart-pause,  i.  122 
Heart-sounds,  i.  118 
Heat,  expenditure  of,  i.  476 
income  of,  i.  475 
source  of,  i.  474 
Heat-centres,  ii.  271 
Heat-dissipation,  conditions  affecting,  i.  485 

estimation  of,  i.  480 
Heat-dyspnosa,  i.  441,  443 
Heat-production,  amount  of,  i.  364 
by  the  heart,  i.  108 
conditions  affecting,  i.  482 
estimation  of,  i.  481 
in  contracting  muscles,  ii.  138 
in  muscles,  ii.  142 
in  nerves,  ii.  96 
in  rigor  mortis,  ii.  160 
relation  of,  to  respiratory  activity,  i.  483 
Heat-rays  of  ether,  ii.  331 
Heat-regulation,  i.  495 
Height  of  contraction,  dependence  of,  on  the 

load,  ii.  Ill 

effect  of  temperature  on,  ii.  136 
Helico-proteid,  composition  of,  i.  579 
Helicotrema,  ii.  376 
Hemianopsia,  anatomical  basis  for,  ii.  240 

from  cortical  lesions,  ii.  255 
Hemi-peptone,  decomposition  of,  by  trypsin,  i. 

303 

definition  of,  i.  293 

Hemisections  of  the  cord  alternating  at  differ- 
ent levels,  ii.  230 

Brown-Se'quard's  paralysis  from,  ii.  233 
degeneration  resulting  from,  ii.  228 
effect  of,  in  man,  ii.  233 

on  sensation  and  motion,  ii.  230 
in  animals,  ii.  234 
physiological  effect  of,  ii.  234 
Hemorrhage,  effect  of,  on  hematopoiesis,  i.  46 
fatal  limits  of,  i.  63 
regeneration  of  the  blood  after,  i.  63 
relation  of,  to  blood -pressure,  i.  91 
saline  injections  after,  i.  64 
Hemorrhagic  dyspno3a,  i.  444 
Hepatin.  i.  .vjs' 
Heredity,  definition  of.  ii.  493 
physical  basis  ,,f.  i.  28 
theories    of.  ii.  .|!»- 

Hering's  theory  of  color  vision,  ii.  336 
Hermann's  theory  of  currents  of  rest,  ii.  148 
Heteromita,  reproduction  in.  ii.  440 


536 


GENERAL  INDEX. 


Hexon-bases,  origin  of,  i.  580 
Hexoses,  i.  562 

Hibernation,  effect  of,  on  the  respiratory  quo- 
tient, i.  438 
Hiccough,  i.  455 

Higher  brain-centres  for  the  heart,  i.  178 
Hinge-joints,  ii.  416 

Hippuric  acid,  nutritive  history  of,  i.  339 
Histidin,  i.  552 
Histohsematin,  i.  44,  578 
Histology  of  striped  muscle,  ii.  104 
Histon,  i.  580 

effect  of,  on  intra vascular  clotting,  i.  61 
Hofacker-Sadler  law,  ii.  484 
Holmgren  method  for  testing  color  vision,  ii. 

339 

Homogentisic  acid,  i.  570 
Homothermous  animals,  i.  467 
Hpropter,  ii.  359 

Hiifner's  method  of  urea  determination,  i.  549 
Human  muscles,  fatigue  of,  with  artificial  stim- 
ulation, ii.  134 
Hunger,  ii.  404 

Hunger-centre,  clinical  evidence  for,  ii.  404 
Hydra,  regeneration  of  lost  parts  in,  ii.  496 
Hydrsemia  from  saline  injections,  i.  69 
Hydraemic  plethora,  effect  of,  on  lymph  secre- 
tion, i.  74 

Hydration,  nature  of  the  process  of,  i.  503 
Hydriodic  acid,  i.  509 
Hydrobilirubin,  i.  320 
Hydrobromic  acid,  i.  509 
Hydrocarbons,  saturated,  i.  531 
Hydrochloric  acid,  occurrence  of,  i.  507 
of  the  gastric  juice,  i.  238 
preparation  of,  i.  507 
properties  of,  i.  508 
secretion  of,  i.  289 
tests  for,  i.  508 
Hydrocumaric  acid,  i.  570 
Hydrocyanic  acid,  i.  542 

action  of,  on  coagulation  of  muscle-plasma, 

ii.  164 
Hydrofluoric  acid,  circulation  of,  in  the  body,  i. 

510 
Hydrogen,  inhalation  of,  i.  440 

occurrence  of,  i.  499 

peroxide,  i.  505 

preparation  of,  i.  500 

properties  of,  i.  500 
Hydrolysis  by  enzyme  action,  i.  282 

definition  of,  i.  504 

of  fats,  i.  305 

of  proteids,  i.  292 
Hydroquinone,  i.  569 
Hymen,  ii.  462 
Hyo-glossus  muscle,  ii.  426 
Hypersesthesia,  homolateral,  after  hemisection 

of  the  cord,  ii.  233 
Hypermetropia,  ii.  313 

range  of  accommodation  in,  ii.  314 
Hyperpnoea,  i.  440 

from  muscular  activity,  i.  442 
Hypertonic  solutions,  physiological  definition 

of,  i.  69 

Hypertonicity,  definition  of,  i.  37 
Hypophysis  cerebri,  function  of,  i.  273 
Hypotonicity,  definition  of,  i.  37 
Hypoxanthin,  i.  553 

of  muscles,  ii.  167 

relation  of,  to  uric-acid  formation,  i.  338 

ICE  calorimeter,  principle  of,  i.  504 
Icterus,  i.  249,  544 
Idants  of  the  germ -plasm,  ii.  503 
Idiomuscular  contraction,  ii.  27,  92,  128 
Idioplasm  as  a  basis  of  heredity,  ii.  499 


Idio-ventricular  rhythm,  i.  152 

Ids  of  the  germ -plasm,  ii.  503 

Illusions,  visual,  in  sizes  of  objects,  ii.  354 

of  space  perception,  ii.  351 
Imbibition  of  water,  i.  504 
Immunity,  inherited,  ii.  498 
Impregnation,  ii.  466 
Incus,  ii.  366 

Independent  irritability  of  muscle,  ii.  25 
Index  of  refraction  of  the  aqueous  humor,  ii. 

303 

of  the  lens,  ii.  303 
of  the  vitreous  humor,  ii.  303 
Indifferent  point  of  polarized  nerves,  ii.  64 
Indirect  vision,  color  sensations  in,  ii.  333 
Indol,  i.  571 

elimination  of,  i.  340 
occurrence  of,  in  feces,  i.  320 
Induced  currents,  making  and  breaking  shocks 

with,  ii.  40 

prevention  of  spread  of,  ii.  44 
electric  currents,  stimulating  effect  of,  ii.  33 
Induction  apparatus,  schema  of,  ii.  33 
Infections,  intra-uterine,  ii.  498 
Infectious  diseases,  germinal  transmission  of,  ii. 

498 
Inferior  laryngeal  nerve,  respiratory  function 

of,  i.  464 

mesenteric  ganglion,  reflex  activity  of,  i.  392 
oblique  muscle,  ii.  299 
rectus  muscle,  ii.  299 
Inflammation,   emigration  of  leucocytes  in,  i. 

83 

Infra-hyoidei  muscles,  i.  405 
Infundibular  body,  function  of,  i.  272 
Inharmonic  overtones,  ii.  386 
Inheritance,  facts  of,  ii.  494 
of  acquired  characters,  ii.  496 
of  diseases,  ii.  498 
theories  of,  ii.  498 

Inhibition  from  cortica*  stimulation,  ii.  224 
in  the  central  nervous  system,  ii.  224 
of  the  heart,  reflex,  i.  172 
Inhibitory  centre,  cardiac,  localization  of,  i.  176 

ton  us  of,  i.  176 
centres,  respiratory,  i.  457 
nerves  of  the  heart,  i.  161 
of  the  intestines,  i.  385 ,, 
of  the  pancreas,  i.  233 
of  the  spleen,  i.  333 
of  the  stomach,  i.  382 
Innervation  of  the  blood-vessels,  i.  192 

of  the  heart,  i.  148 
Inorganic  salts  of  the  blood,  i.  50 
of  urine,  i.  341 

relation  of,  to  blood  coagulation,  i.  56,  57 
to  irritability,  ii.  59 
to  the  heart  beat,  i.  151,  189 
Inosit,  i.  573 
Insanity,  relation  of  brain-weight  to,  ii.  278 

variations  of  muscular  tonus  in,  ii.  220 
Insect  muscle,  time  of  contraction  in,  ii.  108 
Inspiration,  enlargement  of  the  thorax  in,  i. 

398 

muscles  of,  i.  398,  404 
Inspiratory  centre,  i.  457 
Intensity  of  visual  sensations,  ii.  339 
Intercostales  muscles,  respiratory  action  of,  i. 

402,  407 
Intermedius  nerve  of  Wrisberg,  central  path  of, 

ii.  236 

Intermittent  pulse,  i.  141 
Internal  capsule,  grouping  of  fibres  in,  ii.  248 
ear,  anatomy  of,  ii.  371 
rectus  muscle,  ii.  299 
secretion,  definition  of,  i.  265 
of  the  adrenal  bodies,  i.  272 


GENERAL   AV/>AA. 


537 


Internal  secretion  of  the  kidneys,  i.  ^74 
of  the  liver,  i.  265 
of  the  ovaries,  i.  27-1 
of  the  pancreas,  i.  :J(i(-> 
of  the  pituitary  body,  i.  -J73 
of  the  testis,  i.  273 
of  tlu«  thyroids,  i.  27u 
Intestinal  eontent>,  reaction  of,  i.  310 
digestion,  i.  2J>9 
juice,  i.  213 
movements,  i.  382-385 
Intestines,  iuuervatiou  of,  i.  364 

intrinsic  nervous  mechanism  of,  i.  384 
large,  absorption  in,  i.  314 
penduhir  movements  of,  i.  384 
peristalsis  nf.  i.  382 
putrefactive  changes  in,  i.  310 
small,  absorption  in,  i.  313 
vaso-motor  nerves  of,  i.  '206 
Intraeardiac  pressure,  i.  107,  125,  126 

methods  of  measuring,  i.  129,  130 
Intraeranial  pressure,  relation  of,  to  blood-pres- 
sure, ii.  287 

Intra-ocular  images,  ii.  320 
Intrapolar  region,  ii.  62 
Intrapulmonary  pressure,  i.  408 
Intrathoracic  pressure,  i.  397,  409 
Intravascular  clotting,  i.  60,  61 
Intrinsic  nerves  of  the  heart,  i.  148 
Introductory  contractions  of  a  contraction  series, 

ii.  113 

peak  of  tetanus  curves,  ii.  124 
Inversion  of  retinal  images,  ii.  305 
Invertase,  occurrence  of,  i.  308 
Invertebrates,  conduction  in  the  nerves  of,  ii.  91 
Invertine,  definition  of,  i.  280 
Involuntary  muscles,  rigor  mortis  of,  ii.  162 
Iodine,  i.  509 

lodothyrin,  properties  of,  i.  270 
Ionic  theory  of  solutions,  i.  67 
lon-proteid  compounds  of  muscle,  ii.  168 
Iris,  dilator  nerves  of,  ii.  324 
direct  response  to  light  by,  ii.  324 
innervation  of,  ii.  '.'>'-•'> 
movements  of,  in  accommodation,  ii.  309 

rate  of,  ii.  325 
muscles  of,  ii.  323 

relation  of,  to  spherical  aberration,  ii.  315 
Iron,  amount  of,  in  haemoglobin,  i.  39 
excretion  of,  i.  530 
inorganic  absorption  of,  i.  529 
nutritive  history  of,  i.  528 
occurrence  of,  i.  528 
salts,  excretion  of,  i.  356 

nutritive  value  of,  i.  356 
synthesis  of,  into  haemoglobin,  i.  529 
Irradiation  in  the  retina,  ii.  349 
of  medullary  centres,  i.  201 
of  nerve-impulses  in  the  central  nervous  sys- 
tem, ii.  208 
Irrigating  fluids  for  the  isolated  heart,  i.  189, 

191 

Irritability,  definition  of,  ii.  20,  23 
effect  of  blood-supply  on,  ii.  66 
of  constant  current  on,  ii.  62 
of  repeated  stimuli  on,  ii.  65 
of  living  matter,  i.  18 
of  muscle,  ii.  'J."> 
of  nerve-fihn-s.  ii.  21 
of  nerves,  ii.  24 

and  muscles,  conditions  affecting,  ii.  55 
effect  of  section  on,  ii.  69 
of  ova,  ii.  22 
Irritants,  classification  of,  ii.  23 

conditions  determining  their  efficiency,  ii.  28 
effect  of,  on  irritability,  ii.  55 

of  variations  in  strength  of,  ii.  39 


Irritants,  n-latinn  of,  to  the  IV^POIIM-.  ii.  Ml 

Ischa-mia  »!'  heart  mnsele,  i.  181 

Ischio-cavernosi  muscle.-,,  ii.   1  )!» 

Isrhio  caverno.Mis.  action  of.  in  erection,  ii    |>;i 

Iso-lmtyl  alcohol,  i.  .">:;;• 

Iso-butyrie  acid,  i.  :>:;!» 

ISO-dynamic  equivalence  of  foods,  i,  365 

Isolated  apex  of  frog's  heart.  L  186 

conduction  in  nerve-trunks,  ii.  T'.i 
Isolation  of  tin-  heart,  i.  148,  191 
Isomaltose,  i.  .'»»;:> 

Isometric  contractions,  definition  of,  ii.  110 
Iso-pentyl  alcohol,  i.  :>:;!» 
Isotonic  contractions,  definition  of,  ii.  110 

solutions,  i.  36,  69 
Isotouicity,  i.  36,  68 

Isotropic  substance  of  muscle-fibres,  ii.  104 
Iso-valeriaiiic  acid,  i.  539 

JAUNDICE,  i.  249,  544 
Jecorin,  i.  564 

Joints,  classification  of,  ii.  415 
Jumping,  ii.  420 

KARYOKINESIS,  i.  20 

Karyokinetic  figures  in  mature  nerve-cells,  ii. 

202 

Katabolism,  definition  of,  i.  19 
Katelectrotonus,  ii.  (>•_' 
Kathodal  contraction,  ii.  35 
Kathode,  physical  definition  of,  ii.  52 

physiological  definition  of,  ii.  52 
Keratin,  i.  580 
Ketoses,  definition  of,  i.  561 
Keys,  electric,  ii.  30 
Kidneys,  blood-flow  through,  i.  255 

histology  of,  i.  249 

internal  secretion  of,  i.  274 

nerve-endings  in,  i.  251 

vaso-motor  nerves  of,  i.  207,  256 
"  Klopf-versuch  "  of  Goltz,  i.  175 
Knee-kick,  reinforcement  of,  ii.  222 
Krause's  membrane,  ii.  104 
Kymograph,  i.  89 

LABIA  majora,  ii.  462 

minora,  ii.  462 
Labial  consonants,  ii.  437 
Labio-dental  frictionals,  ii.  438 
Labium  tympanicum  of  the  internal  ear,  ii.  377 

vestibulare  of  the  limbus.  ii.  377 
Labor,  nature  of,  ii.  481 

stages  of,  ii.  47!> 
Labor-pains,  ii.  479 

Labyrinth  of  the  ear,  anatomy  of,  ii.  371 
Lactalbumin,  i.  :J'il 
Lactation,  ovtilation  during,  ii.  456 
Lacteal  vessels,  i.  318 
Lacteals,  absorption  through,  i.  311 
Lactic  acid,  i.  545 

fermentation,  i.  545 
occurrence  of,  in  the  stomach,  i.  289 
of  muscles,  ii.  168 
Lacto-globulin,  i.  261 
Lactose,  i.  262,  565 
Laky  blood,  i.  35 
Lamina  spiralis,  ii.  372 

Laminae  of  medullary  tube  in  tin-  fn-tus.  ii.  •?(>.") 
Langerhans,  bodies  of,  i.  232 
Lanolin,  i.  257.  575 
Large  intestine,  digestion  in,  i.  3 
Laryngeal  muscles,  specific  action  of.  ii.  428 

nerve,  recurrent,  ii.  I'.'- 

superior,  ii.  !•_'- 
Laryngoscope,  ii.  I'-!' 
Larynx,  cartilages  of.  ii.  425 

closure  of,  during  muscular  effort,  ii.  423 


538 


GENERAL  INDEX. 


Larynx,  muscles  of,  ii.  425 
nerves  of,  ii.  42b 
structure  of,  ii.  421 
Latent  areas  of  the  cortex,  ii.  261 

characters,  hereditary  transmission  of,  ii,  495 

heat,  definition  of,  i.  504 

period,  effect  of  temperature  on,  ii.  136 

of  tension  on,  ii.  110 
of  cardiac  accelerator  nerves,  i.  170 
of  heart  muscle,  i.  153 
of  motor  end-plates,  ii.  103 
of  muscle,  ii.  103 
of  red  muscle,  ii.  109 
of  retinal  stimulation,  ii.  343 
of  simple  muscular  contraction,  ii.  102 
of  vagus-stimulation,  i.  162 
Latham's  hypothesis  of  the  structure  of  pro- 
toplasm, i.  24 
Laughing,  i.  454 

"Law  of  contraction,"  Pfliiger's,  ii.  50 
Law  of  stimulation  of  human  nerves  by  battery 

currents,  ii.  54 
Lecithin,  i.  559 
amount  of,  in  the  blood,  i.  51 
occurrence  of,  i.  325 
of  bile,  i.  45 
of  milk,  i.  261 
of  nerves,  ii.  169 
Leech  extract,  effect  of,  on  blood  coagulation,  i. 

62 

lymphagogic  action  of,  i.  73 
Lemniscus,  medial,  i.  226 

sensory  paths  entering,  ii.  235 
Lens,  changes  in,  during  accommodation,  ii.  307 
crystalline,  changes  in,  with  old  age,  ii.  314 
curvatures  of,  ii.  303 
opacities  in,  ii.  321 
refractive  index  of,  ii.  303 
thickness  of,  ii.  303 
Lenticular  ganglion,  ii.  311 
Leucin,  action  of,  on  end-plates,  ii.  27 
chemical  properties  of,  i.  540 
formation  of,  in  tryptic  digestion,  i.  303 
nutritive  history  of,  i.  540 
occurrence  of,  i.  540 
Leucocytes,  behavior  of,  in  blood  capillaries,  i. 

82 

classification  of,  i.  47,  48 
emigration  of,  i.  83 

from  the  thymus  gland,  composition  of,  i.  51 
functions  of,  i.  48 
influence  of,  on  blood-plasma,  i.  49 
movements  of,  ii.  19 
origin  of,  i.  49 
Leucocythsemia,  fatty  acids  in,  i.  530 

purin  bases  excreted  in,  i.  557 
Leuconuclein,  effect  of,  on  intravascular  clot- 
ting, i.  61 

Leucophrys  patula,  reproduction  of,  ii.  442 
Levatores  ani  muscles,  expiratory  action   of,  i. 

407 

costarum  breves,  inspiratory  action  of,  i.  402 
Levulic  acid,  i.  538 

fate  of,  in  pancreatic  diabetes,  i.  267 
Levulose,  i.  562 

occurrence  of,  i.  564 
oxidation  of,  in  diabetes,  i.  564 
Lieberkiihn's  crypts,  histology  of,  i.  243 
Liebig's  method  of  urea  determination,  i.  549 
Life,  general  hypothesis  of,  i.  25 

of  the  individual,  stages  of,  ii.  486 
Ligaments  of  the  incus,  ii.  367 

of  the  malleus,  ii.  366 
Ligatures  of  Stannius,  i.  178 
Light,  changes  in  the  retina  produced  by,  ii.  330 
definition  of,  ii.  298 
dispersion  of,  ii.  316,  332 


Light,  monochromatic,  ii.  316 

physical  theory  of,  ii.  331 

rays  of  the  luminiferous  ether,  ii.  331 

sensations,  intensity  of,  ii.  332,  339 

mechanism  for  the  production  of,  ii.  331 
quality  of,  ii.  332 
Light-waves,  lengths  of,  ii.  332 
Limbs,  vaso-motor  nerves  of,  i.  209 
Limbus  of  the  spiral  lamina,  ii.  377 
Lingual  frictiouals,  ii.  438 

nerve,  gustatory  function  of,  ii.  410 
Linguo-palatal  consonants,  ii.  437 
Lipase,  i.  305 
Lipochromes,  i.  574 
Liqueurs,  i.  535 
Liquids,  ii.  436 
Liquor  amnii,  ii.  472 

folliculi,  ii.  454 
Listing's  law,  ii.  299 
Littre,  glands  of,  ii.  448 

Liver,  defensive  action  of,  against  intravascular 
clotting,  i.  61 

extirpation  of,  i.  336 

functions  of,  i.  320 

histology  of,  i.  244,  321 

internal  secretion  of,  i.  265 

lymph  formation  in,  i.  73 

nerve-endings  in,  i.  245 

secretory  function  of,  i.  244 
nerves  of,  i.  247 

urea  formation  in,  i.  331 

vaso-motor  nerves  of,  i.  206 
Living  matter,  elementary  constituents   of,  i. 

499 

general  properties  of,  i.  18 
molecular  structure  of,  i.  23 
Load,  effect  of,  on  latent  period  of  muscle,  ii.  110 

on  the  contraction  curve,  ii.  Ill 
Local  signs  of  sensations,  ii.  394 
Localization,  cutaneous  variations  of,  ii.  395 

in  the  skin,  theory  of,  ii.  395 

of  cell-groups  in  the  cerebral  cortex,  ii.  241 

of  cortical  cell-groups  for  afferent  impulses,  ii. 
252 

of  pain  sensations,  ii.  399 

of  touch  sensations,  ii.  394 

power,  relation  of,  to  mobility,  ii.  394 
Locomotion,  ii.  420 

Locomotor  ataxy,  disturbance   of  equilibrium 
in,  ii.  405 

mechanisms,  action  of,  ii.  414 
Loew's  hypothesis  of  the   structure   of  proto- 
plasm, i.  23 

Long-reed  register  of  the  voice,  ii.  432 
Long  tracts  of  the  cord,  terminations  of,  ii.  235 
Loop  of  Henle,  i.  250 
Loudness  of  the  voice,  factors  determining,  ii. 

430 

physical  cause  of,  ii.  381 

Luminiferous  ether,  rates  of  vibration  of,  ii.331 
Luminosity,  relative,  of  spectral  colors,  ii.  340 
Luminous  sensations,  intensity  of,  ii.  339 
Lungs,  capacity  of,  i.  427 

nerve-supply  of,  i.  465 

structure  of,  i.  396 

vaso-motor  nerves  of,  i.  205 
Lunulse  of  the  semilunar  valves,  i.  Ill 
Lustre  in  visual  sensations,  explanation  of,   ii. 

358 

Lutein,  i.  574 

Luxus  consumption,  i.  348 
Lymph,  i.  33 

amount  of,  i.  146 

definition  of,  i.  70 

formation  of,  i.  71 

gases  of,  i.  419 

glands,  i.  146. 


GENERAL  INDl-IX. 


539 


Lvmph,  mechanical  t  hcory  of  t  he  origin  of,  1.75 

movement  of.  i.  71,  140 

pressure  of,  i.  14G 

-.•cretiou  of,  i.  ~1  I 
Lymphagogues,  action  of,  \.1'.\,  71 
Lymphatic,  system,  nature  of.  i.  145 

Lymphatics  of  tin-  heart,  i.  1><J 

Lymphocytes,  i.  48 

Lysatin,  i.  .">.">! 

Lysatinin,  relation  of,  to  urea  formation,  i.  337, 

551 
Lysin,  i.  552 

M  \rKocKfiiAi.ic  brains,  weight  of,  ii.  275 
Macula  acustica,  ii.  373 

lutea.  ii.  :'••-': 

Macula-    acus'ticse,  relation  of,  to  state  of  equi- 
librium, ii.  407 
Magnesium  carbonate,   i.  527 
nutritive  history  of,  i.  527 
occurrence  of,  i.  527 
phosphates,.!.  527 

Making  contraction,  point  of  origin  of,  ii.  35 
"  Making"  shock,  ii.  31 
Male  births,  relative  number  of,  ii.  483 

prouucleus,  ii.  466 
Mali-s,  rate  of  growth  in,  ii.  488 
Malic  acid,  i.  558 
Malleus,  ii.  365 

ligaments  of,  ii.  366 
Malpighian  corpuscle  of  the  kidney,  structure 

of,  i.  249 
Maltase,  i.  280,  565 

in  starch  digestion,  i.  285 
occurrence  of,  i.  308 
Mammary  glands,  ii.  443,  462 
histological  changes  in,  i.  262 
in  pregnancy,  ii.  477 
normal  secretion  of,  i.  264 
secretory  nerves  of,  i.  263 
structure  of,  i.  261 
Man  nose,  i.  562 

Manometer,  differential,  i.  131 
elastic,  i.  127 
maximum,  i.  107 
mercurial,  i.  87 

Manubrium  of  the  malleus,  ii.  365 
Marsh  gas,  i.  532 
Masseter  muscle,  i.  372 
Mastication,  i.  372 

Masticatory  movements,  effect  of,  on  taste-sen- 
sations, ii.  411 
Mastoid  antrum,  ii.  363 

"  Mastzellen,"  relation  of,  to  colostrum  corpus- 
cles, i.  263 

Maturation  of  germ-cells,  significance  of,  ii.  454 
of  nerve-cells,  ii.  177 
of  spermatozoa,  ii.  445 
of  the  ovum,  ii.  451 

Meat  extracts,  physiological  action  of,  i.  359 
Meats,  composition  of,  i.  278 
Meatus  auditorius  internus,  ii.  373 
Mechanical  stimulation  of  nerves,  ii.  25,  56 
strain,  influence  of,  on  neuroblasts.  ii.  176 
work  of  muscular  contraction,  ii.  138 
Meconium.  biliary  salts  in,  i.  544 
Medial  lemniscus,  ii.  226 

Medullary  sheath,  development  of,  in  the  cen- 
tral nervous  system,  ii.  181 
in  the  peripheral  nerves,  ii.  180 
significance  of,  ii.  180 
tube,  fetal,  ii.  204 

lamina-  of,  in  the  foetus,  ii.  205 
Medullation.  central,  progressive  character  of, 

ii.  1-1 

of  nerve-fibres,  significance  of,  ii.  283 
peripheral,  ii.  180 


Medusas,  rate  of  conduct  ion  in,  ii.  89 

staircase  contractions  in.  ii.  1  TJ 
Melanins.   i.  574 
Melicyl  alcohol,  i.  540 
Membrana  hasilaris,  ii.  374 

flaccida,  ii.  '•',>>'< 

granulosa  of  the  Graafian  follicle,  ii.  -ir.1 

reticulata,  ii.  378 

tectoria.  ii.  .'{77.  :i7'.i 

tympani,  ii.  :;ii| 

Membrane  of  Reissue r,  ii,  :;7t,  379 
Membranous  labyrinth  of  the  ear,  ii.  372 
Menopause,  ii.  459,  490 
Menstruation,  ii.  457 

age  of  onset  of,  ii.  459 

cessation  of,  at  the  climacteric,  ii.  490 

general  disturbances  accompanying,  ii.  459 

in  animals,  ii.  460 

relation  of  ovulation  to,  ii.  456 

theory  of,  ii.  460 
Mental  activity,  relation  of  cerebral  circulation 

to,  ii.  288 

Menthol,  action  of,  on  cold  nerves,  ii.  398 
Mercapturic  acids,  i.  547 
Mercury  manometer,  description  of,  i.  87 
Metabolism,  conditions  influencing,  i.  359 

definition  of,  i.  20 

during  sleep,  i,  361 
starvation,  i.  362 

effect  of  temperature  on,  i.  362 

influence  of  the  cell-nucleus  on,  i.  22 

intensity  of.  in  the  brain,  ii.  288 

methods  of  estimating,  i.  343 
Metaphosphoric  acid,  i.  514 
Methsemoglobin,  i.  44,  573 
Methane,  origin  of,  i.  532 
Methods,  physiological,  i.  31 
Methyl  amido-acetic  acid,  i.  538 

mercaptan,  i.  534 

selenide,  i.  534 

telluride,  i.  534 

violet,  in  testing  for  mineral  acids,  i.  289 
Methylamine,  i.  541 
Meynert's  commissure,  ii.  238 
Micella},  definition  of,  i.  25 
Microcephalic  brains,  weight  of,  ii.  275 
Microcephalies,  ii.  268 
Micturition,  i.  389 

centre  for,  i.  391,  393 

cerebral  control  of,  ii.  270 

nervous  mechanism  of,  i.  392 

reflex  character  of,  ii.  213 
Middle  ear,  ii.  362 

inflammatory  disease  of,  ii.  364 
mechanism  of,  ii.  368 
muscles  of,  ii.  369 
Migration  of  neuroblasts,  ii.  176 
Milk,  composition  of,  i.  261 

mineral  constituents  of,  i.  530 

normal  secretion  of,  i.  264 
Milk-sugar,  i.  565 

Millon's  reaction  for  proteids,  i.  576 
nature  of,  i.  569 
with  phenol,  i.  569 
Mineral  acids,  tests  for,  i.  289 

constituents,  amount  of,  in  the  tissues,  i.  530 
Mitosis,  i.  20 
Modiolus,  ii.  372 
Molecular  weight,  relation  of,  to  physiological 

action,  ii.  Ho 

Molecules,  physical  and  physiological,  i.  25 
Monochromatic  light,  ii.  316 
Mononuclear  leucocytes,  i.  48 
Mons  Veneris,  ii.  462 
Monstrosities,  congenital,  ii.  494 

origin  of,  ii.  483 
Morgagni,  ventricles  of,  ii.  422 


540 


GENERAL  INDEX. 


Morphin,  effect  of,  on  body-temperature,  i.  472 

Morula,  ii.  470 

Motor  areas,  cortical  serial  arrangement  of,  ii. 

247 

degeneration  after  removal  of,  ii.  244 
paralysis  following  removal  of,  ii.  269 
physiological  characters  of,  ii.  243 
subdivision  of,  into  centres,  ii.  247 

centres,  degree  of  separateness  of,  ii.  248 
of  the  human  cortex,  ii.  250 

disturbance  from  hemisection  of  the  cord,  ii. 
230 

end-plates,  latent  period  of,  ii.  103 

transmission  of  excitation  by  means  of,  ii. 
82 

nerves,  fatigue  of,  ii.  96 

rate  of  conduction  in,  ii.  89 
Mouth,  temperature  in,  i.  469 
Movements  of  joints,  varieties  of,  ii.  416 

of  spermatozoa,  ii.  444 

of  the  eyeball,  ii.  298 
Mucin  of  bile,  i.  325 

of  gastric  juice,  i.  288 

of  saliva,  i.  283 

physiological  value  of,  i.  221 

properties  of,  i.  578 

secretion  of,  i.  217 
Mucous  glands,  histology  of,  i.  216 
Miiller's  experiment,  i.  452 
Multiple  conceptions,  ii.  482 

control  from  the  cortex,  ii.  250 
Murexid,  i.  555 
Muscse  volitantes,  ii.  320 
Muscarin,  i.  543 

action  of,  on  the  heart,  i.  150 
Muscle,  accelerator  urinse,  ii.  449 

aryteno-epiglottidean,  ii.  426 

arytenoid,  ii.  426 

bulbo-cavernosus,  ii.   449 

chemistry  of,  ii.  159  , 

ciliary,  in  accommodation,  ii.  309 

crico-arytenoid,  lateral,  ii.  426 

crico-thyroid,  ii.  426 

currents  of  action  in,  ii.  150 
of  rest  in,  ii.  147 

digastric,  i.  372;  ii.  426 

elasticity  of,  ii.  105 

erector  clitoridis,  ii.  464 

external  rectus,  ii.  299 

fatigue  of,  ii.  70 

frog's,  rate  of  conduction  in,  ii.  89^. 

gases  of,  ii.  168 

general  physiology  of,  ii.  17 

genio-hyoid,  i.  372;  ii.  426 

glycogenic  function  of,  i.  330 

human,  rate  of  conduction  in,  ii.  89 

hyo-glossus,  ii.  426 

independent  irritability  of,  ii.  25 

inferior  oblique,  ii.  299 
rectus,  ii.  299 

inorganic  constituents  of,  ii.  168 

internal  rectus,  ii.  299 

involuntary,  properties  of,  i.  370 

limitation  of  the  rate  of  stimulation  in,  ii.  126 

masseter,  i.    327 

mineral  constituents  of,  i.  530 

mylo-hyoid,  i.  372  ;  ii.  426 

nitrogenous  extractives  of,  ii.  166 

non-nitrogenous  constituents  of,  ii.  168 

obliquus  exteruus,  i.  407 
internus,  i.  407 

omo-hyoid,  ii.  425 

posterior  crico-arytenoid,  ii.  426 

pterygoid,  external,  i.  372 
internal,  i.  372 

pyramidalis,  i.  407 

reaction  of,  ii.  159 


Muscle,  red,  capacity  for  tetanic  contraction  of, 

ii.  127 

retractor  lentis,  of  fishes,  ii.  306 
skeletal,  sensory  nerve-endings  in,  ii.  402 
specific  gravity  of,  ii.  159 
stapedius,  ii.  370 
sterno-hyoid,  ii.  425 
sterno-thyroid,  ii.  425 
striated,  histology  of,  ii.  104 
optical  properties  of,  ii.  103 
stylo-hyoid,  ii.  426 
superior  oblique,  ii.  299 

rectus,  ii.  299 
temporalis,  i.  372 
thyro-arytenoid,  ii.  426 
external,  ii.  424 
internal,  ii.  424 
thyro-hyoid,  ii.  425 
transversalis  abdominis,  i.  407. 
trapezius,  i.  405 
Muscle-contraction,  Engelmann's  theory  of,  ii. 

105 

Muscle-plasma,  ii.  161,  163 
Muscle-proteids,  precipitation  temperature  of,  ii. 

166 
Muscles,  abdominales,  action  of,  in  vomiting,  i. 

387 

respiratory  function  of,  i.  407    - 
absolute  force  of,  ii.  141 
action  of,  upon  the  bones,  ii.  417 
classification  of,  ii.  18 
degeneration  of,  after  section  of  motor  nerves, 

ii.  48,  54,  70 
endurance  of.  ii.  76 
erectores  penis,  ii.  449 

spinse,  i.  405 
expiratory,  i.  407 
glycogeu  of,  i.  330 
human,  fatigue  of,  with  artificial  stimulation, 

ii.  134 

infrahyoidei,  i.  405 
inspiratory,  i.  399,  405 
intercostal,  i.  402,  407 
ischio-cavernosi,  ii.  449 
levatores  ani,  i.  407 

costarum,  i.  402 
of  mastication,  i.  372 
of  the  eye,  ii.  299 
of  the  iris,  ii.  323 
of  the  middle  ear,  ii.  369 
pectorales,  i.  405 
quadrati  lumborum,  i.  399 
rate  of  conduction  in,  ii.  89 
rhomboidei,  i.  405 
scaleni,  i.  401 
serrati  postici,  i.  399,  402 
sterno-cleido-mastoid,  i.  404 
tensor  tympani,  ii.  369 
thermogenic  function  of,  i.  490 
triangulares  sterni,  i.  407 
vaso-motor  nerves  of,  i.  210 
Muscle-serum,  ii.  166 
Muscle-sounds,  ii.  132 
Muscle-spindles,  ii.  402 

Muscle-structure,  influence  of,  on   its  contrac- 
tion, ii.  107 
Muscle-tonus,  ii.  143 

Muscular  contractions,  effect  of  drugs  on,  ii.  137 
of  support  on  the  height  of,  122 
of  temperature  on,  ii.  136 
graphic  record  of,  ii.  99 
influences  affecting,  ii.  107 
post-mortem,  ii.  160 
source  of  energy  in,  ii.  74 
effort,  closure  of  larynx  in,  ii.  423 
exercise,  effect  of,  on  metabolism,  i.  359 
on  the  pulse-rate,  i.  121 


GENERAL   INDEX. 


541 


Muscular   exercise,    effect    of,    on    the    rate   of 

respiration,  i.  U»i 
on  the  respiratory  exchanges,  i.  433 

quotient,  i.    \'.\^ 
on  the  sweat  glands,  i.  260 
on  the  venous  circulation,  i.  1)5 
inhibit  ion  from  cortical  stimulation,  ii.  224 
movements,  relations  of  antagonistic  muscles 

in,  ii.  418 

sensations,  cortical  area  for,  ii.  253 
definition  of,  ii.  390 

effect  of  hemisectiou  of  the  cord  on,  ii.  235 
in  estimation  of  weights,  ii.  403 
nature  of,  ii.  401 

path  of  conduction  for,  in  the  cord,  ii.  235 
psychological  value  of,  ii.  391 
work,  effect  of  stimulants  on,  ii.  7"> 
Musical  sounds,  characteristics  of,  ii.  387 
tones,  beats  of,  ii.  386 

limits  in  the  pitch  of,  ii.  282 
production  of,  ii.  381 
Mycoderma  aceti,  i.  537 
Mydriatics,  ii.  325 
Mylo-hyoid  muscle,  i.  372  ;  ii.  426 
Myo-albuinin,  ii.  166 
Myo-albumose,  ii.  166 
Myogen-fibrin,  ii.  164 
Myoglobulin,  ii.  166 

Myogonic  theory  of  the  causation  of  the  heart- 
beat, i.  150 
Myogram,  ii.  34 
definition  of,  ii.  100 

effect  of  temperature  on  the  form  of,  ii.  136 
Myograph,  ii.  35 

description  of,  ii.  100 
double,  of  Heriug,  ii.  36 
Myohsematin,  i.  578 ;  ii.  166 
Myopia,  ii.  313 

range  of  accommodation  in,  ii.  314 
Myosin,  ii.  163 
absorption  of,  i.  315 
ferment,  ii.  161,  163 
Myosin-fibrin,  ii.  164 
Myosinogen,  ii.  163 

temperature  of  heat  coagulation  of,  ii.  165 
Myotics,  ii.  325 
Myxcedema,  i.  269 

NATIVE  albumins,  i.  577 

Nausea  from  disturbance  of  equilibrium,  ii.  405 
Near-point  of  vision,  ii.  312 
Negative  after-images,  ii.  346 
pressure  in  the  auricles,  i.  137 
in  the  heart,  i.  98 
in  the  thorax,  i.  95 
in  the  veins,  i.  94 
variation  in  muscles,  rate  of  propagation,  ii. 

152 

relation  of,  to  the  contraction,  ii.  153 
of  muscle-currents,  ii.  150 
of  nerve-currents,  ii.  154 
of  the  beating  heart,  i.  153 
Nerve,  auriculo-temporal,  i.  218 
chorda  tympani,  i,  194,  219 

gustatory  function  of,  ii.  410 
coronary,  of  the  tortoise,  i.  164 
depressor,  i.  172,  203 
facial,  secretory  fibres  of,  i.  219 
general  physiology  of,  ii.  17 
glossopharyngeal,  gustatory  function  of,  ii.  410 

secretory  fibres  of.  i.  218 
Jacobson's,  i.  218 
lingual,  secretory  fibres  of,  i.  219 
oculomotor,  ii.  '.\'l'-\ 
recurrent  laryngeal,  ii.  428 
small  superficial  petrosal,  i.  218 
superior  laryngeal,  ii.  428 


Nerve,  vakils,  eanliae  branches  of,  i.  l.V.) 
gastric  branches  of,  i.  :;-l 
intestinal  branches  of,  i.  !{.<> 
pulmonary  branches  of,  i.   Jiif, 
repiratory  functions  of.  i.  !.">!! 
secretory  fibres  of,  i 

trophic  influence  of,  on  the  heart,  i.  166 
Nerve-cells,  atrophy  of.  from  disuse,  ii.  195 

changes  in,  with  age,  ii.  mo 

chemical  changes  in.  ii.  1!M 

classification  of,  ii.  177 

connection  between,  ii.  206 

degeneration  of  the  cell-bodies  of,  ii.  199 

diaxonic,  ii.  178 

effect  of  exercise  on,  ii.  76 

fatigue  of,  ii.  191 

generation  of  impulses  in,  ii.  188 

growth  of,  ii.  176 

histological    change    in,    due    to    functional 
activity,  ii.  I!'-.' 

human,  size  of,  ii.  174 

internal  structure  of,  ii.  179 

maturation  of,  ii.  177 

morphology  of,  ii.  173 

number  of,  in  the  central  nervous  system,  ii. 
888 

nutrition  of,  ii.  190 

nutritive  control  of,  over  nerve-fibres,  ii.  198 

of  animals,  size  of,  ii.  175 

of  spinal  ganglia,  development  of,  ii.  178 

peculiarities  of,  ii.  174 

pyramidal,  ii.  178 

rate  of  discharge  from,  ii.  189 

regeneration  of,  ii.  201 

relation  of  size  and  function  in,  ii.  175 

senescence  of,  ii.  182,  490 

significance  of  the  branches  of,  ii.  186 

summation  of  stimuli  in,  ii.  190 

volume  relation  of,  ii.  175 

Nerve-elements,  primitive  segmental  arrange- 
ment of,  ii.  205 
Nerve-endings  in  the  liver,  i.  245 

in  the  salivary  glands,  i.  220 

in  the  skin,  ii.  392 
Nerve-fibres,  reaction  of,  ii.  170 

classification  of,  ii.  21  — — 

cortical,  increase   in   the  number  of,  during 
growth,  ii.  282 

fatigue  in,  ii.  95 

functions  of,  ii.  21 
Nerve-impulse,  definition  of,  ii.  25 

direction  of  the  passage  of,  ii.  184 

electrical  variation  accompanying,  ii.  183 

generation  of,  ii.  187 

in  peripheral  nerves,  ii.  183 

peripheral  diffusion  of,  ii.  218 

reversed,  in  spinal  ganglion-cells,  ii.  185 

theories  of,  ii.  97 

transmission  of,  from  neurone  to  neurone,  ii. 

-.'( )T 

Nerve-muscle  preparation  of  a  frog,  ii.  34 
Nerves,  action  currents  in,  ii.  153 

auditory,  central  path  of,  ii.  237 

augmentor,  of  the  heart,  i.  167 

cardiac,  i.  148 

cervical  sympathetic,  i.  193 

chemistry  of,  ii.  H>!> 

cross-suturing  of,  ii.  200 

current  of  rest  in.  ii.  149 

degeneration  of,  after  section,  ii.  69,  78 

depressor,  of  the  heart,  i.  !?•-' 

fatigue  of,  ii.  7~> 

glossopharyngeal,    central    conduction    paths 
for.  li.238 

in  man,  stimulation  of.  ii.  .",1 

law  of  stimulation  of,  with  galvanic  current, 
ii.  50 


542 


GENERAL  INDEX. 


Nerves,  limitation  of  the  rate  of  stimulation  in, 

ii,  126 

lingual,  gustatory  function  of,  ii.  410 
medullation  of,  ii.  180 

non-medullated,  rate  of  conduction  in,  ii.  90 
of    common     sensation,    central    conduction 

paths  of,  ii.  230 
of  cutaneous  sensations,  central   conduction 

paths  of,  ii.  233 
of  dermal  sensation,  area  of  distribution  of,  ii. 

231 

of  invertebrates,  rate  of  conduction  in,  ii.  91 
of  taste,  nuclei  of  origin  of,  ii.  236 
of  temperature,  ii.  397 
of  the  bile  vessels,  i.  248 
of  Wrisberg  (intermedius),  central   path   of, 

ii.  236 

olfactory,  central  paths  of,  ii.  241 
optic,  central  paths  of,  ii.  238 
phrenic,  i.  463 
rate  of  conduction  in,  ii.  89 
secondary  degeneration  of,  ii.  197 
septal,  of  the  frog's  heart,  i.  166 
specific  energy  of,  ii.  232 
splanchnic,  i.  173 
trigeminal.  i.  463 

central  paths  of,  ii.  238 

vagus,  course  of  the  afferent  fibres  in,  ii.  236 
Nerve-trunks,  isolated  conduction  in,  ii.  79 
Nervi  erigentes,  ii.  464 

intestinal  branches  of,  i.  385 
Neukomm's  test  for  bile  acids,  i.  545 
Neuridin,  i.  543 
Neurin,  i.  543 

Neuroblast,  development  of,  ii.  176 
Neurogenic    theory  of   the    causation    of    the 

heart-beat,  i.  149 
Neuro-keratin,  i.  580 

of  nerves,  ii.  169 
Neuromuscular  spindle,  ii.  390 
Neurone,  definition  of,  ii.  173 
Neurones,  ii.  21 

afferent,  to  the  spinal  cord,  ii.  203 
changes  in  number  and  size  of,  ii.  280 
conduction  in,  ii.  97 
connection  by,  ii.  206 
double  conduction  in,  ii.  185 
increase  in  number  of,  during  growth,  ii.  282 
internal  structure  of,  ii.  179 
polarity  of,  ii.  184 
total  number  of,  ii.  283 
Neutral  salts,  effects  of,  on  blood  coagulation,  i. 

62 

Neutrophiles,  i.  47 

Nicotin,  action  of,  on  end-plates,  ii.  27 
on  intestinal  movements,  i.  384 
on  secretory  nerves,  i.  229 
on  sympathetic  ganglia,  ii.  219 
Nissl  method  for  nerve-cells,  ii.  195 
substance,  iron  in,  ii.  191 

of  nerve-cells,  ii.  179 
Nitric  oxide,  i.  512 

haemoglobin,  i.  39,  512 

Nitrogen  equilibrium,  definition  of,  i.  344,  512 
free,  of  muscles,  ii.  168 
history  of,  in  the  body,  i.  512 
inhalation,  i.  440 
occurrence  of,  i.  510 
of  the  feces,  i.  320 
preparation  of,  i.  510 
tension  of  the  blood,  i.  417 
Nitrogenous  equilibrium,  definition  of,  i.  344, 

512 
excreta  of  milk,  i.  262 

of  sweat,  i.  259 
extractives  of  muscle,  ii.  166 
of  the  spleen,  i.  333 


Nitrogenous  metabolism,  estimation  of,  i.  343 
Nitrous  oxide,  inhalation  of,  i.  440 

properties  of,  i.  512 
Nodal  point  in  the  simplest  dioptric  system,  ii. 

301 

Noaud  vital,  i.  456 ;  ii.  236 
Noises,  definition  of,  ii.  388 
Non-medullated  nerves,  rate  of  conduction  in, 

ii.  90 

stimulation  fatigue  of,  ii.  180 
Non-polarizable  electrodes,  ii.  29 
Nose,  anatomy  of,  ii.  408 

respiratory 'tract  of,  ii.  408 
Nucleic  acid,  i.  579 
Nuclein  bases,  i.  552 

composition,  i.  556,  579 
Nucleo-histon  of  the  blood-plates,  i.  49 

relation  of,  to  intravascular  clotting,  i.  61 
Nucleo-proteids,  classification  of,  i.  577 

properties  of,  i.  579 
Nucleus,  functions  of,  i.  22 

relation  of,  to  oxidation,  i.  503 
Nutrition  of  living  matter,  i.  18 
of  nerve-cells,  ii.  190 
of  the  embryo,  ii.  475 
Nutritive    control    of   nerve-cell    bodies    over 

nerve-fibres,  ii.  198 
value  of  albuminoids,  i.  349 
of  carbohydrates,  i,  353 
of  fats,  i.  350 
of  proteids,  i.  276,  345 
of  salts,  i.  354 
of  water,  i.  354 
Nymphse,  ii.  462 
Nystagmus  after  cerebellar  injury,  ii.  272 

OBLIQUUS  externus,    respiratory   action   of,   i. 
407 

interims,  respiratory  action  of,  i.  407 
Occlusion  of  the  bile-duct,  effect  of,  i.  249 
Oculomotor  nerve,  ciliary  fibres  of,  ii.  311 

relation  of,  to  the  iris,  ii.  323 
Odors,  ii.  410 
CEdema,  i.  148 

of  the  glottis,  ii.  422 
(Esophagus,  deglutition  in  the,  i.  374 
Oils,  effect  of,  on  gastric  secretion,  i.  241 

on  pancreatic  secretion,  i.  236 
Old  age  of  the  central  nervous  system,  ii.  295 
Olefines,  i.  542 
Oleic  acid,  i.  541-560 
Olfactory  area  of  the  cortex,  ii.  253 

cells,  ii.  408 

epithelium,  ii.  408 

nerves,  central  paths  of,  ii.  241 

paths  to  the  brain,  ii.  409 

stimuli,  conditions  affecting,  ii.  409 

tracts,  section  of,  in  sharks,  ii.  264 
Omo-hyoid  muscle,  ii.  425 
Oncometer,  i.  255 

Ontogenetic  development  of  nerve-cells,  ii.  177 
Onychodromus,  reproduction  of,  ii.  442 
Oocyte,  ii.  451 

Oophorin  tablets,  action  of,  i.  274 
Opening  of  the  chest,  effect  of,  on  heart,  i.  115 
Ophthalmometer,  ii.  304 
Ophthalmoscope,  ii.  326 

Opium,  effect  of,  on  respiratory  rhythm,  i.  425 
Optic  commissure,  decussation  of  optic  fibres  in, 
ii,  238 

nerve,  currents  of  action  in,  ii.  154 

nerve-fibres,  number  of,  ii.  330 

nerves,  central  paths  of.  ii.  238 
cortical  centres  of,  ii.  240 
efferent  fibres  of,  ii.  240 

thalami,  functions  of,  ii.  271 
Optical  constants  of  the  eye,  ii.  303 


643 


Optical  illusion*  in  binocular  vision,  ii.  T>!» 
of  span-  perceptions,  ii.  .Til 

properties  of  striated  mn>cle.  ii.  In;; 
Optograms.  ii.  ."i^11 
Organ  of  Corti,  structure  of,  ii.  377 

of  Golgi  in  tendons,  ii     |n-j 
11  <  )ri:anej\veiss,"   i.  '.\\*> 

<  )rirani/ation  in  the  central  nervous  system,  ii. 
286 

relation  of  educability  to  the  establishment 

of.  ii.  -.'Hi 

Organs,  growth  of,  ii.  486 
Ornithin.  i.  552 
Orthophosphorie  acid,  i.  51-1 
Os  orbicnlare  of  the  incus,  ii.  366 
Osa/.ones  of  glycoses.  i.  T.ti-J 
Oscillatory  activity  of  the  retina,  ii.  344 
Osmosis,  definition  of,  i.  <>f> 

relation  of,  to  secretion,  i.  213 
Osmotic  pressure,  definition  of,  i.  65 
method  of  determining,  i.  67,  68 
relation  of,  to  concentration,  i.  66 
Osones,  preparation  of,  i.  562 
Ossicles,  auditory,  ii.  365 

of  the  ear,  movements  of,  ii.  367 
Osteomalacia,  i.  524,  525 

ovariotomy  in,  i.  274 
Osteoporosis,  i.  525 
Otitis  media,  ii.  364 
Otoconia,  ii.  374 
Otoliths,  ii.  374 
Ova,  ii.  440 

number  of,  in  human  ovary,  ii.  451 
Ovaries,  ii.  443 

effect  of  removal  of,  on  menstruation,  ii.  459 
»     internal  secretion  of,  i.  274 
Ovariotomy,  effects  of,  i.  '274 
Ovary,  structure  of,  ii.  454 
Overtones,  definition  of,  ii.  383 

inharmonic,  ii.  386 
Oviducts,  ii.  443.     See  Fallopian  tubes. 
Ovnlation,  ii.  -I.") 
Ovum,  chemistry  of,  ii.  450 

fertilization  of,  ii.  466 

human,  structure  of,  ii.  449 

maturation  of,  ii.  451 

physiological  properties  of,  ii.  22 

segmentation  of.  ii.  407 

stages  in  the  maturation  of,  ii.  452 
Oxalate  solutions,  effect  of,  on  blood  coagula- 
tion, i.  63 
Oxalic  acid,  i.  557 
Oxaluric  acid,  i.  r> ."•."> 
Oxidases,  i.  281 
Oxidation,  i.  501 

physiological,  Hoppe-Seyler's  theory  of,  i.  505 

Traube's  theory  of,  i.  502 
Oxidizing  enzymes,  i.  280 
Oxybutyric  acid,  i.  548 
Oxycholin,  i.  543 
Oxygen,  alveolar  tension  of,  i.  413 

occurrence  of,  i.  500 

preparation  of,  i.  501 

properties  of,  i.  501 

storage  of,  in  muscle,  ii.  169 

supply,  relation  of,  to  irritability,  ii.  68 

tension  in  the  blood,  i.  -115 

respiratory  effects  of  varying,  i.  440 
Oxygen-absorption,  coefficient  of,  i.  415 

conditions  affecting,  i.  429 

cutaneous,  i.  422 

estimation  of,  i.  428 
Oxygen-dyspnrea,  i.  444 
Oxytaemoglobin,  composition  of,  i.  38 

dissociation  of,  i.  415,  501 
Oxyntic  cells  of  gastric  glands,  i.  237 
Oxyphenyl-acetic  acid,  i.  570 


Oxyphenyl-amido-propionic  acid,  i.   .">?<) 

Oxyphiles,    i.   17 

<  )/.one  inhalation,  i.   1  |n 

preparation  of.  i 

properties  ot,  i.  ;,n:j 

PACINIAN  body,  ii.  :;!»! 
of  the  pem>.  ii.  449 
Pain  nerves,  evidence  for  the  existence  of,  ii.  232 

points  of  the  skin,  ii.  400 

sensations  of.  ii.  :;!»!» 

Pains,  transferred  or  sympathetic,  ii.  400 
Pale  striped  muscle,  physiological  peculiarities 

of.   ii.    1U!» 

Palmitic  acid,  i.  541 . 
Pancreas,  anatomy  of,  i.  231 
extirpation  of,  i.  266 
grafting  of,  i.  ~Ji>7 
histology  of,  i.  231 
innervation  of,  i.  232 
internal  secretion  of,  i.  266 
mineral  constituents  of,  i.  530 
secretory  changes  in,  i.  233 
vaso-inotor  nerves  of,  i.  207 
Pancreatic  diabetej^LJ267,  353,  563 
fistulae,  preparation  ofpfc  300 
juice,  amyloly tic  action  of,  i.  305 
artificial,  i.  30i- ... 
collection  of,  i.  300 
composition  of,  i.  232,  299 
fat-splitting  power  of,  i.  305 
secretion,  composition  of,  i.  232,  299 
histological  changes  during,  i.  233 
nervous  mechanism  of,  i.  232 
normal  mechanism  of,  i.  235 
reflex  character  of,  i.  236 
relation  of,  to  the  character  of  the  food,  i. 

237 

Pangenesis,  Darwin's  theory  of,  ii.  501 
Papain,  i.  280 

Papilla  foliata  of  rabbits,  ii.  410 
Papillary  muscles,  i.  110 
Parabamic  acid,  i.  555 
Paracasein,  i.  296 
Paradoxical  contraction,  ii.  157 
Paraffins,  i.  531 
Paraformic  aldehyde,  i.  533 
Paraglobulin,  amount  of,  in  the  blood,  i.  53 
composition  of,  i.  53 
functions  of,  i.  53 
origin  of,  i.  53 
*  properties  of,  i.  53 

Parallax,  use  of,  in  estimation  of  distance,  ii.  3."i» 
Paralysis  after  removal  of  motor  areas,  ii.  2<!!i 
agitans,  ii.  296 
Brown-Sequard's,  ii.  233 
homolateral,  after  hemisection   of  the  cord, 

ii.  233 

Paralytic  secretion,  i.  229 
Paramcecium,  reproduction  in,  ii.  440 
PHramyosinogen.  ii.  163 

temperature  of  heat  coagulation  of,  ii.  165 
Parapeptone,  definition  of,  i.  292 
Parannclein.  i.  579 
Parathyroids,  anatomy  of,  i.  268 

function  of,  i.  269 
Paresis  following   removal   of  the  cerebellum, 

ii.  272 

from  injury  to  motor  areas,  ii.  269 
Parotid  gland,  anatomy  of,  i.  217 

innervation  of.  i.  218 
Partial  tones,  definition  of,  ii.  383 
Parturition,  ii.  J7!> 

spinal  centre  of,  ii.  481 
Pate  de  foie  pr:«<.  i.  ",t;n 

Paths  of  conduction  in   the  cord,  clinical  evi- 
dence on,  ii.  234 


544 


GENERAL  INDEX. 


Pause,  compensatory,  of  the  heart,  i.  156 
Pauses,  respiratory,  i.  424 
Pectoral  muscles,  respiratory  action  of,  i.  405 
Pendular  movements  of  the  intestines,  i.  384 

vibrations,  ii.  381 
Penis,  ii.  443 

structure  of,  ii.  448 
Pentamethylene-diamin,  i.  543 
Pentoses,  i.  562 
Pepsin,  i.  237,  238 

effect  of,  on  blood  coagulation,  i.  63 

preparation  of,  i.  291 

properties  of,  i.  290 

Pepsin-hydrochloric  acid,  action  of,  i.  292 
Pepsinogen  granules,  i.  242 
Peptic  digestion,  i.  292,  294 
Peptones,  absorption  of,  in  the  stomach,  i.  313 

definition  of,  i.  292,  295 

effect  of,  on  blood  coagulation,  i.  62 

properties  of,  i.  294,  577 

Pepton-injection,  effect  of,  on  lymph  formation, 
i.  73 

toxicity  of,  i.  316 
Perfusion  cannula,  i.  187 
Perilymph,  ii.  372 
Periodic  reflexes,  ii.  216 
Peripheral  nerves,  medullation  of,  ii.  180 

reference  of  special  sensations,  ii.  400 

reflex  centres,  i.  178 
Peristalsis,  definition  of,  i.  372 

intestinal,  i.  382 

of  the  stomach,  i.  379 

of  the  ureters,  i.  389 
Permeability  of  the  capillary  walls,  i.  70 
Peroxide  of  hydrogen,  i.  505 
Pettenkofer's   reaction    for  bile    acids,   i.   324, 

544 

Pexinogen  granules,  i.  242 

Pfliiger's  hypothesis  of  the  structure  of  proto- 
plasm, i.  23 

law  of  contraction,  ii.  50 
Phagocytosis,  i.  48 
Phakoscope,  ii.  308 

Phalangar  process  of  the  rods  of  Corti,  ii.  378 
Pharynx,  deglutition  in,  i.  373 
Phenaceturic  acid,  i.  569 
Phenol,  i.  569 

elimination  of,  i.  340 
Phenyl-acetic  acid,  i.  569 
Phloridzin  diabetes,  i.  563 
Phosphates,  i,  514 
Phosphene,  pressure,  ii.  305,  331 
Phosphoric  acid,  salts  of,  i.  514 
Phosphorus,  nutritive  history  of,  i.  515 

occurrence  of,  i.  513 

peroxide,  i.  514 

poisoning,  i.  513 

preparation  of,  i.  513 

properties  of,  i.  513 
Photometry,  ii.  345 
Phrenic  nerve,  current  of  action  in,  ii.  154 

nerves,  i.  463 

Phylogenetic  development  of  nerve-cells,  ii.  177 
Physical  molecules,  definition  of,  i.  25 
Physiological  anodes,  definition  of,  ii.  52 

division  of  labor,  i.  22 

molecules,  i.  25 

kathode,  definition  of,  ii.  52 

observations  on  afferent  paths  in  the  cord,  ii. 
229 

rheoscope,  ii.  148,  151 

salt  solution,  ii.  59 

in  transfusions,  i.  64 
Physiology,  definition  of,  i.  17 

human,  definition  of.  i.  30 

methods  employed  in,  i.  30 

subdivisions  of,  i.  17,  29 


Physostigmin,  action  of,  on  accommodation,  ii. 

311 

on  the  eye,  ii.  325 
Pia  mater,  weight  of,  ii.  274 
Pigment  epithelium,  retinal,  movements  of,  ii. 

330 
retinal,  relation  of,  to  adaptation  of  the  eye, 

ii.  326 
Pigments,  biliary,  i.  45,  245,  322,  530,  574 

blood-,  i.  37,  44,  573 
Pilocarpin,  action  of,  on  salivary  glands,  i.  229 

on  sweat-glands,  i.  260 

Pilomotor  mechanism,  relation  of,  to  thermo- 
lysis, i.  494 

Pi  nee  myographique,  ii.  87 
Pineal  gland,  calcification  of,  in  old  age,  ii.  491 
Pinna  of  the  ear,  ii.  362 
Pitch,  limits  of  perception  of,  ii.  382 
of  musical  tones,  ii.  381 
of  the  voice,  ii.  430,  432 
Pituitary  body,  anatomy  of,  i.  272 
functions  of,  i.  273 
internal  secretion  of,  i.  273 
extracts,  action  of,  i.  272 
membrane,  ii.  408 
Pivot-joint,  ii.  417 
Placenta,  ii.  474 
Placental  transmission  of  infectious  diseases,  ii. 

498 

villi,  ii.  474 

Plain  muscle,  histology  of,  i.  369 
physiology  of,  i.  370 
tone  of,  i.  371 

Plant-cells,  assimilation  in,  i.  18 
Plants,  regeneration  of  lost  parts  in,  ii.  496 
Plasma  of  blood,  i.  33,  50 

oxygen  absorption-coefficient  of,  i.  416 
Plastic  food-stuffs,  definition  of,  i.  346 
Plethysmograph,  i.  196 
Pneumatic  cabinet,  i.  453 
Pneumogastric  nerve.     See  Vagus. 
pulmonary  branches  of,  i.  465 
respiratory  function  of,  i.  459,  460 
Pneumograph,  i.  423 
Pohl's  mercury  commutator,  ii.  36 
Poikilothermous  animals,  i.  467 
Polar  amphiaster  of  the  ripening  egg,  ii.  453 
bodies,  ii.  451 

of  the  ovum,  ii.  453 
Polarity  of  neurones,  ii.  184 
Polarization,  after-effects  of,  ii.  65 

physiological,  of  neuroblasts,  ii.  176 
Polarizing  current,  effect  of,  on  conductivity, 

ii.50 

on  muscles,  ii.  61 
on  nerves,  ii.  62 
Pole-changers,  ii.  36 
Polynucleated  leucocytes,  i.  48 
Polypncea,  i.  441 
Polyspermy,  ii,  471 
Pomum  Adami,  ii.  425 
Portal  vein,  vase-motor  nerves  of,  i.  209 
Positive  after-images,  ii.  346 
variation  of  the  heart  during  vagus  stimula- 
tion, i.  164 

Posterior  association  centre,  ii.  257 
Post-ganglion ic  fibres  of  the  sympathetic  sys- 
tem, ii.  219 

Post-mortem  rise  of  temperature,  i.  497 
Posture  sense,  ii.  399 

Potassium  carbonates,  nutritive  history  of,  i.  520 
chloride,  nutritive  history  of,  i.  519 
cyanide,  i.  542 
occurrence  of,  i.  519 
phosphates,  nutritive  history  of,  i.  520 
relation  of,  to  heart  muscle,  i.  151 
salts,  action  of,  on  muscles,  ii.  138 


GENERAL    1M>KX. 


545 


Potassium  suits,  relation  of,  to  irritability,  ii.  .",!» 
toxicity  of,  i.  iV.'o 

sulphocyanide.  detection  of,  i.  284 
occurrence  of,  i.  '2*'.\,  ~>l'* 
of  tlir  urine,   i.  r>()7 

thiocyanide,  i.  •">  |-J 
Potential  energy  of  food,  i.  ill! I 
Preformat  ion  theory  of  heredity,  ii.  500 
Pre-ganul  ionic  fibres  of  the  sympathetic  system, 

ii.  -Jl!> 

Pregnancy,  effects  of,  on  the  mother,  ii.  477 
I'n-sbyopia,  ii.  .'!!  I 
Pressor  nerves,  i.  202 

Pressure,  eil'ert  of,  on  irritability  of  nerves,  ii. 
66 

influence  of,  on  conductivity,  ii.  92 

intracanliac.  i.  107 

intrathoracic,  i.  396,  409 

intraventriciilar,  i.  125 

of  the  lymph,  i.  146 
Pressure-points  of  the  skin,  ii.  396 
Pressure-sensations,  fusion  of,  ii.  394 
Pressure-sense,  delicacy  of,  ii.  392 

of  the  tympanic  membrane,  ii.  382 
Primary  position  of  the  eye,  ii.  299 

taste-sensations,  ii.  412 
Principal  foci  in  a  dioptric  system,  ii.  302 

point  of  the  simplest  dioptric  system,  ii.  301 

ray  in  the  simplest  dioptric  system,  ii.  301 
Processus  brevis  of  the  malleus,  ii.  365 

gracilis  sive  folianus  of  the  malleus,  ii.  365 
Projection    system    of   fibres,  origin    of,  from 

central  cells,  ii.  205 
Pronucleus,  female,  ii.  453 

male,  ii.  466 

Propeptones,  definition  of,  i.  292 
Propionic  acid,  i.  538 
Proptosis  after  cerebellar  injury,  ii.  272 
Propyl  alcohol,  i.  536,  538 
Prostate  glands,  ii.  443 
histology  of,  ii.  448 
secretion  of,  ii.  446 
Prostatic  fluid,  ii.  446,  448 
Protagon,  i.  559 

of  medullary  substance,  ii.  170 
Protamin,  nature  and  origin  of,  i.  24 
Protamins,  properties  of,  i.  580 
Proteid,  affinity  of  cell-substance  for,  i.  568 

circulating,  definition  of,  i.  346 

metabolism  during  starvation,  i.  363 
effect  of  muscular  work  on,  i.  360 
end-products  of,  i.  337 
Proteid-absorption,  mechanism  of,  i.  316 
Proteids,  absorption  of,  i.  315 

classification  of,  i.  576 

color  reactions  of,  i.  576 

combined,  classification  of,  i.  579 

combustion  equivalent  of,  i.  365 

diffusion  of,  i.  70 

dynamic  value  of.  i.  475 

effect  of,  on  glycogen  formation,  i.  328 

gastric  digestion  of,  i.  292 

general  reactions  of,  i.  ">?.") 
significance  of,  i.  24 

living,  theoretical  structure  of,  i.  23,  24 

molecular  structure  of,  i.  581 

nutritive  value  of,  i.  :.'7»i,  :Jl.~> 

of  milk,  i.  •„><)! 

of  muscle,  precipitation  temperatures   of,  ii. 
166 

of  muscle-serum,  ii.  166 

of  the  blood,  i   49,  50 

origin  of  fat  from,  i.  351 

osmotic  pressure  of,  i.  69 

putrefaction  of.  in  the  intestines,  i.  310 

rapidity  of  oxidation  of,  i.  347 

relation  of.  to  muscular  work,  ii.  74 


IVoleids.  simple,  classification  of,  i.  .~>7<; 
substitutes  for,  in  t  he  diet,  i.  348 
synthesis  of,  i.  :>!-. 
tryptic  digestion  of,  i.  303 
vegetable,  i.  .~>?7 
Proteo lysis,  i.  -»i:; 
tryptic,  i.  303 
value  of,  i.  :<!."> 

Proteolytic  enzymes,  definition  of,  i.  280 
Proteose  injection,  effects  of,  i.  316 
Proteoses,  definition  of,  i.  292 

properties  of,  i.  .777 
Protbrombin.  i.  58 
Protoplasm,  i.  17,  499 
Pseudo-mucoid,  i.  578 
Pseudoscopic  vision,  ii.  318,  .'!.">7 
Psychical  powers  of  the  spinal  cord,  ii.  215 
Psycho-physic  law,  ii.  340 

of  Fechner,  ii.  393 
Pterygoid  muscles,  i.  372 
Ptomaines,  chemical  structure  of,  i.  542 
Ptyalin,  i.  221,  280 
action  of,  i.  284,  286,  566 
occurrence  of,  i.  284 
Puberty,  ii.  489 
Pulmonary  circulation,  i.  78,  103 

innervation  of,  i.  205 
ventilation,  forces  concerned  in,  i.  413 
Pulse,  arterial,  cause  of,  i.  93 
celerity  of,  i.  142 
definition  of,  i.  139 
dicrotic  wave  of,  i.  143 
extinction  of,  i.  94 
frequency  of,  i.  121,  141 
regularity  of,  i.  141 

respiratory  variations  in  the  rate  of,  i.  451 
size  of,  i.  141 
tension  of,  i.  141 
transmission  of,  i.  140 
relation  of,  to  body-temperature,  i.  471 
respiratory,  i.  96 
Pulse-curve,  i.  142 
Pulse-rate,  diurnal  variations  of,  121 
Pulse-volume  of  the  heart,  definition  of,  i.  105 
Pupil,   changes  during  accommodation   in,  ii. 

311 

in  size  of,  ii.  323 
condition  of,  in  sleep,  ii.  325 
dilator  nerves,  ii.  324 
size  of,  in  old  age,  ii.  314 
Pupillary  reflex  to  light,  ii.  323 
Purin,  i.'553 
bases,  i.  552 

in  leucocythsemia,  i.  557 
Purkinje-Sanson's  images,  ii.  307 
Purkinje's  figure,  ii.  321 
phenomenon,  ii.  340 

explanation  of,  ii.  342 
Purposeful  reflexes,  ii.  215 
Putrefaction,  intestinal,  products  of,  i.  310 
Putrescin,  i.  543 
Pyin,  i.  579 

Pyramidal  fibres,  number  of,  ii.  246 
nerve-cells,  development  of,  ii.  178 
tracts  in  the  cord,  ii.  245 
geminal  fibres  of,  ii.  245 
size  of,  ii.  252 
Pyramidalis    muscle,   expiratory  action    of,   i. 

407 

Pyridin.  i.  .",71 
Pyrocatechin,  i.  569 

QUADRATI  lumborum,  respiratory  action  of,  i. 
399 

(Quadruplets,  ii.  483 
Quality  of  musical  tones,  ii.  383 
of  the  voice,  ii.  430 


546 


GENERAL  INDEX. 


Quinine,  action  of,  on  coagulation   of  muscle- 
plasma,  ii.  164 

hydrochlorate,  action   of,  on  salivary  glands, 
i.  222 

Quintuplets,  ii.  483 

RACE,  relation  of  brain-weight  to,  ii.  278 
Range  of  accommodation,  normal,  ii.  312 
Rarefied  air,  respiration  of,  i.  452 
Rate  of  conduction  in  heart  muscle,  i.  154 
in  muscles,  ii.  87 
in  nerves,  ii.  89 
of  excitation,   effect   of,   on   the  contraction 

curve,  ii.  Ill 

of  heart-beat,  variations  of,  i.  121 
of  progress  of  the  food  in  the  intestines,  i.  314 
of  respiratory  movements,  i.  425 
of  stimulation  in  voluntary  contractions,  ii. 

133 

limitations  of,  for  muscle,  ii.  126 
required  to  tetanize,  ii.  125 
of  transmission  of  the  pulse,  i.  140 
Reaction,  influence  of,  on  action  of  ptyaliu,  i. 

286 

of  bile,  i.  322 
of  blood,  i.  34 
of  degeneration,  ii.  48,  54 
of  gastric  juice,  i.  288 
of  intestinal  contents,  i.  310 
of  muscles,  ii.  159 
of  nerve-cells,  ii.  191 
of  nerve-fibres,  ii.  170 
of  pancreatic  juice,  i.  232,  300 
of  succus  entericus,  i.  308 
of  sweat,  i.  342 
of  urine,  i.  250,  334 
time,  ii.  291 

in  old  age,  ii.  491 

Rectus  abdominis,  expiratory  action  of,  i.  407 
Recurrent  laryngeal  nerve,  ii.  428 

sensibility  of  the  anterior  roots,  ii.  204 
Red  corpuscles,  behavior  of,  in  the  capillaries,  i. 

81 

color  of,  i.  35 
composition  of,  i.  51 
disintegration  of,  i.  45 
form  of,  i.  35 
function  of,  i.  35 
number  of,  i.  35 
origin  of,  i.  45,  46,  333 
size  of,  i.  35 
structure  of,  i.  35 
variations  in  the  number  of,  i.  46 
Red-striped  muscles,  physiological  properties  of, 

ii.  109 

Reduced  eye,  ii.  304 
Reduction,  i.  502 

processes  in  the  animal  body,  i.  536 
Reflex  acceleration  of  the  heart,  i.  177 
actions,  simple,  ii.  208 
arc,  ii.  209 
coughs,  i.  455 
discharge  of  bile,  i.  248 
frog,  ii.  209 

inhibition  of  the  heart,  i.  172 
secretion  of  gastric  juice,  i.  239 
of  pancreatic  juice,  i.  236 
of  saliva,  i.  230 
segmental  reaction,  ii.  210 
stimulation  of  the  nervous  system,  ii.  208 
tonus  of  muscular  tissues,  ii.  220 
vaso-motor  changes,  i.  202 
Reflexes,  co-ordinated,  ii.  211 

co-ordination  of  the  efferent  impulses  in,  ii. 

214 

effect  of  location  of  stimulus  on,  ii.  209 
of  strength  of  stimulus  on,  ii.  210 


Reflexes  from  the  isolated  cord  in  man,  ii.  213 

lumbar  cord,  ii.  213 
in  different  vertebrates,  ii.  212 
in  man,  ii.  216 
latent  period  of,  ii.  211 
of  a  purposeful  character,  ii.  215 
periodic,  ii.  216 
simple,  ii.  208 
spinal,  ii.  212 

reinforcement  of,  ii.  222 
summation  of  stimuli  in,  ii.  211 
through  sympathetic  ganglia,  vaso-motor,  i. 

200 

voluntary  control  of,  ii.  214 
Refractive  index  of  the  aqueous  humor,  ii.  303 
of  the  lens,  ii.  303 
of  the  vitreous  humor,  ii.  303 
media  of  the  eye,  ii.  302 
surfaces  of  the  eye,  ii.  303 
"Refractory  period  "  of  nerves,  ii.  57,  66 

of  the  heart,  i.  156,  158 

Regeneration  of  blood  after  hemorrhage,  i.  63 
of  lost  parts,  ii.  496 
of  nerves,  ii.  78,  199 
Registers  of  the  voice,  ii.  432 
Regular  astigmatism,  ii.  317 
Reinforcement  of  reflexes,  ii.  222 

of  the  knee-kick,  ii.  222 
Reissner,  membrane  of,  ii.  374,  379 
Rejuvenescence  by  sexual  reproduction,  ii.  442 
Relaxation  of  muscle,  nature  of,  ii.  99 
Rennin,  i.  238 

action  of,  on  milk,  i.  296 
occurrence  of,  in  gastric  juice,  i.  295 
of  the  kidneys,  i.  274 
preparation  of,  i.  295 
Reproduction,  asexual,  ii.  439 
of  leucophrys  patula,  ii.  442 
of  living  matter,  i.  18,  20 
of  onychodromus,  ii.  442 
of  stylonichia,  ii.  442 
sexual,  ii.  440 

elements  of,  ii.  440 
theory  of,  ii.  441 

Reproductive  organs,  female,  ii.  449 
internal  secretions  of,  ii.  462 
male,  ii.  443 

vaso-motor  nerves  of,  i.  208 
process,  ii.  463 

Residual  air,  definition  of,  i.  427 
Resonance  of  the  ear,  ii.  388 
Resonants,  ii.  436 

Resonators,  analysis  of  sounds  by,  ii.  385 
Respiration,  artificial,  i.  446 
associated  movements  of,  i.  408 
cutaneous,  i.  422 
definition  of,  i.  395 
heat  dissipated  in,  i.  488 
intensity  of,  i.  429 
internal',  i.  422 
nervous  mechanism  of,  i.  455 
rhythm  of,  i.  423 

Respiratory  activity,  conditions  affecting,  i.  429 
centres,  i.  455 

afferent  nerves  to,  i.  459 
conditions  influencing,  i.  458 
foetal,  i.  464 
rhythmicity  of,  i.  458 
food-stuffs,  definition  of,  i.  346 
movements,  circulatory  effects  of,  i.  447 
duration  of,  i.  424 
effect  of,  on  blood-pressure,  i.  448 
on  venous  circulation,  i.  95,  96 
frequency  of,  i.  425 
special,  i.  453 
nerves,  afferent,  i.  460 
efferent,  i.  463 


547 


R<  -piratory  pauses,  i.   I'.M 
pressure,  i.  408 
quotient,  i.  410 

(luring  hibernation,  i.  434 
relation  of.  to  the  diet,  i.  l^C, 
variations  of.  i.  437 
sounds,  i.   K'H 

Resuscitation  from  drowning,  i.  445 
Kete  inirabilc  of  the  Ifalpighian  corpuscles,  i. 

249 

vasenlosiim  of  the  testis,  ii.  447 
Retina,  changes  pioduced  in,  by  light,  ii.  330 
circulation  in.  ii.  .'!'-'•.' 
histology  of.  ii.  '•'>'"* 
oscillatory  activity  of,  ii.  344 
•pace-perceptions  by,  ii.  348 

structure  of,  ii.  327 
Retinal  currents,  ii.  331 

images,  inversion  of,  ii.  305 

si/e  of,  ii.  305 

stimulation,  sifter-effect  of,  ii.  345 
fati-ne  in,  ii.  344 
latent  period  of,  ii.  343 
laws  of,  ii.  343 

rise  to  maximum  for  different  colors,  ii.  343 
vessels,  demonstration  of,  ii.  321 
Reversion  to  ancestral  characters,  ii.  495 
Rhamnose,  i.  562 
Rheocord,  ii.  41 
Rheometer,  i.  99 
Rheonome,  ii.  31 
Rheoscope,  physiological,  ii.  148 
Rheoscopic  frog,  ii.  148 
Rheostat,  ii.  40 
Rhinencephalon,  ii.  241 
Rhomboideus  muscles,  respiratory  action  of,  i. 

405 

Rhythm  of  the  respiratory  movements,  i.  423 
Rhythmic  activity  of  the  vaso-constrictor  cen- 
tre, i.  201 
Rhythmicity  of  the  heart,  abnormal,  i.  152 

cause  of,  i.  148 

Ribs,  respiratory  movements  of.  i.  400 
Rickets,  i.  356,  525 
Right  lymphatic  duct,  i.  145 
Rigor  caloris,  ii.  57,  164 
contracting  in,  ii.  128 
effect  of  fatigue  on,  ii.  165 
mortis,  ii.  159 

chemical  changes  accompanying,  ii.  162 
contracture  of,  ii.  128 
disappearance  of,  ii.  162 
influence  of  the  nervous  system  on,  ii.  220 
nature  of  changes  in,  ii.  161 
Rima  glottidis.  ii.  423 
respiratoria,  ii.  423 
vocalis,  ii.  423 

Ringer's  solution  for  the  heart,  i.  190 
Ritter's  opening  tetanus,  ii.  37,  61 

tetanus,  ii.  132 
Rivinus,  ducts  of,  i.  217 
Rod-and-cone  layer,  function  of,  ii.  327 
Rod-pigment,  ii,  339.     See  Visual  purple. 
Rods  and  cones,  function  of,  n.  341 

number  of,  ii.  330 
of  Corti,  ii.  377 
retinal,  function  of,  ii.  341 
Rosel  von  Rosenhof  on   spontaneous  changes  in 

form  of  living  organisms,  ii.  19 
Rotation,  movements  of,  ii.  416 
Roy's  tonometer,  i.  iss 
Running,  ii.  421 
Rut  of  animals,  ii.  460 

\\ AROSE,  i.  564 

Sacculus  of  (ho  internal  ear,  ii.  373 
Saccus  endolymphaticus,  ii.  373 


Saddle-joint,  ii.  41»! 
Saliva,  composition  of,  i.  220,  283 
mineral  constituents  of.  i.  ~>:;o 
properties  of.  i.  •j-jn.  888 
uses  of,  i.  '.'-Mi 
Salivary  corpuscles,  i.  283 
glands,  i.  :_»ir» 

anatomy  of,  i.  217 
histolou'ical  changes  in,  i.  226 
histology  of,  i.  :.'!!» 
nerves  of,  i.  218,  •_'•_> I 
vaso-motor  nerves  of,  i.  222 
secretion,  action  of  drugs  on,  i.  229 
cerebral  control  of,  ii.  'J70 
normal  mechanism  of,  i.  '.'".<> 
Salkowski's  reaction  for  cholesterin,  i.  575 
Salmin,  i.  580 

Salt  solution,  physiological  injection  of,  i.  64 
Salt-licks,  i.  355 
Salts,  absorption  of,  i.  318 
inorganic,  of  muscle,  ii.  168 

relation  of,  to  irritability,  ii.  58 
lymphagogic  action  of,  i.  73 
nutritive  value  of,  i.  276,  354 
of  heavy    metals,   action  of,  on  nerve    and 

muscle,  ii.  60 

Santorini,  cartilage  of,  ii.  422,  425 
Saponification  of  fats,  i.  306,  558 
Saprin,  i.  543 
Sarcin,  i.  553 

Sarcode  of  sponges,  contractility  of,  ii.  20 
Sarcolactic  acid,  i.  546 

formation  of,  in  rigor  mortis,  ii.  161,  162 
in  clotting  of  muscle-plasma,  ii.  164 
relation  of,  to  fatigue  contracture,  ii.  131 
Sarcoplasm,  ii.  104 
Sarcosin,  i.  538 
Saturation  of  colors,  ii.  342 
Scala  media,  ii.  375 
tympani,  ii.  372,  375 
vestibuli,  ii.  372,  375 

Scaleni  muscles,  inspiratory  action  of,  i.  401 
Schneiderian  membrane,  ii.  408 
Scombrin,  i.  580 
Scrotum,  ii.  443 

Sebaceous  glands,  structure  of,  i.  257 
secretion,  composition  of,  i.  342 
function  of,  i.  258 
physiological  value  of,  i.  342 
Sebum,  composition  of,  i.  257 
Secondary  degeneration  of  nerves,  ii.  197 
position  of  the  eye,  ii.  :1W 
tetanus,  ii.  150 
Secreting  glands,  electrical  changes  in,  i.  231 

histological  changes  in,  i.  226 
Secretion,  antilytic,  i.  230 
biliary,  i.  248 

capillaries  of  the  gastric  glands,  i.  238 
definition  of,  i.  211 
gastric,  240 

histological  changes  during,  i.  226 
internal,  definition  of,  i.  211 
intestinal,  i.  243 
mammary,  i.  264 
nierliaiiism  of,  i.  213 
pancreatic,  i.  235 
paralytic,  i.  229 

psychical,  of  gastric  juice,  i.  239 
relation  of,  to  intensity  of  stimulus,  i.  223 
salivary,  i.  230 

cerebral  control  of,  ii.  270 
sebaceous,  i.  257,  342 
sweat,  i.  259 
urinary,  i.  251 

Secretions,  general  characteristics  of.  i.  213 
Secret  oiroL'iies  for  the  gastric  {.'lands,  i.  359 
Secretory  centre,  salivary,  i.  230 


548 


GENERAL  INDEX. 


Secretory  fibres  proper,  definition  of,  i.  224 
nerves,  evidence  for,  i.  222 
fatigue  of,  ii.  96 
mode  of  action  of,  i.  225 
of  the  adrenal  bodies,  i.  272 
of  the  kidneys,  i.  251 
of  the  liver,  i.  247 
of  the  mammary  glands,  i.  263 
of  the  pancreas,  i.  232 
of  the  stomach,  i.  239 
of  the  sweat  glands,  i.  259 
salivary,  endings  of,  i.  220 
significance  of,  i.  214 
stimulation  of,  i.  222 
Segmental  arrangement  of  nerve-elements,  ii. 

206 

reactions,  reflex,  ii.  210 
Segmentation,  ii.  467 
Segmeritation-centrosomes,  ii.  469 
Segmentation-nucleus,  ii.  466 
Semen,  composition  of,  ii.  445 
Semicircular  canals,  membranous,  ii.  373 
of  the  bony  labyrinth,  ii.  371 
relation  of,  to  equilibrium,  ii.  405 
section  of,  ii.  405 
Semilunar  valves,  i.  110 
Seminal  vesicles,  ii.  443 
function  of,  ii.  448 
secretion  of,  ii.  446 
Semi-vowels,  ii.  436 
Senescence  of  nerve-cells,  ii.  18%^~\ 
of  the  central  nervous  system,  ir!  295 
phenomena  of,  ii.  486 
Sensation,  cutaneous,  definition  of,  ii.  390 
muscular,  definition  of,  ii.  390 
of  after-pressure,  ii.  394     \ 
of  light,  mechanism  for  the  production  of,  ii. 

331 

of  temperature,  ii.  397 
Sense  of  equilibrium,  ii.  404 

of  touch,  ii.  392 
Sensory  areas  of  the  cortex,  determination  of,  ii. 

253 
conducting  paths  in  the  spinal  cord,  ii.  234 

continuation  of,  in  the  brain,  ii.  235 
cortical  areas  in  man,  ii.  255 

motor  responses  from,  ii.  253 
relative  functional  importance  of,  ii.  270 
disturbance  from  hemisection  of  the  cord,  ii. 

230 

impulses,  path  of,  in  the  central  nervous  sys- 
tem, ii.  226 
relation    of,   to    the    maintenance    of   the 

erect  posture,  ii.  419 
nerve-endings  in  skeletal  muscle,  ii.  402 
in  tendon,  ii.  402 
in  the  skin,  ii.  391 

nerves,  influence  of,  on  respiration,  i.  463 
of  the  heart,  i.  172 
rate  of  conduction  in,  ii.  91 
relation  of,  to  the  respiratory  centre,  i.  459 
reflex  influence  of,  on  the  pulse-rate,  i.  175 
paths,  degeneration  of,  after  section  of  the 

dorsal  roots,  ii.  227 
in  the  central  nervous  system,  ii.  226 
regions  of  the  cortex  cerebri,  ii.  252 
stimulation,  relation  of,  to  sleep,  ii.  291 
Septal  nerves  of  the  frog's  heart,  i.  166 
Serous  cavities,  i.  146 
Serrati  postici   inferiores,  respiratory  function 

of,  i.  399 

superiores,  inspiratory  action  of,  i.  402 
Serum,  bactericidal  action  of,  i.  36 
globulicidal  action  of,  i.  36 
osmotic  pressure  of,  i.  68 
toxicity  of,  i.  36 
Serum-albumin,  action  of,  on  carbonates,  i.  517 


Serum-albumin,  amount  of,  in  the  blood,  i.  52 

composition  of,  i.  52 

functions  of,  i.  52 

properties  of,  i.  52 
Sex,  characters  of,  ii.  442 

influence  of,  on  heat  production,  i.  482 
on  pulse-rate,  i.  121 
on  respiration,  i.  430 

of  offspring,  determination  of,  ii.  483 

origin  of,  ii.  441 

relation  of  body-temperature  to,  i.  470 

of  brain-weight  to,  ii.  276 
Sexual  characters,  ii.  442 

glands,  accessory,  ii.  445 

organs,  ii.  443 

reproduction,  ii.  440 

congenital  variations  resulting  from,  ii.  501 
theory  of  origin  of.  ii.  441 
Shark,  reflexes  in,  ii.  2i2 

removal  of  cerebral  hemispheres  in,  ii.  263 
Shivering,  i.  362,  491 
Shrapnell's  membrane,  ii.  365 
Siamese  twins,  ii.  483 
Silicic  acid,  properties  of,  i.  519 
Silicon,  i.  519 
Simple  muscular  contraction,  duration  of,  ii.  102, 

108 
explanation  of,  ii.  101 

proteids,  i.  576 

Simultaneous  contrast,  ii.  347 
Singing  voice,  ii.  434 
Sinuses  of  Valsalva,  i.  Ill 
Size,  increase  of  the  embryo  in,  ii.  487 

influence  of,  on  pulse-rate,  i.  121 

of  nerve-cells,  ii.  175 
Skatol,  i.  572 

elimination  of,  i.  340 

occurrence  of,  in  feces,  i.  320 
Skiascopy,  detection  of  astigmatism  by  means 

of,  ii.  319 
Skin,  functions  of,  i.  341 

glands  of,  i.  257 

tactile  areas  of,  ii.  395 
Sleep,  ii.  291 

cause  of,  ii.  292 

condition  of  the  pupils  in,  ii.  325 

curve  of  intensity  of,  ii.  294 

effect  of  loss  of,  ii.  295 
on  metabolism,  i.  361 
on  respiration,  i.  424 
on  the  respiratory  quotient,  i.  438 

responsiveness  to  stimuli  in,  ii.  293 
Smegma  prseputii,  i.  257 
Smell,  ii.  408 

comparative  physiology  of,  ii.  409 

subjective  sensations  of,  ii.  410 
Smooth  muscle,  rate  of  conduction  in,  ii.  89 
Snails,  regeneration  of  lost  parts  in,  ii.  496 
Sneezing,  i.  454 
Snoring,  i.  455 
Sobbing,  i.  454 
Sodium  ammonium  phosphate,  i.  523 

carbonates,  i.  522,  523 

chloride,  nutritive  history  of,  i.  521 

phosphates,  i.  522 

sulphate,  i.  522 
Somatic  death,  ii.  491 
Somatoplasm,  definition  of,  ii.  496 
Somatopleure,  ii.  472 
Sound,  physical,  ii.  381 

quality  of,  ii.  383 
Sound-waves,  amplitude  of,  ii.  381 

composite,  ii.  384 

limits  of  perceptions  of,  ii.  382 

production  of,  ii.  381 
Space  illusions,  ii.  354 
Space-perception  from  visual  sensations,  ii.  347 


GENERAL  INDI.X. 


Special  respiratory  movements,  i.  .r>:; 
Specialization  of  functions,  i.  21 
Specitic  energies  of  nerves,  doctrine  of,  ii. 
er.eriry  of  I  In-  opiic  nerve,  ii.  331 
gravity  of  blood,  i.  .'II 

of  blood-corpuscles,  i.  34,  35 

of  muscle,  ii.  !.">!> 

of  tbe  eiicephalon,  ii.  ~7."> 

of  the  nervous  system  at  different  ages,  ii. 

284 

of  urine,  i.  •_'."">  1 
beat,  definition  of,  i.  477 

of  tbe  human  body,  i.  504 
nerve-energy,  doctrine  of,  ii.  399 
Spectral   colors,   incomplete  saturation    of,    ii. 

348 

Spectroscope,  i.  40 
Spectrum,  ii.  332 
definition  of,  i.  40 

luminous  intensity  of  the  colors  of,  ii.  340 
of  CO-htemoglobin,  i.  44 
of  luernoglobin,  i.  4'J 
of  oxyhsemoglobin,  i.  41 
solar,  i.  41 

top,  Benham's,  ii.  344 
Speech,  dependence  of,  on  hearing,  ii.  431 

elements  of,  ii.  433 
Speech-centre,  ii.  257 
Spermaceti,  i.  540 
Sperm-aster,  ii.  467 
Spermatids,  ii.  445 
Spennatocytes,  ii.  445 
Spermatozoa,  ii.  440 
contractility  of,  ii.  20 
discovery  of,  ii.  443 
entrance  of,  into  the  uterus,  ii.  465 
locomotion  of,  ii.  465 
maturation  of,  ii.  445 
movements  of,  ii.  444 
structure  of,  ii.  443 
Spermin,  ii.  445 

physiological  action  of,  i.  273 
Sperm-nucleus,  ii.  466 
Spherical  aberration,  ii.  315 
Sphincter  ani,  cerebral  control  of,  ii.  270 
antri  pylorici,  i.  377 
iridis,  ii.  3-_»:i 
pylori,  i.  377,  381 
urethra,  i.  390 

contraction  of,  in  erection,  ii.  464 
vesicse  internus,  i.  390 
Sphincters  ani,  i.  386 
Sphygruogram,  i.  143 
Sphygmograph,  i.  142 
Sphyginoraanometer,  i.  141 
Sphygmometer,  i.  141 

Spinal  centres  for  vaso-motor  nerves,  i.  199 
cord,  afferent  paths  of,  ii.  229 
central  neurones  of,  ii.  203 
degeneration  of,  from  hemlsection,  ii.  228 
efferent  neurones  of,  ii.  203 
motor  tracts  of,  ii.  245 
reflexes  jjji  man  after  section  of,  ii.  213 
schematic? cross-section  of,  ii.  202 
weight  of,  ii.  274 

ganglion-cells,  development  of,  ii.  178 
nerve-roots,  section  of,  ii.  198 
Spiral  ganglion  of  the  ear,  ii.  376 
ligament  of  the  cochlea,  ii.  379 
Spirometer,  i.  427 

Splanchnic  nerves,  gastric  fibres  of,  i.  382 
influence  of,  on  blood-pressure,  i.  173 

on  respiration,  i.  463 
intestinal  fibres  of,  i.  385 
stimulation  of,  i.  173 
Spleen,  composition  of,  i.  333 
function  of,  i.  322 


Spleen,  innervation  of,  i.  :;:;:; 

movements  of.  i.  :;-.".' 

vaso-motor  nerves  of,  i.  -.'07 
Staircase  contraction-,,  ii.  Uii.  1  TJ 
relation  of.  to  tetanus,  ii.  U  I 
Standing,  ii.  11* 
Stannius's  ligatures,  i.  178 
Stapedius  muscle,  ii.  370 
Stapes,  ii.  ::i;7 
Starch,  i.  r,i;<; 

digestion  of,  i.  284,  305 

hydrolysis  of,  by  acids,  i.  286 

by  amylolytic  ferments,  i.  285 
Starvation,  effect  of,  on  metabolism,  i.  362 
on  the  nervous  system,  ii 

glycogeu  disappearance  during,  i.  331 

nutrition  during,  i.  .'!.'>( i 

phosporus  excretion  in,  i.  516 

potassium  excretion  in,  i.  520 
Static  equilibrium,  organs  of,  ii.  407 
Stature,  relation  of  brain-weight  to,  ii.  276 
Steapsin,  i.  232,  -J-o 

demonstration  of,  i.  306 

occurrence  of,  i.  305 
Stearic  acid,  i.  541 
Stenson's  duct,  i.  217 

experiment,  ii.  67 
Stercorin,  i.  575 
Stereoscope,  ii.  356 

Sterno-cleido-mastoid   muscles,  respiratory  ac- 
tion of,  i.  404 
Sterno-hyoid  muscle,  ii.  425 
Sterno-thyroid  muscle,  ii.  425 
Sternum,  respiratory  movements  of,  i.  401 
Stethograph,  i.  423 
Stimulants,  effect  of,  on  muscular  work,  ii.  75 

of  the  sweat  glands,  i.  260 

physiological  action  of,  i.  357 
Stimulation  fatigue  of  non-medullated  nerves, 
ii.  180 

of  the  cortex,  ii.  241 

Stimuli,  artificial,  effect  of,  on  the  heart,  i.  156 
classification  of,  ii.  23 

chemical,  of  muscle,  ii.  131 

conditions  determining  efficiency  of,  ii.  28 

effect  of  changing  intensity  of,  ii.  32 
of  their  repetition  on  irritability,  ii.  65 
of  varying  strength  of,  ii.  39 

galvanic,  contracture  effect  of,  in  muscles,  ii. 
131 

variations  in  intensity  of,  ii.  31 
Stokes' s  reagent,  composition  of,  i.  43 
Stomach,  absorption  in,  i.  312 

extirpation  of,  i.  299 

glands  of,  i.  237 

immunity  of,  to  its  own  secretion,  i.  297 

innervation  of,  i.  381 

movements  of,  i.  377,  378 

musculature  of,  i.  377 
Strabismus,  ii.  300 
Strise  acusticse,  ii.  237 

gravidarum,  ii.  477 
Stromuhr  of  Ludwig,  i.  99 
Strontium,  i.  526 
Strychnin,  action  of,  on  diffusion  of  impulses  in 

the  cord,  ii.  217 
on  end-plates,  ii.  27 
on  sympathetic  ganglia,  ii.  219 

effect  of,  on  body-temperature,  i.  472 

tetanus,  ii.  217 
Sturin,  i.  580 

Stylo-hyoid  muscle,  ii.  426 
Stylonychia,  reproduction  of,  ii.  442 
Sublingual  gland,  anatomy  of,  i.  217 
Submaxillary  gland,  anatomy  of,  i.  217 
Successive  contrast,  ii.  346 
Succinic  acid,  i.  557 


550 


GENERAL  INDEX. 


Succus  entericus,  i.  243 

action  of,  ou  carbohydrates,  i.  309 
collection  of,  i.  308 
digestive  action  of,  i.  308 
ferments  of,  i.  308 
Suction  action  of  the  heart,  i.  134 
Sudorific  drugs,  i.  260 
Suffocation.     See  Asphyxia. 
Sugar  injections,  lyruphagogic  action  of,  i.  73 
of  muscles,  ii.  167    . 
use  of,  in  muscular  work,  ii.  74 
Sugars,  absorption  of,  i.  313,  317 

consumption  of,  by  the  tissues,  i.  353 
effect  of,  on  glycogen  formation,  i.  328 
synthesis  of,  i.  533 
Sulphates  of  the  urine,  estimation  of,  i.  506 

origin  of,  i.  506 
Sulph-hsemoglobin,  i.  506 
Sulphur,  elimination  of,  i.  340 
metabolism  of,  i.  507 
neutral,  i.  506 
occurrence  of,  i.  505 
Sulphuretted  hydrogen,  inhalation  of,  i.  440 

properties  of,  i.  506 
Sulphuric  acid,  i.  506 
Sulphurous  acid,  i.  506 

Summation  of  contraction  in  muscle,  ii.  121 
of  stimuli  in  nerve-cells,  ii.  190 

in  reflex  action,  ii.  211 
Superior  laryngeal  nerve,  ii.  428 

nerves,  influence  of,  on  respiration,  i.  459, 

462 

oblique  muscle,  ii.  299 
rectus  muscle,  ii.  299 
Supernumerary  digits,  ii.  494 
Supplemental  air,  definition  of,  i.  427 
Suprarenal  capsules,  extirpation  of,  i.  271 
Sustentacular  cells  of  the  crista  acustica,  ii.  374 
Suture,  ii.  414 
Swallowing,  i.  375 

action  of  the  epiglottis  in,  ii.  422 
Sweat,  amount  of,  i.  258,  342 
composition  of,  i.  259,  342 
nitrogenous  constituents  of,  i.  512 
Sweat-centres,  spinal,  i.  261 
Sweat-glands,  secretory  nerves  of,  i.  259 
stimulation  of,  i.  260 
structure  of,  i.  258 
Sweat-nerves,  i.  259 

Sweat-secretion,  action  of  drugs  on,  i.  260 
Sylvian  heat-centre,  ii.  271 
Sympathetic  ganglia,  action  of  nicotin  on,  ii.  219 

of  strychnin  on,  ii.  219 
nerves,  cardiac,  i.  168,  171 
pulmonary,  i.  466 

reflex  influence  of,  on  the  pulse-rate,  i.  175 
to  the  iris,  ii.  324 
pains,  ii.  400 

secretory  fibres  to  the  pancreas,  i.  232 
to  the  salivary  glands,  i.  218,  222 
system,  connection  of,  with  the  cerebro-spinal, 

ii.  218 

post-ganglionic  fibres  of,  ii.  219 
pre-ganglionic  fibres  of,  ii.  218 
vaso-motor  centres,  i.  200 
vibration,  ii.  385 
Symphysis,  ii.  414 
Syndesmosis.  ii.  414 
Synovial  fluid,  ii.  415 
Synthesis  of  proteids,  i.  518,  582 

of  sugars,  i.  563 

Synthetic  processes  of  plants,  i.  518 
Syntonin,  absorption  of,  i.  315 

occurrence  of,  in  peptic  digestion,  i.  292 
Syphilis,  hereditary  transmission  of,  ii.  498 
Systole,  auricular,  i.  124.  136 
ventricular,  i.  123 


TACTILE  areas  of  the  skin,  ii,  395 

corpuscle,  ii.  390,  392 
Tartar,  i.  524 
Taste,  nerves  of,  ii.  410 

organs  of,  ii.  430 
Taste-buds,  ii.  410 

Taste-nerves,  nuclei  of  origin  of,  ii.  236 
Taste-perceptions,  conditions  affecting,  ii.  411 
Taste-sensations,  conditions  which  influence,  ii. 

411 
primary,  ii.  412 

distribution  of.  ii.  413 
Tauriu,  i.  507,  543 

in  muscles,  ii.  167 
Tea,  nutritive  value  of,  i.  357 
stimulating  action  of,  ii.  75 
Tectorial  membrane,  ii.  379 
Tegmen  of  the  tympanum,  ii.  364 
Temperature,  axillary,  i.  468 

body-,  effect  of,  on  respiratory  activity,  i.  432 
influence  of  drugs  on,  i.  472 
lowering  of,  i.  472 
variations  of,  i.  469 
effect  of,  on  enzymes,  i.  281 
on  heat  dissipation,  i.  487 
on  metabolism,  i.  362 
on  muscular  contraction,  ii.  136 
on  sweat  glands,  i.  260 
on  the  respiratory  quotient,  i.  438 
on  tryptic  digestion,  i.  301 
external,  effect  of,  on  respiration,  i.  426 
on  respiratory  exchanges,  i.  432 
on  thermotaxis,  i.  496 
influence  of,  on  conductivity,  ii.  92 
on  heat  production,  i.  483 
on  irritability,  ii.  56 
on  ptyalin,  i.  286 
on  rigor  mortis,  ii.  161 
limits  of  muscular  contraction,  ii.  136 
nerves,  ii.  397 
of  animals,  i.  467 
of  respired  air,  i.  410 
of  the  blood  from  the  brain,  ii.  288 
post-mortem  rise  of,  i.  497 
regulation  of,  i.  473 

rise  of,  from  lesions  of  corpora  striata,  ii.  271 
sense,  ii.  397 
spots  of  the  skin,  ii.  398 
topography  of,  i.  468 
Temporal  muscle,  i.  372 
Tendon  reflexes  after  cerebellar  injury,  ii.  272 

sensory  nerve-endings  in,  ii.  402 
Tension,  effect  of,  on  contraction  curve,  ii.  109 
on  irritability  of  nerves  and  muscles,  ii.  56 
on  latent  period,  ii.  110 
of  the  blood-gases,  i.  415 
Tensor  tympani  muscle,  ii.  369 
Tentacles  of  Actiniae,  contractility  of,  ii.  20 
Terminal  arborizations,  definition  of,  ii.  174 
Tertiary  positions  of  the  eye,  ii.  299 
Testicular  extracts,  action  of,  i.  273 
Testis,  ii.  443 
ducts  of,  ii.  447 
histology  of,  ii.  446 
internal  secretion  of,  i.  273 
Tetanic  contractions,  height  of,  ii.  120 

relative  intensity  of,  ii.  126 
Tetanomotor,  ii.  56 
Tetanus,  ii.  66 
analysis  of,  ii.  123 
complete,  ii.  120 

curves,  introductory  peaks  of,  ii.  124 
explanation  of,  ii.  121 
from  strychnin  poisoning,  ii.  217 
incomplete,  ii.  117 
normal  physiological,  ii.  132 
of  the  heart,  i.  165 


GENERAL   /.V/>A'A'. 


551 


Tetanus  of  the  muscles,  ii.  127 

rate  of  stimulation  required  for,  ii.  125 

Ritter's,  ii.  37,  61 

secondary,  ii.  150 

voluntary,  ii.  133 

Wundfs.'ii.  37.  <il 
Tetramethylene-diamin,  i-  543 
Thalamus,  Vortical  connections  of,  ii.  271 

heat -cent  re  of.  ii.  271 
Theobromin,  i.  "),">:> 
Theophyllin,  i.  553 
Thermal  energy  liberated  in  muscle,  ii.  141 

stimulation  of  nerve,  ii.  25 
Tln-i-i no-accelerator  centres,  i.  492 
Thermogeneds,  i.  477 

mechanism  of,  i.  489 
Thermogenio  centres,  i.  491 

n.-rvcs.  i.  490 

tissues,  i.  490 

Thermo-inhibitory  centres,  i.  492 
Thermolysis,  i.  485 

mechanism  of,  i.  494 
Thermopile,  ii.  1  I:.' 
Thermotaxis,  i.  489,  495,  496 

relation  of  cerebrum  to,  ii.  270 
Thiolactic  acid,  i.  547 
Thirst,  ii.  404 
Thiry-Vella  fistula,  i.  308 
Thoracic  duct,  i.  145 
Thorax,  effects  of  opening,  i.  115 

movements  of,  in  respiration,  i.  397 

negative  pressure  in,  i.  396 
Thrombin,  i.  58,  280 
Thrombus,  i.  60 
Thymic  acid,  i.  579 
Thyro-arytenoid  muscles,  ii.  424,  426 
Thyroglobulin,  i.  509 
Thyro-hyoid  muscle,  ii.  425 
Thyroid  cartilage,  ii.  425 

extract,  injection  of,  i.  269,  270 

gland,  relation  of,  to  growth  of  the  central 

nervous  system,  ii.  289 
Thyroidectomy,  i.  269 
Thyroids,  anatomy  of,  i.  267 

extirpation  of,  i.  268 

functions  of,  i.  268 

grafting  of,  i.  269 

internal  secretion  of,  i.  270 
Thyroiodine,  i.  509 
Tidal  air,  volume  of,  i.  426 
Tigroid  of  nerve-cells,  ii.  179 
Timbre  of  musical  tones,  ii.  383,  387 
Time  intervals,  perception  of,  by  the  ear,  ii.  388 

of  a  complete  circulation,  i.  79 
Tinctures,  definition  of,  i.  535 
Tissue  death,  ii.  492 
Tissue-proteid,  definition  of,  i.  346 
Tissue-respiration,  i.  422 
Tissues,  growth  of,  ii.  486 
Tobacco  smoke,  action  of,  on  nerves,  ii.  60 
Tones,  combinational,  ii.387 

differential,  ii.  387 

fundamental,  ii.  383 

loud  ness  of,  ii.  381 

pitch  of,  ii.  381 

simple,  ii.  381 

Tongue,  distribution  of  taste-sensations  on,  ii. 
413 

vaso-motor  nerves  of,  i.  204 
Tonicity  of  involuntary  muscle,  i.  371 

of  vaso-con stricter  centre,  i.  199 
Tonograph,  definition  of,  i.  127 
Tonometer,  i.  188 

Tonus,  muscular,  in  the  insane,  ii.  220 
reflex  origin  of,  ii.  220 

of  muscles,  ii.  143 

ventricular,  during  vagus  stimulation,  i.  163 


Touch  illusions,  ii.  3!Hi 
sensations,  ii.  :i!i-j 

locali/.at  ion  of.  ii.  3!M 
Tractus  solitarius,  ii.  •_':;«; 
Transfusion  of  blood,  i.  64 
Transversalis    abdominis     muscle,   respiratory 

action  of,  i.  407 

Trapi'/.ius  muscle,  respiratory  action  of,  i.  405 
Traulie-Hering  waves,  i.  201 
Tremors,  ii.  132 
Triangulares  sterni  muscles,  expiratory  action 

of,  i.  407 
Trigeminal  nerves,  central  paths  of,  ii.  238 

influence  of,  on  respiration,  i.  463 
Ti  imethylamine,  i.  541 
Trioses,  i.  559 
Triplets,  ii.  483 

Trommer's  test  for  carbohydrates,  i.  562 
Tropseolin  00  test  for  mineral  acid,  i.  289 
Trophic  impulses  to  muscles,  ii.  70 

influence  of  neurones  on  one  another,  ii.  197 

of  the  vagi  on  the  heart,  i.  167 
nerves  of  the  muscles,  ii.  70 
of  the  salivary  glands,  i.  224 
pulmonary,  i.  466 
Trypsin,  i.  232 

effect  of,  on  blood  coagulation,  i.  63 
extracts,  preparation  of,  i.  301 
properties  of,  i.  301 
Trypsinogen,  i.  235 

granules,  i.  235 
Tryptic  digestion,  products  of,  i.  302 

value  of,  i.  304 
Tryptophan,  i.  574 
Tubules,  uriniferous,  i.  250 
Tubuli  recti  of  the  testis,  ii.  447 
Tunicin,  i.  566 
Turtle's  striped  muscle,  time  of  contraction  in, 

ii.  108 

Twins,  ii.  482 
Tympanic  membrane,  ii.  364 

effect  of  destruction  of,  ii.  370 
pressure-sensations  of,  ii.  382 
vibrations  of,  ii.  370 
Tympanum,  ii.  363 

mechanics  of,  ii.  368 
Tyrosin,  i.  570 

formation  of,  in  tryptic  digestion,  i.  303 

ULTIMUM  moriens,  ii.  492 
Umbilical  arteries,  ii.  474 

vein,  ii.  474 

Umbo  of  the  tympanic  membrane,  ii.  365 
Unconsciousness,  ii.  293 
Unipolar  excitation  for  localized  excitation,  ii. 

45 

nerve-cells,  development  of,  ii.  178 
stimulation,  ii.  30 

principles  of,  ii.  43 
Units,  calorimetric,  i.  477 
Unorganized  ferments,  definition  of,  i.  279 
Urea,  amount  of,  in  sweat,  i.  335 
in  the  blood,  i.  51 
in  the  urine,  i.  335 
antecedents  of,  i.  335 
elimination  of,  i.  252 
estimation  of,  i.  549 
formation  of,  after  removal  of  the  liver,  i.  337 

in  the  liver,  i.  331 
in  muscles,  ii.  lb? 
origin  of,  in  the  body,  i.  550 

in  the  liver,  i.  266 
preparation  of,  i.  548 
from  proteid,  i.  337 
presence  of,  in  sweat,  i.  337 
properties  of,  i.  549 
Ureters,  movements  of,  i.  371,  389 


552 


GENERAL  INDEX. 


Urethra,  ii.  443 

structure  of,  ii.  448 
Uric  acid,  formation  of,  i.  338 
in  the  liver,  i.  322 
in  the  spleen,  i.  333 
in  muscles,  ii.  167 
molecular  structure  of,  i.  554 
occurrence  of,  i.  338 
origin  of,  in  birds,  i.  557 

in  mammals,  i.  338,  556 
preparation  of,  i.  555 
properties  of,  i.  555 
Urinary  bladder,  innervation  of,  i.  392 

movements  of,  i.  390 

pigments,  origin  of,  from  haemoglobin,  i.  45 
secretion,  normal  stimulus  for,  i.  255 
relation  of,  to   the   blood-flow  through  the 

kidney,  i.  253 

Urine,  acidity  of,  after  meals,  i.  290 
composition  of,  i.  250,  334 
ethereal  sulphates  of,  i.  572 
secretion  of,  i.  251 
Uriuiferous  tubules,   secretory  function   of.  i. 

252 

structure  of,  i.  250 
Urobilin,  i.  574 
Uterus,  ii.  443,  456 
Utriculus  of  the  internal  ear,  ii.  373 

VAGINA,  ii.  443,  462 

Vagus,  anabolic  action  of,  on  the  heart,  i.  166 
anatomy  of,  in  the  dog,  i.  159 
cardiac  branches  of,  i.  159 
central  path  of  the  afferent  fibres  in,  ii.  236 
effect  on  the  heart,  nature  of,  i.  166 
gastric  branches  of,  i.  381 
inhibition,  dependence  of,  on  the  character 

of  the  stimulus,  i.  165 
intestinal  branches  of,  i.  385 
nerve,  fatigue  of,  ii.  96 

pulmonary  branches  of,  i.  465 
rate  of  conduction  in,  ii.  90 
relation  of,  to  apnoea,  i.  442 
respiratory  function  of,  i.  459 
pneumonia,  i.  466 
secretory  fibres  of,  to  the  pancreas,  i.  232 

to  the  stomach,  i.  239 
stimulation,  auricular  effects  of,  i.  164 
effect  of,  latent  period  of,  i.  162 
on  the  heart,  i.  152,  163 
on  the  ventricle,  i.  162 
Valsalva's  experiment,  i.  452 

sinuses,  i.  Ill 

Valves,  auriculo- ventricular,  i.  108 
of  lymphatic  vessels,  i.  146 
semilunar,  i.  110 
Valvulse  conniventes,  value  of,  in  absorption,  i. 

314 
Variation  of  the  offspring  in  reproduction,  ii. 

500 

Variations,  somatic,  classification  of,  i.  497 
Vas  deferens,  ii.  447 
Vasa  deferentia,  ii.  443 

efferentia  of  the  testis,  ii.  447 
Vaseline,  i.  531 
Vaso-constrictor  centre,  rhythmical  activity  of, 

i.  201,  451 

nerves,  discovery  of,  i.  193 
Vaso-dilator  nerves,  discovery  of,  i.  194 
Vaso-motor  centre,  medullary,  i.  198 
centres,  spinal,  i.  199 
sympathetic,  i.  200 
nerves,  anatomy  of,  i.  198 

methods  of  investigating,  i.  195 
of  the  brain,  i.  203 
of  the  cranial  vessels,  ii.  286 
of  the  generative  organs,  i.  208 


Vaso-motor  nerves  of  the  head,  i.  204 
of  the  heart,  i.  206 
of  the  intestines,  i.  206 
of  the  kidneys,  i.  207,  256 
of  the  limbs,  i.  209 
of  the  liver,  i.  206 
of  the  lungs,  i.  205,  466 
of  the  muscles,  i.  210 
of  the  pancreas,  i.  207 
of  the  portal  system,  i.  209 
of  the  salivary  glands,  i.  222 
of  the  spleen,  i.  207 
of  the  tongue,  i.  205 
of  the  veins,  i.  195 
special  properties  of,  i.  197 
reflexes,  i.  201 

through  the  vagi,  i.  172 
Vegetable  foods,  composition  of,  i.  278 

proteids,  i.  577 

Veins,  effect  of  compression  of,  on  lymph  forma- 
tion, i.  72 

entrance  of  air  into,  i.  97 
rate  of  flow  in,  i.  101 
vaso-motor  nerves  of,  i.  209 
Velocity  of  blood-flow,  i.  99-101 
Venae  Thebesii,  i.  184 
Veno-motor  nerves  of  the  limbs,  i.  209 
Venous  blood-flow,  effect  of  the  auricles  on,  i. 

137 

circulation,  i.  95,  96 
pressure,  i.  91,  94 
pulse,  respiratory,  i.  96 
Ventilation,  principles  of,  i.  439 
Ventral  nerve-roots,  number  of  fibres  of,  ii.  230 
Ventricles,  independent  rhythm  of,  i.  152 
of  Morgagni,  ii.  422 
of  the  brain,  capacity  of,  ii.  274  , 

work  done  by,  i.  106,  107 
Ventricular  bands,  ii.  422 
cycle,  analysis  of,  i.  133 
diastole,  duration  of,  i.  123 
pressure-curves,  analysis  of,  i.  128 
pressures,  i.  125 
systole,  duration  of,  i.  123 

Veratria,  action  of,  on  coagulation  of  muscle- 
plasma,  ii.  164 

on  muscular  contraction,  ii.  129,  137 
on  nerves  and  muscles,  ii.  60 
effect  of,  on  muscular  contraction,  ii.  128 
Vernix  caseosa,  i.  258 
Vessels  of  Thebesius,  i.  186 
Vertigo  in  diseases  of  the  ear  labyrinth,  ii.  406 
Vestibular  root  of  the  auditory  nerve,  central 

path  of,  ii.  237 

Vestibule  of  the  bony  labyrinth,  ii.  371 
Vibrations  of  the  tympanic  membrane,  ii.  370 
transmission  of,   through   the   labyrinth,   ii. 

376 

Villus,  intestinal,  structure  of,  i.  318 
Viscero-motor  nerves  to  the  intestines,  i.  385 
Viscosity  of  irrigating  media  for  the  heart,  i. 

191 

Vision,  binocular,  ii.  356 
far-point  of,  ii.  312 
indirect,  ii.  341 
near-point  of,  ii.  312 
pseudoscopic,  ii.  357 
sterescopic,  ii.  357 
Visual  area  of  the  cortex,  ii.  253 

impulses,  place  of  origin  of,  in  the  retina,  ii. 

327 

judgments  of  distance,  ii.  348 
of  size,  ii.  350 

and  distance,  ii.  354 
purple,  i.  575;  ii.  330 

adaptation  of  the  eye  by,  ii.  326 
sensation,  intensity  of,  ii.  339 


GENERAL  INDKX. 


Vital  capacity  of  the  lungs,  i.  427 

force,  definition  of.  i.  25 
Vitrlliu,  composition  of,  i.  .~>?;i 
Vitelline  membrane,  absence  of,  in  human  ova, 

ii.  450 
Vitreous  humor,  opacities  in,  ii.  321 

refract  ivf  index  of,  ii.  303 
Vocal  cords,  false,  ii.  422 

true,  ii.  -12.'$ 
Voice,  ii.  430 

changes  at  puberty  in,  ii.489 
etfect  of  am-  on,  ii.  431 
pitch  of,  ii.  \'.\-l 
registers  of.  ii.  432 

Voice-production,  ii.  421 

mechanism  of,  ii.  431 
Voices,  classification  of,  ii.  433 
Volta,  ii.  •> 
Voltaic  pile,  ii.  28 
Voluntary  control  of  the  heart,  i.  178 

muscular  contractions,  fatigue  of,  ii.  134 
tetanic  character  of,  ii.  133 

reactions,  atl'erent  paths  of,  ii.  226 
anatomical  mechanism  of,  ii.  226 
compared  with  reflex,  ii.  225 
Vomiting,  i.  387 

causes  of,  i.  388 

centre  for,  i.  389 

nervous  mechanism  of,  i.  388 
von  Gudden's  commissure,  ii.  238 
Vorticella,  movements  of,  ii.  20 
Vowel-sounds,  ii.  431 

differences  in  quality  of,  ii.  385 

production  of,  ii.  434 
Vulva,  ii.  443,  462 

WALKING,  ii.  420 

Wallerian  degeneration,  changes  of  excitability 

in,  ii.  69 

of  nerve-fibres,  ii.  197 
of  nerves,  ii.  69 

Wandering  cells,  definition  of,  i.  48 
Water,  absorption  of,  i.  313,  318 
amount  lost  through  the  lungs,  i.  410 
distribution  of,  i.  503 
effect  of,  on  pancreatic  secretion,  i.  236 
elimination  of,  i.  340 
imbibition  of,  i.  504 


Water,  latent  heat  of,  i.  504 
nutritive  value  of.  i.  -_'7i;.  :;:,» 
percentage  of,  in  brain  and  cord,  ii.  274 
properties  of,  i.  503 
pure,  toxic  action  of,  on  nerves  and  muscles, 

ii.  r,- 
Weber's  law,  ii.  340 

applied  to  pressure-sensations,  ii.  393 
Weight  of  embryo,  increase  of,  ii.  487 
of  the  brain  and  spinal  cord,  ii.  274 
decrease  of.  in  old  age,  ii.  :.'J»i; 
relation  of,  to  social  environment,  ii.  277 
of  the  child  at  birth,  ii.  487 
Weissmann's  theory  of  heredity,  ii.  502 
Wharton's  duct,  i.  217 
Whispering,  ii.  436 

Whistling  register  of  the  voice,  ii.  433 
White  matter  of  the  central  nervous  system, 

water  contents  of,  ii.  274 
William's  frog-heart  apparatus,  i.  188 

valve,  i.  187 
Wines,  i.  :.:;:> 
Wirsuug's  duct,  i.  231 
Womb.     See  Uterus. 
Work  done  by  contracting  muscles,  conditions 

affecting,  ii.  139 

by  muscular  contraction,  curve  of,  ii.  140 
by  the  heart  ventricles,  i.  106,  107 
Worms,  segmental  nervous  system  of,  ii.  212 
Wrisberg,  cartilages  of,  ii.  422,  425 
Wundt's  tetanus,  ii.  131 
closing,  ii.  37,  61 

XANTHIN,  i.  553 

of  muscles,  ii.  167 

physiological  significance  of,  i.  339 
Xantho-proteid  reaction,  i.  576 
Xylose,  i.  562 

YAWNING,  i.  454 

Young-Helmholtz  theory  of  color  vision,  ii.  335 

ZOLLNER'S  lines,  ii.  351 
Zona  pel lucida.  ii.  454 

of  the  ovum,  ii.  450 
radiata  of  the  ovum,  ii.  449,  450 
Zymogen  granules,  definition  of,  i.  228 

of  the  pancreas,  i.  235 


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